The Boiler as Part of the Wet System

To repeat an earlier statement, the precise type of boiler or the fuel it uses is rarely a matter of any great importance, except to those who sell fuel. The only example of boilers which are not completely interchangeable is that of a system of heating only having all the heat emitters capable of shutting down at the same time. Such a system could not have a solid fuel boiler, which must have a buffer, usually the hot water cylinder, to mop up surplus heat during the slowing down period.

Pressure jet oil fired boilers are not often tolerated indoors, and to that extent are not fully interchangeable with other types. But that is only because of their operating noise, and not relevant to the present context. It is convenient when listing the principal types of boiler available to put them in fuel categories.

Gas: There is the free standing type, with smart casing if it goes in the kitchen, without case if it will go under the stairs or in the attic. The back boiler is allied to a gas fire and is not seen, but is generally much the same boiler as the free standing one, perhaps shortened. The wall hung boiler represents a new breed, with its own technology.

Oil is represented mainly by free standing types, of which there are three: pressure jet, wall flame and vaporiser, depending upon the type of combustion. Oil is taking from solid fuel some of the job of heating combination ranges of the Aga type, and there is even a unit fired by oil which closely resembles a gas fired back boiler with fire.

Solid fuel I.e. a grade of coal, coke or prepared smokeless fuel. Without access to statistics, if such exist, it is probably true to say that there are more back boiler type appliances than any other type using solid fuel domestically. The choice among bigger and more instrumented appliances is now quite limited, and for an assessment at any given time one should consult the Solid Fuel Advisory Service, Hobart House, London SW1. Solid fuel has many admirable qualities, but it lacks the ability to be instantaneously controlled which its competitors have, and it is not therefore always best suited to work in systems which depend upon quick positive control. In the open fire/back boiler combination, where the only control is a manually operated damper, it does well.

The open fire/back boiler, even the closeable fire with back boiler, has been victimised over the years. Far too often it has, no doubt because of its relatively low cost, become associated with other forms of cost cutting which in these enlightened times should not be tolerated. The most usual is the direct system which we have seen to be responsible for bad internal troubles inside systems. By all means have a back boiler unit if you wish, but do not skimp the system on that account. Also, be aware of what you are buying in quantitative terms. Remember, as much heat as you need?

A typical boiler for a typical house would be rated at least 13kW or 45 000 Btu/h for full central heating. A typical open fire/back boiler is rated at about 7.5kW or 25 000 Btu/h. It is unlikely that this or any solid fuel appliance will maintain its rated output through all the changes of fuelling and clinkering, burn up and run down of fire bed intensity and so on. So this unit is necessarily associated with an installation with limited requirements, perhaps partial heating, or selective heating.

Gas Boilers

To look at boilers in the sort of detail which might count when deciding to buy, let us start with that which is numerically most in demand – the gas boiler. If we are to look for a reason for this success, perhaps the foremost is that a gas boiler is easy. It is so easy that it will go for a year entirely unattended. No fuel to order, store, carry; no lighting procedure; rarely any noise or temperamental behaviour. These are points which will appeal to those who are planning for retirement and an easy life.

In spite of the growing variety, the standard gas boiler is still based upon a cast iron heat exchanger. This is the free standing boiler, with or without stove enamelled steel case, usually situated in the kitchen or under the stairs. There is no real limit to the places where a boiler may be placed, though bedrooms should be avoided on account of the disconcerting ‘click’a thermostat can make in the quiet small hours.

It is customary to make the boiler casing to standard kitchen unit height. In spite of this being the subject of a British Standard, it is surprising how ideas vary about what an acceptable height is. The safe course, in order to match kitchen furniture already in place, is to determine the actual height. A short boiler can be stood on a metal plate, or tile, to increase its height.

Most manufacturers try to keep the back-to-front dimension down as well, still pursuing the elusive ‘kitchen unit’ standard. But the real stress is laid upon width, and with certain essential components to be housed it is obvious that the makers cannot go on cutting down all three dimensions. So, in the end, it may take some juggling to get the kitchen looking uniform.

A word of warning against too much emphasis upon neatness of packaging of kitchen cabinets etc. with the boiler. Read the boiler instructions carefully. It is very likely that there must be 50mm space at one or both sides. Some boilers take their essential air in at the back, and it is therefore vital for ample air to be available at the back of the boiler. The flue, if it goes away from the back, must not be in contact with any combustible material. The free space in front of the boiler, if necessary through an opening door, must be equal to the total length of the boiler, so that the burner may be withdrawn during servicing.

