Introduction to central heating systems

Before studying any subject it is necessary to establish guide lines, to define it. This is as true of central heating as anything else. For the name proves, on examination, to have been used quite loosely.

Central heating was first called ‘central’ for the best of reasons, that it was heating from a central source, usually a coal fired boiler. This compared with the unsystematic methods which had been in use since the end of the Roman occupation, often one open coal fire in the house, or in the last 150 years the coal fired range, which cooked as well. Larger houses with better-off occupants would generally have several open fires, quite often one in each room, which accounts for the multiple chimney stacks in buildings of the period.

The earliest domestic central heating was cumbersome, usually wholly dependent upon the meagre forces of gravity for its circulation, which made it equally dependent upon a good standard of installation so as to offer a minimum of resistance. Otherwise unexplained noises in the house were customarily and usually justly attributed to the central heating.

This state of affairs ended abruptly following work done by a research association in 1956. They showed, in the course of finding more outlets for coal, that a lot of heat could be obtained from a small boiler, with small pipes, if the water were pumped around to the radiators. This was the ‘small bore’ system, whose advent led to a wide range of activities. For instance pump manufacturers were forced to look into the availability and reliability of suitable models, and controls manufacturers came to life. But it was the oil industry which seized upon the system and gave it a high market rating. Only later did gas join in seriously, in due course to establish a lead as suppliers of fuel for boilers. Solid fuel, through coke and special coals, maintained a steady but low market share.

Meantime, it was not to be expected that the electricity industry would sit by and watch its competitors making hay in such a potentially vast market. They could not compete in the boiler market, and had to look elsewhere. This led to the development of the off-peak storage heater, which also offered some solution to their own generating problem, of what to do with plant necessary during the day but standing expensively idle at night. Electricity as heat energy could be stored in massive receivers overnight, so that it would leak out during the day and warm the house. The first efforts led to underfloor heating, in which concrete floors formed the heat reservoir.

Underfloor heating could rarely be applied to any but new housing, which was too limiting. So there arrived the unit heater which was, and is, a rather heavy free standing unit of manageable size, incorporating a heating coil and heat absorbing material, and scientifically insulated to permit a calculated rate of heat leakage to air.

At that stage the electrical effort was just as surely random and non-central as the old coal fire had been. But it was sold quite firmly under the central heating banner, which is why the name has assumed a portmanteau quality. Only later did electricity produce the central unit, Electricaire, which is in effect a very large unit heater with ducts to other rooms and a fan to distribute warmed air.

We have often had a pedantic urge to give up using the term ‘central heating’ but the difficulty lies in knowing what will take its place. ‘Whole house heating’ is tempting but fails because it is untrue in the case of partial central heating, itself a respectable and well documented variant of central heating. To refer to ‘modern heating’ is too vague, too reminiscent of someone’s advertising. So in the end we come back to Central Heating, which despite its imperfections is widely understood.

There is a way to classify heating systems broadly, which divides them into ‘wet’ and ‘dry’. All the systems to be described in this post fall into one of those categories and, broad as they are, they are very important in one respect. With a wet system it may be assumed that the domestic hot water supply, I.e. hot water to taps, is taken care of, or at any rate can be covered without additional fitments. On the other hand with a dry system additional equipment is necessary to supply domestic hot water. More of that in a preceding post1.

Wet systems employ water as the heat conveying fluid. Water is extremely suitable for this duty. It is cheap, usually freely available, not outstandingly corrosive, is relatively harmless if it escapes from the system, and it has a high capacity for heat. It must of course be confined within pipes, and we have already mentioned that large pipes began giving way to small pipes from 1956. More recently, small pipes have been sharing the market with very small pipes, called microbore, and this indicates how the pump industry has progressed.

Wet systems have a boiler, which puts the heat into the water. We will discuss boilers in a preceding post. In order to take the heat out of the water at points where it is required, the most common device is the radiator. Everyone knows what a radiator is, and it is important in installation and in room furnishing to remember that it does radiate anything up to half of its total heat output. For that reason it must always be able to ‘see the room’, never hidden away in analcove or behindheavy furniture. For the rest it is however a convector.