It is not a part of the responsibility of anyone employing an installer to decide between the merits of conventional and balanced flue. This is a matter which tends to decide itself, in relation to the site. If there is a good flue or chimney, and if the boiler may conveniently be joined to it and be acceptable in that position, then a conventionally flued model is the obvious choice. A balanced flue should not be regarded as an equal alternative, but as the second choice if a conventional flue is not available. This advice is strengthened by the fact that balanced flue models cost more to buy. It is appropriate at this point to mention the use of the boiler thermostat. Some quaint ideas persist about this, for instance that using a very low temperature setting helps to keep the boiler from wearing out. Unfortunately for this theory the truth is quite opposite. A setting below 60°C encourages the boiler to wear out since it permits condensation in the flueways. When direct systems were common, the problems connected with going to higher temperatures were just as serious. In hard water areas, the higher the temperature the more troublesome the scale deposition. That is why the 140°F was first selected, as a compromise. But now that systems are indirect the prohibition is removed, and we recommend the use of the thermostat for seasonal modulation of boiler output. In hard winter set it high, not less than 80°C, and indeed up to 90°C. This enables the boiler, and the heat emitters, radiators etc, to give their maximum output. In moderate winter weather this temperature may be relaxed, perhaps to the 70-75°C range. The milder beginning and end of the heating season are the times for the lower setting. Such attentions are not strictly necessary, but one should never forget the usefulness of the upper range. Instead of getting annoyed with the inadequacy of the boiler, turn up the thermostat to make it work harder.

Ignition

A distinction must be made between first lighting, and the routine automatic lighting of the main burner which occurs under the influence of the boiler thermostat. The second is standardised, employing a small pilot flame which also heats the thermal junction – the flame safety device. The adjustment of the pilot is a part of the boiler design and should never be altered except by an expert. Spark ignition is becoming more common, but is not an alternative, since a given model or make of boiler has one or the other built in. Spark ignition saves the surprising annual cost of running a pilot. Pilot ignition has no moving parts to wear out. The lighting of the pilot introduces outdoors, making no demands upon the room where it is situated. So if for any reason there is likely to be difficulty in getting an adequate air supply to the boiler, good conventional flue notwithstanding, then in that case a balanced flue model would be preferable.

The basic controls on a gas boiler are nowadays almost exclusively electrical. Despite the degree of dependence implied, the risk is small and the benefits, in crisp positive action, considerable. Thus, the boiler thermostat and the gas control valve, and the programmer if fitted, are all electrical. But it is comforting to note that the most basic safety device, the flame failure mechanism, does not depend upon current electricity. It works entirely off the pilot flame, which heats a small bimetal junction, generating an electromagnetic potential which holds open a key gas valve. If the pilot goes out that key gas valve cannot remain open, and nothing can open it. So however much the boiler instruments call for heat, no gas will flow unless the pilot is alight to ignite it.

A boiler which is engaged in heating is a part of a circuit which itself has to be controlled. There is a good deal of sense and usefulness in lumping most of those controls on to the boiler, so that it becomes the ‘engine room’ of the system. The commonest such addition is the programmer, whose principal duty is to control the starting and stopping times of the boiler, whether it is required to give domestic hot water only or a full service of hot water and central heating.

There are simple programmers with four or five programmes on offer, and there are magnificent units with a great many choices. We will not attempt to indicate which is preferable, and indeed it is a matter for personal choice, influenced perhaps by consideration of how the household operates in its coming and goings. The only pertinent observation to make from experience is that people with say ten programmes to choose from rarely use more than three or four.

A fully equipped boiler usually houses a junction box, into which are wired such external services as the pump and the room thermostat which, along with the clock in the programmer, controls its activity. some variety of method, though too much should not be made of this, for it will normally occur only once or twice in a season. The room sealed appliance, I.e. balanced flue or Se-duct, is never nowadays lit manually. In anything like an on-terminal wind it is impossible. In the earliest days models were made for manual lighting, and users were advised to hang their hat over the terminal temporarily. But six floors up? With that exception, the main methods of lighting a pilot are: (1) Manual, using a taper or match, never a bulky spill of paper. (2) By glow coil, mains fed through a transformer. Many multifunctional controls operate at 12 volts, and the transformer handles everything. The glow coil method is losing popularity, partly because like all incandescent coils it has a measurable life. (3) By piezo-electric igniter. The quite unexpected result of hitting a certain crystalline chemical a sharp blow is to produce a very high voltage, which can be made to jump a gap as a spark. This is the piezo effect and is built into a small triggering device in the form of a pull or push button. It is entirely independent of any outside source of energy. Piezo igniters are now being sold as hand held units, like the old flint lighter for cookers. They could therefore be used for manual lighting.

Servicing

The routine servicing of a gas boiler involves partial dismantling, removal of burner and of any cleaning cover necessary to enable the heat exchanger to be thoroughly brushed or scraped. A particular area for attention is in the gas injector and at the burner tips. The entrainment of air can lead to a collection of air borne dust and fibres, and a condition called ‘linting’ can occur. This interferes with the designed air/gas ratio, hence with good combustion. The prohibition against using a paper spill for lighting is to keep paper ash out of the burner. though bedrooms should be avoided on account of the disconcerting ‘click’ a thermostat can make in the quiet small hours.

It is customary to make the boiler casing to standard kitchen unit height. In spite of this being the subject of a British Standard, it is surprising how ideas vary about what an acceptable height is. The safe course, in order to match kitchen furniture already in place, is to determine the actual height. A short boiler can be stood on a metal plate, or tile, to increase its height.

Most manufacturers try to keep the back-to-front dimension down as well, still pursuing the elusive ‘kitchen unit’ standard. But the real stress is laid upon width, and with certain essential components to be housed it is obvious that the makers cannot go on cutting down all three dimensions. So, in the end, it may take some juggling to get the kitchen looking uniform.

A word of warning against too much emphasis upon neatness of packaging of kitchen cabinets etc. with the boiler. Read the boiler instructions carefully. It is very likely that there must be 50mm space at one or both sides. Some boilers take their essential air in at the back, and it is therefore vital for ample air to be available at the back of the boiler. The flue, if it goes away from the back, must not be in contact with any combustible material. The free space in front of the boiler, if necessary through an opening door, must be equal to the total length of the boiler, so that the burner may be withdrawn during servicing.

It is not a part of the responsibility of anyone employing an installer to decide between the merits of conventional and balanced flue. This is a matter which tends to decide itself, in relation to the site. If there is a good flue or chimney, and if the boiler may conveniently be joined to it and be acceptable in that position, then a conventionally flued model is the obvious choice. A balanced flue should not be regarded as an equal alternative, but as the second choice if a conventional flue is not available. This advice is strengthened by the fact that balanced flue models cost more to buy. So if for any reason there is likely to be difficulty in getting an adequate air supply to the boiler, good conventional flue notwithstanding, then in that case a balanced flue model would be preferable.

The basic controls on a gas boiler are nowadays almost exclusively electrical. Despite the degree of dependence implied, the risk is small and the benefits, in crisp positive action, considerable. Thus, the boiler thermostat and the gas control valve, and the programmer if fitted, are all electrical. But it is comforting to note that the most basic safety device, the flame failure mechanism, does not depend upon current electricity. It works entirely off the pilot flame, which heats a small bimetal junction, generating an electromagnetic potential which holds open a key gas valve. If the pilot goes out that key gas valve cannot remain open, and nothing can open it. So however much the boiler instruments call for heat, no gas will flow unless the pilot is alight to ignite it.

A boiler which is engaged in heating is a part of a circuit which itself has to be controlled. There is a good deal of sense and usefulness in lumping most of those controls on to the boiler, so that it becomes the ‘engine room’ of the system. The commonest such addition is the programmer, whose principal duty is to control the starting and stopping times of the boiler, whether it is required to give domestic hot water only or a full service of hot water and central heating.

There are simple programmers with four or five programmes on offer, and there are magnificent units with a great many choices. We will not attempt to indicate which is preferable, and indeed it is a matter for personal choice, influenced perhaps by consideration of how the household operates in its coming and goings. The only pertinent observation to make from experience is that people with say ten programmes to choose from rarely use more than three or four.

A fully equipped boiler usually houses a junction box, into which are wired such external services as the pump and the room thermostat which, along with the clock in the programmer, controls its activity.

It is appropriate at this point to mention the use of the boiler thermostat. Some quaint ideas persist about this, for instance that using a very low temperature setting helps to keep the boiler from wearing out. Unfortunately for this theory the truth is quite opposite. A setting below 60°C encourages the boiler to wear out since it permits condensation in the flueways. When direct systems were common, the problems connected with going to higher temperatures were just as serious. In hard water areas, the higher the temperature the more troublesome the scale deposition. That is why the 140°F was first selected, as a compromise. But now that systems are indirect the prohibition is removed, and we recommend the use of the thermostat for seasonal modulation of boiler output. In hard winter set it high, not less than 80°C, and indeed up to 90°C. This enables the boiler, and the heat emitters, radiators etc, to give their maximum output. In moderate winter weather this temperature may be relaxed, perhaps to the 70-75°C range. The milder beginning and end of the heating season are the times for the lower setting. Such attentions are not strictly necessary, but one should never forget the usefulness of the upper range. Instead of getting annoyed with the inadequacy of the boiler, turn up the thermostat to make it work harder.

Ignition

A distinction must be made between first lighting, and the routine automatic lighting of the main burner which occurs under the influence of the boiler thermostat. The second is standardised, employing a small pilot flame which also heats the thermal junction – the flame safety device. The adjustment of the pilot is a part of the boiler design and should never be altered except by an expert. Spark ignition is becoming more common, but is not an alternative, since a given model or make of boiler has one or the other built in. Spark ignition saves the surprising annual cost of running a pilot. Pilot ignition has no moving parts to wear out. The lighting of the pilot introduces some variety of method, though too much should not be made of this, for it will normally occur only once or twice in a season. The room sealed appliance, I.e. balanced flue or Se-duct, is never nowadays lit manually. In anything like an on-terminal wind it is impossible. In the earliest days models were made for manual lighting, and users were advised to hang their hat over the terminal temporarily. But six floors up? With that exception, the main methods of lighting a pilot are: (1) Manual, using a taper or match, never a bulky spill of paper. (2) By glow coil, mains fed through a transformer. Many multifunctional controls operate at 12 volts, and the transformer handles everything. The glow coil method is losing popularity, partly because like all incandescent coils it has a measurable life. (3) By piezo-electric igniter. The quite unexpected result of hitting a certain crystalline chemical a sharp blow is to produce a very high voltage, which can be made to jump a gap as a spark. This is the piezo effect and is built into a small triggering device in the form of a pull or push button. It is entirely independent of any outside source of energy. Piezo igniters are now being sold as hand held units, like the old flint lighter for cookers. They could therefore be used for manual lighting.