But a convector by that name is a device which gives next to no radiation, only a stream of warm air. Convectors may be natural, I.e. without fan assistance; or they may be fan con-vectors, which give very little warm air when not at work, but a lot when a thermostat or similar device switches on the fan. A skirting heater, which the Americans tend to call a baseboard heater, is a form of convector, and a very efficient one in terms of maintaining even room heating, horizontally and vertically. An interesting application of a wet system is that in which it serves as the heat source for a dry system. This is covered in a preceding post.

Dry systems are, not too surprisingly, all those which do not use water as the heat conveying fluid.

A high proportion of dry systems depend upon the direct heating of air, which is then circulated. Having pointed out the advantages of water as a medium, let us list the advantages of air. It is even cheaper and more readily available than water, it is entirely non-corrosive, is not only beneficial but essential to life, is not a hazard if it leaks from a conveying pipe or duct, and it does not freeze. Against those qualities the only negative is that it has a low capacity for heat.

Though we will deal with warm air systems in more detail later it may be noted here that there are two broad types. There is the random type, in which warm air is generated and left to find its own way around, mainly by drift, possibly with a little assistance. This system can be quite satisfactory in certain styles of dwelling, principally those which have a substantial vertical factor. It is not very well suited to most bungalows, which have no noticeable vertical factor. The other type is the ducted system, which conveys warmed air to specific points for discharge, and then arranges by means of return air ducts to bring most of it back for reheating.

The dry systems which do not employ air directly as the heating medium are principally electrical. That is to say, electricity is used to heat a solid or a liquid, and in one way or another this material passes on its heat to the surroundings. In most cases it does so by creating a current of warmed air, which then circulates. This includes oil filled electric radiators; storage radiators which are mainly based upon natural circulation but may be obtained with plain damper control or with fan control; underfloor heating, which is no longer being installed but deserves a mention. Storage radiators qualify for a reduced tariff, off-peak or White Meter.

A form of electric heating in which air circulation plays no direct part is the low temperature radiant panel. This is generally a structural item and may be fitted to wall or ceiling. It constitutes a large area source of low intensity radiation which is invisible but can be sensed by anyone in its path.

So far we have classified central heating according to fuel and method of operation, but there is another division — by its scope. There are degrees of central heating and some of the terms used are full, partial, background and selective heating.

These phrases were coined early in the development of modern central heating, and although they are not written into any legal definition or standard they are well understood in the heating trade. Let us look at them more closely.

Full central heating: a term applied to a system which will maintain all rooms at their specified temperatures simultaneously, when the outdoor temperature is 30°F or — 1°C.

Partial central heating: will maintain specified rooms at their specified temperatures simultaneously, when the outdoor temperature is 30°F or —1°C. The specified rooms are those with heat emission apparatus installed.

Background central heating: similar to full central heating but the specified temperatures are much lower; the assumption being that other measures will be taken to boost the temperature at particular points.

Selective central heating: will maintain only a proportion of the rooms at full central heating conditions simultaneously. This assumes that the whole house is equipped with heat emitters, so that the disposition of rooms chosen to be heated may vary. This is a very sensible and economic system for a house which is, as most are, not fully occupied at almost any time.

Clearly one may ring some changes on this basic list, for example with partial background heating, or selective background heating. The important thing is that if a contractor is being employed the type of heating chosen shall be written into the contract. In all the cases mentioned there is one other factor to be detailed, and that is the specified temperatures. These are for the client to select, though some guidance is offered below. In specifying temperatures it is necessary to consider the cost of higher temperatures on the economy of running the system. The salient fact is that the cost of unnecessary heat grows out of proportion to the average cost, and the rule should be therefore to have (1) as much heat as necessary, but no more (2) heat when wanted but at no other time (3) heat where wanted but nowhere else.

Points 2 and 3 show the importance of good thermostatic control and on/off controls of heat emitters. Point 1 requires an assessment of reasonable temperatures, and reasonable values may be gathered from the following list.