Servicing

The routine servicing of a gas boiler involves partial dismantling, removal of burner and of any cleaning cover necessary to enable the heat exchanger to be thoroughly brushed or scraped. A particular area for attention is in the gas injector and at the burner tips. The entrainment of air can lead to a collection of air borne dust and fibres, and a condition called ‘tinting’ can occur. This interferes with the designed air/gas ratio, hence with good combustion. The prohibition against using a paper spill for lighting is to keep paper ash out of the burner.

All electrical connections should be examined, and so far as possible instruments and controls checked for operation. It is however usual in routine servicing to assume that if controls are known to be operating satisfactorily they are not in need of detailed attention. Servicing includes taking care of anything specifically mentioned as being in need of attention. Cover plates almost always have a gasket, of asbestos or similar, to form an air tight seal. These gaskets should be renewed.

A good service engineer will always leave the appliance at work, unless instructed otherwise, and will draw the owner’s attention to anything he should know, such as a particular item which ought to receive attention at the next service visit.

Change and innovation are characteristics of a progressive technology, and there is always something happening in the total field of gas boilers and the like. Frequently it is not greatly significant, just enough to give a manufacturer a talking point, but one fairly recent change seems to be in a more significant category, in part because it has to do with that important household commodity, space. The innovators named it Mighty Mouse, though a change of company ownership resulted in the less memorable renaming to Worcester Delglo.

The ex-MM is based upon a gas fired boiler, output 13.4 kW or 45 000 Btu/h. Instead of serving a hot water cylinder in some remote airing cupboard, it incorporates its cylinder within the case, thus saving not only space but heat loss from pipework. More than that, it employs a larger than usual heat exchanger, giving a high heat make-up rate into a reduced volume of stored water. The makers claim that it will serve a bath every 12 minutes.

It has conventional and balanced flue models, and is British Gas approved.

The Gas Fired Back Boiler

Unlike the free standing boiler, which may be made in any size, the back boiler has limits imposed upon it by the aperture available. The free standing boiler is made right up to the generally accepted but arbitrary limit for domestic equipment of 44kW or 150 000 Btu/h. A back boiler is still struggling to beat 13kW or 45 000 Btu/h. But in terms of usefulness that is only a part of the story. The gas fire will almost always give ample warmth for the room in which it is situated, and the total effective output of the unit could be placed at say 4.5kW higher.

The problem of how to get more than a pint into the pint pot of a British Standard chimney opening is almost the only technical problem left. Details of flue sharing with the fire, remote lighting of the pilot or burner, controls for the boiler incorporated in the fire, have all been satisfactorily settled. It must be noted however that the pairing of a fire and a boiler is fundamentally work for a manufacturer. If you have a gas fire you cannot go out and buy a boiler, and couple the two together. In case it is not obvious, this application is for conventional flue only, and it is noteworthy for two reasons. First, it makes the most of the limited space in modern dwellings, by taking no space at all. Second, it forces upon the user a source of radiant heat. This is in some respects superior to convected warmth though impractical as a source of ‘central’ heat.

All the comments already made about ignition and servicing apply, with the extra proviso that the fire must be removed in order to get at the boiler.

Wall Hung Boiler

A development even more recent than the back boiler is the wall hung boiler, another concession to the urge to save space. Weight was very much in mind at the start, and in some cases no doubt still is. Cast iron was abandoned in favour of alloy coated copper for the heat exchanger, and a fair comparison would be with an oversized Ascot type water heater. With this went very small waterways and an entirely new concept of urgency, necessary when applying a lot of heat to a little water. This had to be kept moving rapidly, and so was continuously pumped. A change in control methods had to be made, to ensure that the burner would not light unless the pump were working, and that the pump would stay working for long enough after the burner went out to purge the ‘after heat’.

But now ways have been found to blend the old technology into the new package, and we have wall hung boilers which have cast iron heat exchangers and are suited to gravity circulation as well as to pumped primaries.

It must be kept in mind however that though the conditions may be relaxed they are most carefully worked out. When fitting one of these, or indeed any boiler, the maker’s instructions should be read and followed very carefully. These instructions are a part of the official approval and always a condition by which the guarantee is valid. In other words if you do not fit it as instructed you cannot expect any sympathy if it misbehaves.

Boilers on Bottled Gas

In theory — and we stress that point — there is no reason why almost any gas appliance ever devised should not be made in a bottled gas version. But in practice the list is quite short, for two reasons. One is that manufacturers are under no obligation to work out the necessary design characteristics in fulfilling their prime obligation, which is to the approvals authority, British Gas. Some do, and no doubt other makers would if they could command the very considerable work force needed. A second reason may be that conversion characteristics have been worked out as a laboratory exercise, but no parts manufactured. If anyone is interested in converting an existing appliance, or is particularly taken by one model of a new one, the only sure way to find out if parts or even information are available is to ask the technical staff of the manufacturer concerned. Given the information, the gas supplier might be able to do the conversion.

In spite of the difference in combustion and combustion equipment, bottled gas appliances are operationally similar to those using natural gas in all relevant respects. The technicalities are already built in and do not concern the user. Installation might be slightly different in that the required size of the gas pipe supplying the appliance can be smaller for bottled gas.

Some notes on bottled gas appear in a preceding post. The only query likely to arise concerns the size of storage container, which may depend on the number and type of appliance to be supplied. This is best resolved by taking the advice of the supplier, which will be based upon national experience. The aim is to strike a balance between keeping the cost of hire down, and taking advantage of any bulk discount obtainable — and getting a reasonable time of running out of one container.

Oil Fired Boilers

Perhaps the most important distinction between types of oil boiler divides those which can be used indoors or out from those which are best suited to be away from the house. The latter are the pressure jets, and perhaps the name jet will suggest why this type of burner is best given a place of its own to operate in. Of the rest, those which depend entirely upon natural operation are silent, the rest make some noise however slight. It is always worth asking, in any given case, whether there is a history of the model making more noise as it ages and wears.

The principal types of vaporising burners, as they are called, are the wall flame and the pot type, for natural or fan assisted draught. The wall flame and the fan assisted pot type must have electricity, but it is usual nowadays to electrify all types, because as in the case of gas it makes control so much easier and so much more standard.

Another common feature of these boilers is that they burn kerosene, which is 28 sec or Class C fuel oil. They operate in a routine way under the control of a boiler thermostat, and it depends upon the detail of the controller whether the burner works on/off or on high/low flame.

The incorporation of other controls, a clock or programmer for instance, present no difficulty so long as the boiler is electrified, for controls of this kind simply control electric circuits. A clock, for instance, is only a means of switching on or off, and so stopping or starting an operation.

We will not make a serious attempt to describe how these boilers operate. They are not suitable subjects for amateur adjustment or even servicing, and so the details are not relevant. It should be sufficient to know that all the vaporisers have a method of converting the liquid fuel to a vapour, in which form it burns. The method of conversion differs considerably from one type to the next. It is most important that the boiler is not neglected. For instance signs of sooting might indicate that a primary air port has become blocked, perhaps by something falling across it. Irregularities not understood should be reported, and as with gas boilers it is desirable to have an understanding, if not actually a service contract, with a trusted engineer.

Not all models of this type of boiler are available over the entire domestic range, nominally up to 44kW or 150 000 Btu/h. The starting range is about 8 kW or 27 000 Btu/h. Forced draught vaporisers go up to about 23 kW or 80 000 Btu/h, and there are wall flame or rotary vaporisers up to the maximum.

A point to watch for and to get settled at the earliest stage concerns the type of combustion control and the maker’s instructions about the system. Boilers of the vaporising type which have on/off control are in general in line with any other instantaneously controlled boiler unit, such as gas. They are safe for use in a ‘heating only’ system if necessary. But boilers which have high/low flame control, in which the low flame may be compared to an exaggerated pilot, generate an amount of heat in their ‘at rest’ position, which must be dissipated if overheating is to be avoided. It is customary to employ this surplus warmth usefully, through the primary circuit which feeds the domestic hot water cylinder, the cylinder being an essential part of such a circuit. The heat emitter is a towel rail or small radiator, usually in the bathroom, which is always in circuit. Being in the hot water primary it is independent of any circulating pump which may be on the heating circuit proper. The need to know about any such requirement before ordering materials and running pipes, will be obvious.

We have already described the pressure jet boiler as a type unsuited to being indoors, and this is bound to bring pained reaction from some manufacturers. Operational noise occupies a major part of their development engineers’ attention, and someone is always claiming to have beaten the bogey. But in the end it is the owner’s sensitivity of hearing which settles the matter, and we would advise anyone in doubt to ask for a practical demonstration. But let us add one other note of caution. The site can sometimes influence the nature and level chimney which runs by a bedroom, where the flue gases have to make a sharp change of direction. The effect, particularly in the quiet of night, is of distant battle, muffled but insistent. It is a clear warning to pay attention to the flue as part of the whole installation.