Living rooms 70 – 75°F

Dining room 65 – 70°F

Bedrooms 55 – 65°F

Bathroom 65°F

Hall 60°F

Kitchen, if not receiving sufficient ‘free’ warmth from cooking etc, to be kept at approx 65°F.

Three things must be stressed about this list: first that it is for guidance only; secondly that a decision must be made about the maximum required temperature in each room, and either built into the contract or used by the client doing his own design. The third factor, which deserves to precede all others in timing, is the basis of good running economics, and the way to contribute to the national effort to conserve energy. It is, in short, to require less energy for a given result, by losing less. This means, insulate! Insulate to the maximum extent, knowing that money spent at an early stage on insulation will mean less outlay on heating equipment since smaller units will be needed; and it will mean savings on running cost for ever more. We shall have more to say on that important topic later. The reason for thinking of insulation even before heating is that the heat balance must be made on the insulated structure in order to secure all the economies in apparatus.

Before leaving the subject of preferred temperatures, and the desirability of avoiding too much warmth, we can deal with another offshoot of this. Heating ‘dries the air’. A rise in temperature brings about a fall in relative humidity, so that the air feels drier. In moderation this is no more than a passing nuisance to healthy people, who acclimatise as they would if they moved to, say, the Transvaal where the prevailing humidity is very low but the climate excellent. The effect of dryness increases as the temperature rises, and it is an additional reason for keeping the temperatures down.

The word ‘humidifier’ is often mentioned, and it describes a device which adds water to the air. We would certainly oppose the proposition that all central heating should be accompanied by humidification, and instead we list three categories where it is usually justifiable. (1) For people who suffer from sinus complaints. We go no further than that, and recommend that they seek medical advice by way of confirmation. (2) Where antique furniture is involved. Furniture, unlike people, cannot adapt, and antiques might be spoiled due to shrinkage. (3) In some cases, in older type timber framed buildings. Although the changes in temperature and humidity which take place are no more than those which occur seasonally and rarely do any harm, they do very often cause quite alarming noises to issue from the structure. It is solely to mitigate these that humidification might be considered.

Humidifiers are sold in three classes, cheap, moderate and expensive. The first is little more than a wick or similar evaporative device, though almost always made quite attractive in appearance. The prototype is a towel hanging from a radiator into a bucket of water. Commercial models of this type are suitable for most purposes, and bring about quite good results. At the extreme end of cost are models of considerable complication, extensively instrumented, electrically operated. The moderate range falls somewhere between those two extremes. So much for shortage, or apparent shortage, of moisture in the air. But what about the times when there seems to be too much, when it forms as condensation, stains wall paper, ruins paint and curtains, mists windows and turns clothes mouldy in wardrobes?. There is no magic machine which will reduce the relative humidity irrespective of temperature, but there are two ways to avoid condensation. One is to prevent wall surfaces, in particular outdoor wall surfaces, and windows from becoming cold enough to cause it. The other is to ventilate, to remove enough of the moisture laden air as it forms to keep the concentration within manageable limits. These matters we will deal with when we come to Insulation.

There is no such thing as a best heating system. What is best for any one situation depends upon such things as what it is expected to do, where it has to go, and what may not be available.

One such, and a common feature, concerns the space required compared to the space available. An outstanding example of this is a warm air system with full ducting. Ducts average about 250 x 200 mm or 10 x 8 in. Very few houses can tolerate apparatus of such a size running down a corridor or across a ceiling. For that reason, a full warm air duct system is always regarded as something to be built into a new house, not added to an existing one. At construction stage ducting can be built into the structure and ‘lost’. Warm air systems put into existing property tend to use stub ducting, I.e. short runs leading only into rooms adjacent to the heat unit. Underfloor electric heating was of course associated with new building, since it was almost wholly incorporated in concrete floors. Its successor, the storage radiator, is sometimes described by owners of rather small rooms as bulky or cumbersome. Some manufacturers have gone as far as the natural limitations of the subject will allow in reducing the depth of projection into the room. In such cases an unbiased observer might say that the unit takes no more space than would a small bookcase, and it has a safe surface temperature. It can therefore be assimilated into the room.