Pressure jets differ from other forms of combustion, in that the flame is not divided between a number of burner jets or spread over an area. It is concentrated in one jet, rather like the water from a fireman’s hose, and like that example it issues under pressure. The fuel oil is ejected under pressure from the pump which is incorporated in the burner, and at the point of release is atomised by being given a swirling motion as it passes through a very finely machined and calibrated nozzle. The combustion air, also under pressure and controlled, is admitted and admixed in a very precise way, within the burner assembly. By choice of nozzle, by adjustment of the pump pressure and the air inlet damper, a high degree of precision is obtainable. One of the principal objects of regular service is to ensure that the set conditions are maintained: to keep the nozzle free from foreign matter which might have got past the filters; to see that there is no interference with the air ways.

Pressure jets are on/off burners, and ignition is by spark from a pair of electrodes bridging the exit of the fuel nozzle.

The brain and heart of a pressure jet is the control box. It houses the boiler thermostat, for instance, and when this is satisfied it cuts the current to the burner pump and fan. When the thermostat again calls for heat it sets the spark ignition going, as a preliminary to starting oil pump and fan. If the burner does not light it persists for a given number of seconds, then stops. There is then a pause, known as a purging period, to allow unburnt oil vapour to escape. The control box will then initiate another firing period of similar length to the first. If this fails the burner will shut down, and a red light will show. It is a signal to see what might be wrong, and nothing can be restarted until the user has manually reset the burner by pressing a green button.

At this stage someone is sure to ask how it is that the control box knows that the burner has not lit. When it sees a flame it generates a very small electric current, enough to inform the control box. If that information is missing the control box reacts in the way described. If it does, the first and most obvious step to take is to make sure that the ‘eye’ is clean. Quite often a shut-down is due to nothing worse than a thin film of carbon or soot on the glass. But, such is the overall reliability of the device, the second most common cause of flame failure is running out of fuel. This might be due to a blocked filter or pipe line rather than an empty tank.

Pressure jet boilers are generally less sensitive about flue conditions than the vaporising types, for the very good reason that they create a pressure in the combustion chamber to start the evacuation process going.

The smallest pressure jet boilers are in the 14 kW or 50 000 Btu/h range, but they then go on into sizes far above the domestic range. They belong therefore to the larger installation.

Pressure jet burners may be specified for one class of fuel only, I.e. Class C or D, which is kerosene or gas oil. Or they may be classed as suitable for either. In that case it is necessary to see that all the conditions relative to the class of fuel being used are observed. These conditions usually refer to the rating of the nozzle, the oil pump pressure, and then during setting up the correct adjustment of the air damper to go with the oil input. It must not be expected that a change can be made from one grade of oil to another without such adjustments.

If the fuel oil store happens to be buried, or at a lower level than the burner, the pressure jet burner offers one advantage. When considering fuels reference is made to the two-pipe system for getting a supply of oil to the burner. The pump for the purpose is the oil pump on the pressure jet burner, with only a simple adjustment to be made during installation.

Combined units

Oil, like gas and solid fuel, has its hearth fitted unit with a fire in the front and a boiler at the back. The similarity with gas is the more obvious, since the fire resembles a gas fire in appearance. A typical model is as described below.

The boiler is a natural draught vaporising type, with high/low control, and the notes about installation and dissipation of surplus heat apply to it. The total rated output of the model shown is 10.9 kW or 37 400 Btu/h, of which the amount going to water may be varied, from 5.1 kW to 9.3 kW, the rest of course going to the room as radiation and convection. The method by which this ratio is determined could not be simpler.

By means of a knob at the side, the angle at which the radiant section rests is varied, to bring more or less of it into direct heat. The whole of the section is glass fronted.

Because of the limitations imposed by the chimney size, this type of unit is unlikely to exceed 12 kW or 40 000 Btu/h rated output. Within that total will be ample output for most rooms in which the unit is likely to be situated. The oil supply is usually piped in from outdoor storage. But it might interest in particular anyone seeking a ‘can take it with you’ unit to find one which has an incorporated fuel tank.

Flue Stabiliser

A flue stabiliser is usually in the form of a flap damper which is free to swing, from shut to some way open, under the influence of changing chimney draught or ‘pull’. Its purpose is to stabilise the amount of draught at the base of the chimney, when the natural draught emanating from the top of the chimney is fluctuating. A surge of draught causes the damper to swing open, and air enters through the open damper, sufficient to satisfy the extra demand caused by the draught increase. In consequence the extra demand is not passed on to the base, and so to the boiler or other appliance fitted to the chimney.

Oil fired appliances in particular work best in stable draught conditions, and of those the vaporising type are most sensitive. They are generally fitted with an integral draught stabiliser in the flue offtake, and this should be able to swing freely.

Pressure jet boilers, due to the start they get from having a slightly pressurised combustion chamber, do not often have a built-in stabiliser, and makers commonly state that in normal conditions none is needed. But in extreme conditions, such as one might meet on an open site in, say North Cornwall, high winds might cause high and fluctuating draughts in a chimney. In such cases, and in cases where vaporising boilers are not already equipped, a separate stabiliser would have to be fitted in the chimney.

When this is done, it is imperative that it shall be fitted in the same room as the boiler or appliance. It cannot be for instance in the room above. It is usually put up near the ceiling. The reason for this condition is that it must share the conditions which apply to the burner, on both sides of the damper, inside and outside.

Solid Fuel Boilers

The breadth of choice of heating equipment with solid fuel is more than with other fuels. In consequence of this there is a greater responsibility upon the seller of apparatus to declare the maximum performance, or rating, of every appliance. It is when this responsibility is avoided, and too much emphasis placed upon the simple evocative phrase ‘central heating’ that troubles can start. The law recognises that the buyer has a duty to look after himself, and in this market a potential buyer should always enquire, first, what is the official rating of an appliance.

Then, except in the case of automatically fed hopper charged boilers, he should allow for the difference between test conditions, charging every 1 ½ hours, and the real life conditions into which the appliance would fit. It would not be unreasonable to take 10% off the rated output for that.

Because the result of such a preliminary enquiry will be to narrow the field of choice at the outset, let us start there.

The open fire with back boiler will have a boiler output in the region 3 kW or 10 000 Btu/h. If described as high output this could go as high as 7 kW or even higher. This is ‘rated output’ of course. Except in the smallest house this is for partial or background or selective heating only. (2) The closed or closeable room heater with back boiler. We must consider this working as a closed unit since the opening of the front causes a significant drop in performance. The maximum rating for such a unit is 13 kW or 43 500 Btu/h which if sustained could satisfy a good proportion of dwellings. To that output must be added the convection and radiation from the heater which is probably adequate for the room where it is situated. (3) The free standing manually fed boiler. This is probably the type with which older people are most familiar, perhaps when it was in the form of a cylinder, of cast iron with a lid on the top. Originally coke boilers, they will burn any smokeless fuel, the correct size grade being that recommended by the manufacturer. The use of bituminous or house coal is allowed only if the firebed is specially formed as a ‘Smoke Eater’. This type of boiler is frequently used as an incinerator of kitchen rubbish. We cannot stop that practice merely by disapproving of it, but may we urge moderation. Years ago we were puzzled by the lady who complained that whenever she burned cabbage stalks she got excessive clinker formation. It did not seem like a common property of cabbage. After a time it was traced to the fact that the cabbage stalks tended to damp the fire down, and the owner opened the damper wide to brighten it up again. Hence the heavy clinker. Manually fed boilers for domestic use fall into a category roughly between 5 and 15 kW, or 17 and 50 000 Btu/h rated output. (4) The free standing hopper fed boiler is way ahead of the others in operational ease and efficiency. It provides the working conditions in which the purest of the solid fuels, anthracite, will burn, and in return gives steady output and very limited interference from ash. It is a demanding boiler, in that it must have a good flue and that size of fuel is critical. The price of anthracite is matched to the size, and most of the hopper fed boilers currently available use peas or grains. The density of packing of these sizes means that a small electric fan is needed to force air through the fuel bed. This offsets any saving on fuel cost but gives an extra measure of control over combustion. One side benefit is to extend greatly the length of time the boiler can stand idling, without the fire going out. A clock is wired into the fan circuit, timed to cut in for a short period at intervals, in order to brighten the fire. If there is a natural draught boiler available at the time of going to print, it will use a larger fuel, probably beans. Any boiler in this class which is allowed to burn bituminous fuel will have a Smoke Eater grade.

The frequency of filling the hopper, and of removing the ash, are competitive features between makes. But bearing in mind that there is no poking or riddling to do – and indeed it would be wrong to do it – then attention once daily should not be excessive. In most models it is arranged that the ash when withdrawn is nearly cold, and so not giving rise to air borne dust.

These are the larger boilers, from about 13 kW or 40 000 Btu/h upward and into the commercial sizes. They are therefore readily available for full central heating in any domestic premises.

We have already commented upon what might be called the social grading of these types of boiler, but it bears repeating. There is no justification for the belief that the first two, and perhaps the third, can satisfactorily be connected to direct water systems. The need for an indirect system is as urgent for them as it is for the last, the hopper fed boiler. It is the heat, not the type of boiler, which damages the system.

Despite their wide differences, the four types of boiler have two things in common: (1) They all have to be lit manually. This may not be the paper and sticks method, and it is quite common to use a gas poker, or an electric poker which generates a stream of very hot air to blow on to the fuel. Or again there are patent firelighters. The manufacturer will almost always have something useful to say on the subject, in his instructions. (2) They all have the phenomenon of response lag, which causes unwanted heat to be generated after the command to stop. Consequently they all must be provided with a buffer or heat leak. In most cases, certainly where the outputs are in the lower range, the hot water cylinder will absorb the normal surplus. But in case of doubt, and for larger boilers, it is desirable to include a radiator or towel rail in the hot water primary circuit, to be left on continuously.

At the top end of the domestic range is a class of boiler which is industrial in design, and unquestionably needs to be given a boiler house. It requires an independent hopper, facing the boiler, the two being connected by a screw feed mechanism which provides a slow but constant supply of fuel to the base of the fire. This is called under-feed stoking.