Still considering forms of electric heating, radiant wall and ceiling panels take no noticeable space, but like warm abducting must be thought of as structural items.

Let us examine the space and facilities demanded by wet systems. Until quite recently the boiler would stand alongside the refrigerator or the washing machine and look very like either.

Or, still the same size, it might be situated under the stairs, or in a spare room. The criterion was often where it could get access to a flue, either of the conventional or the balanced type. Nowadays, a boiler may be hidden behind the gas fire, so taking no useful room at all; or hung on the kitchen wall like a moderately sized cupboard.

Radiators cannot be reduced in size since surface area is fundamental to their performance. Within that context however they have been streamlined, brought as near to the wall as possible without impairing their efficiency, and have had projections smoothed away. The old pattern column radiator is scarcely available now, though it, and the double and treble panel versions of the panel radiator, continue to supply the answer to that other problem, of where there is enough space for projection but too little wall area for the required size of single panel. Radiator manufacturers do their best to meet problems of wall space by offering a choice of heights of radiator. For example the heat emission from a radiator measuring 4 x 1 is just about the same as that from a panel 2×2. Shortage of wall space may be overcome in two other ways. The skirting heater increases the projection of a normal skirting board but otherwise uses no wall space at all, and the fan convector may be assumed to occupy a wall area only one sixth or less of the area of a panel radiator of comparable output.

In an extreme case, where the wall width was only two feet, but the height ample, we have known a long radiator of limited height to be fitted vertically instead of horizontally in order to get the required area into a room. It shows what can be done, but for technical reasons this ploy is not highly recommended.

The problem of fitting an appropriate type of system to a given set of conditions has other aspects besides space. Some may be overcome. For instance, no gas main within miles? Then there is bottled gas. Some are not overcome so easily, if at all. Take for instance a flat in a multistorey block. It will almost certainly have no conventional flue. Whether a balanced flue would be allowed to pass through the building wall must be determined by reference to the conditions of sale, or lease, or let. Such a flat would quite certainly not be able to accommodate an oil fuel storage tank, and that limitation is much more widely applicable, covering all premises which have no garden or very little garden.

When we come to the point of deciding which form of central heating to choose, the choice is not so wide and bewildering as might at first appear. Let us begin by supposing that the whole of available heating stretches from A to Z. Then, by the time we have eliminated those sections of the market which cannot be accommodated by us, the market has shrunk to, say, A to P. Then, we find that we do not much care for, it may be electricity, or gas. That would reduce the choice to A to J. Within the new short range we must then be careful to see that what we look at has earned the Approval of the appropriate authority: SFAS for solid fuel, British Gas for gas, DOBETA for oil, either BEAMA or the Electricity Council for electricity, MARC for radiators. All goods which are sold in appointed places, such as gas and electricity showrooms, can be taken for granted. Nowhere else should one fail to enquire about Approval. There are, it is true, cheapjack wares about, but decreasingly so. More to the point are the imported items which are reaching us. There are of course many excellent items being imported, but an item is usually designed for the conditions it will meet in its home country, which are not necessarily those of the UK. It could therefore be successful at home but not here. Any item which is both reputable and suited to UK conditions will have been submitted for UK approval. You will see therefore that whether an item is British made or imported, you need to satisfy yourself that it has Approval. More than that, if you place your heating contract in the hands of an installer, make sure that he chooses and uses Approved items on your behalf.

Practical points to emerge from this post are: (1) Not all types of heating system can be accommodated in every home. (2) There are two broad types of system — wet and dry. (3) The range of choice within ‘wet’ and ‘dry’ allows for at least one example of each to be suited to any home. (4) Before setting out to acquire a heating system the householder should be quite clear about the standard of heating he is seeking, e.g. whether full, partial or background; and the maximum temperatures to be maintained. (5) When buying apparatus make sure that it complies with an official Approval scheme wherever appropriate.