The smallest size likely in this would be about 24 kW or 80 000 Btu/h. Apart from the amount of space needed, it may be claimed for this type of boiler that it does away with even daily attention such as a gravity fed hopper boiler needs. Offsetting that is the cost of operating the screw feed.

In comparison with other fuels, the lighting of solid fuel is a chore, and one to be avoided for as long as possible. There is the added disadvantage that an appliance which has gone cold takes a relatively long time to warm up again, because the fire is slow in getting hold. It is of considerable importance therefore to know how long any appliance will keep going between re-fuellings, so that it may be left and not go out. The obvious and recurring period of neglect is overnight.

Though it is clear that the period will depend upon the rate of burning, whether the boiler is working hard or idling during the period, it is usual to assume that we are considering an idling period, overnight being again typical.

The hopper fed boilers present no problems in fuel availability, since the hopper capacity is almost always capable of riding over at least a day, usually longer at low or nil load. In fact the problem with such boilers is more to prevent them dying of inaction.

The manually fed free standing boiler and the closed room heater are usually designed to have enough room for fuel so that with a nearly closed damper they will idle on for many hours.

The open fire rarely has any such provision, even though the need for continuity of combustion is there. Consequently the makers, if in doubt, supply a deepening bar, a removable plate which is slipped in at night to increase the height of the front of the fire, so creating a larger pocket for fuel. The nature and usefulness of the deepening bar if supplied are examined by the approval authority, and if satisfactory are included in the approval. Intending purchasers might like to note that they have a perfect right to see the precise terms of any approval granted to an appliance and the seller should make this available to them.

It is usual to remove the deepening bar during normal usage hours, not least because it will shield off some of the radiation from the room. Along with its use as a means of prolonging an active fire goes partial closing of the damper which is below the fire bed. This greatly reduces the amount of air able to get under the fire bed, while one role of the deepening bar is to deflect off the air which otherwise would sweep across the top of the fire – both being responsible for some fuel corn-sumption.

Many modern units have one more damper, fitted in the throat or go-away flue of the appliance. The purpose of this damper is to control by throttling the amount of draught or ‘pull’ which the chimney is exerting upon the fire, which in practical terms means the amount of air it is persuading the fire to use. One operation in preparation for idling is to part close this damper, so reducing the demand made by the chimney. All the preparations are designed to reduce activity, and not for nothing is a fire in this condition said to be slumbering. The maker’s instructions will generally give an indication of the best settings for dampers, though it will be obvious that no absolute values can be given since chimney draughts vary from place to place and from time to time. Fully shutting dampers will almost always result in the fire going out.

Electric Boilers

Electricity does not come readily to mind when talking about boilers. Nevertheless there are two kinds of electric boiler which must be mentioned in the domestic field.

One, the electrode boiler, seems never to have become a serious item on the market, and may be disregarded. The other, known as the Centralec system, is available and would be of particular interest to those who are debarred from having any kind of flue on their premises. Centralec is basically a storage heater of the storage radiator type, which instead of passing its stored heat into the air passes it into water, under control. The water is, of course, the wet heating system, which after it leaves the Centralec unit is exactly the same as any other wet system.

The heater being of the storage pattern qualifies for the use of cheap off-peak current, which is charged at off-peak hours. The capacity of the unit is then made to match the heat requirements over the whole 24 hours. The unit may be obtained with a fitted circulating pump, or an external pump may be used.

One of the advantages of a wet system over a storage radiator system is in greater precision of control over heat output. In Centralec we find electric storage allied to finer control, another advantage.

The Size of the Boiler

A custom has grown up, from father to son as it were, of calculating the size of boiler required for a job, and then adding 10% or even 20%, and we have known even more. But it is neither necessary nor good practice. It arose originally because the waywardness of the solid fuel boiler in the user’s hands was recognised, and an allowance made for that. As a cover against bad calculations it was welcomed by some workers, who usually excused it as a hedge against extra bad weather.

We have already mentioned that in the case of manually fed solid fuel boilers one should take a conservative view of the real output in comparison with the rated output. But that is the only case. In all other circumstances oversizing — for that is what it is — is wrong, leading to waste of money and early wear on the boiler. Since the greater part of the heating season is far from being as severe as that on which the calculations are based, we already have a situation, with a boiler of correct size, where it takes a number of rest periods during working hours because it has done its job quickly. Every time it goes off, which it might do for a total of say 30 minutes every hour, things start to cool, and heat is dissipated mostly uselessly. When it goes on again that heat has to be made good, and all this counts against the running cost. Consequently, to make the boiler even bigger is simply to add to the down times, the cooling and the loss.

To anyone tempted still to think of the extreme case of bad weather let us say this. Life is almost wholly made up of quite ordinary weather, bad though it may be. When the wolves are howling outside it is a crisis situation in which you can afford to make a temporary concession. Decide to see it out by having heating in only say two rooms, instead of four or five or six. For that limited duty the boiler will be adequate. But do not live the rest of your life paying extravagantly for the remote possibility that some such crisis might occur.

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