Automatic Controls for central heating systems

To anyone approaching the subject of central heating there are two kinds of controls. There are those you must have and so there is no decision to be made. And there is the other sort, which may again be put in two ranks, those which are to be strongly recommended, and those which if fitted will make some contribution.

The most anyone needs to know about the first category is how best to use them to obtain the most benefit, since like a willing horse they may need to be given guidance. The second category is more complicated since it brings in choice, decision making about whether to spend money, and so one needs to know what each type can contribute to comfort or economy or both.

Controls which are obligatory are built into the appliance. For example it is not possible to buy a gas fired boiler without a boiler thermostat and flame failure device; nor a pressure jet boiler without a control box and boiler thermostat. A further safeguard lies in the fact that if these parts were somehow to be removed from those boilers lighting would be impossible, because they are inextricably built into the lighting process.

A similar degree of interdependence exists in a free standing solid fuel boiler of comparable status. But it does not extend to the more elementary type of unit represented by the back boiler to either open or openable fire. This appliance always has a damper but it is manually controlled in most cases, offering no automatic safeguard against too much or too little fire. There are, however, models with thermostatic damper, and a model which has fan assisted draught is very readily controlled by thermostat. In the same type of appliance, where there exists a choice in the ratio of heat to air and heat to water, the controlling damper is manual.

Among obligatory controls the boiler thermostat is the only one which offers a chance to make routine adjustments. The easy solution is to ignore it, and leave it always on one setting, for example 70°C or 160°F. It can however be used with advantage, in the following ways. (1) It must never be set below 60°C or 140°F. (2) It should rarely be set above 90°C or 190°F, or in the case of solid fuel boilers 80°C or 180°F. (3) Within those limits it may be used to spread the boiler’s working time over the total ‘on’ time, to avoid long periods of being shut down on the thermostat. It is better for a boiler to work for 40 minutes in every hour than 20 minutes. To achieve this, adjust the temperature setting to the climate, for instance with maximum temperature in hard weather, tailing off to near minimum at each end of the heating season. (4) If the system does not have a cylinder thermostat on the hot water cylinder, and there are very young or very old residents who might be at risk, avoid high temperature settings above so that the domestic hot water stays at a hot but not scalding level. (5) But note, in relation to, that if the dependents have the use of a thermostatic hot and cold water mixing valve, this works best when supplied with really hot water.

The thermostat which is incorporated with an electric immersion heater is a simple example. But the device is very often fitted separately, to control the water heating activities of a gas or oil fired boiler. This it does either by being wired back to the appliance so that it shuts it off when the water is up to temperature; or it operates a solenoid valve in the flow or return primary, thus preventing the flow of any more water into the cylinder.

The first method is neater, though on the face of it open to the objection that a satisfied hot water cylinder would shut down the central heating. By various ingenious ways not to be explained here, this is avoidable. The second method avoids it without prearrangement but introduces an extra solenoid valve, which would usually be fitted somewhere in the vicinity of the hot water cylinder. It would be possible to arrange diversion through a three-way valve at such times, so that the primaries continued to flow through the towel rail only. But the cost and complication would rarely justify such a step.

A cylinder thermostat when fitted has a specific job, that of allowing the stored hot water temperature to be at its own best temperature, not dependent upon what seems best for the central heating. It was a shock and an unresolved puzzle to become involved in an American offshore contract calling for exactly the reverse of that situation!

If the holding of a safe temperature of stored water is not important, it should be remembered that in times of heavy demand, as when a lot of guests want baths, that it is thermal storage that counts, not just gallonage. Thus, hot water at 80°C or 180°F will go about 25% further than the same water at 60°C or 140°F, and if a cylinder thermostat is not fitted, this is a crisis which justifies using the boiler thermostat to boost the hot water storage.

The programmer serves both heater and heating system, and is a device of more or less operational complexity usually attached to the boiler, though it may be supplied detached, to fix to a convenient wall. The essentials of a programmer are a clock, boiler thermostat, wiring to pump and multiswitching. It is usual nowadays to use it as a junction for other items; for example the room thermostat which controls the pump and would otherwise be wired directly to it; cylinder thermostat; frost or low limit thermostat.

Usually the clock has four levers or tappets, which enable the user to choose two periods during 24 hours when the boiler shall start itself up and later shut down. The switching, reduced to a single knob, permits the user to decide whether, during the two working periods, he will have domestic hot water and central heating, or domestic hot water only. Note that unless the boiler has pumped primary flow the programmer will not permit heating without hot water. We have stated elsewhere our belief that most people manage very successfully with very few programmes.

Elementary facts about using a programmer include such matters as noting the difference between night and day on the clock face, and turning the dial in the direction shown when altering the time setting. After a shut down for any reason, or a power cut, failure to reset the clock could lead to unexpected working hours.

The most commonly met control away from the boiler is the room thermostat. In most domestic premises there is one only, and it therefore has to act as the representative of the rest of the premises while being answerable only to the room in which it is fitted. This is not as chancy as it sounds, since the design will have allowed for the fact that while it may be set at say 15°C or 60°F in the hall, the emitters in the living room are of a capacity to bring that room up to 20°C or 70°F, and conversely, supposing the roomstat to be in the living room and set to 70°.

There are elementary common sense rules about the precise location of a roomstat. It must sense average conditions, not being tucked away in a corner where little air circulation takes place; not being chilled by being near a draughty door or window; not being artificially warmed by an adjacent lamp or television set or direct sunlight — or of course a radiator! Although the traffic through doors is a factor against using the hall, it nevertheless remains a very useful place, on the grounds that it is the one part of the house which should always be warmed. If the thermostat is in the living room this can never be allowed to go cold, without having an effect on the rest of the house.

The ordinary roomstat is an on/off device. But there is another, called a setback, which instead switches from normal to a low setting. It gives the user the ability to choose what the high and low settings are, and also the period of the setback, usually up to about 10 hours, after which it reverts to the high or normal setting. When there are special reasons for some warmth to be maintained all night, perhaps through illness, then the setback thermostat is well worth considering. The effect it produces with refined fuels, of a low level of maintained temperature, is almost precisely what occurs as a matter of course with solid fuel heaters which work at low level instead of going out.

Anyone who has a programmer has a clock, to control the times of operation. But for those who do not, a clock may be obtained separately, and for convenience wired into the roomstat circuit, since both are doing the same job, of controlling the boiler on/off. Other applications of a clock may be made, such as controlling the pump only if for any reason it is desired to keep the boiler always at work. Another example is that of zone control. This must start with the pipe runs, so that each zone is self-contained as a circuit off the main flow and return. A typical zone might be the first floor, the bedrooms, which have different needs from the living rooms. In that example, all heating would be cut off the first floor at, say 7 a.m. This would be the job of the clock, operating a solenoid valve. Such an arrangement is made into a unit in the Satchwell Minival.

Although the commonest clock is the four tappet type, variations abound. There is for instance day omission, which allows everything to stay dormant perhaps over a weekend, but brings it on automatically on Monday morning. Then there is the over-ride, a simple means of making a change of routine for one occasion only. Thus, an unexpected call out when an evening at home was planned, and the ‘on’ of heating can be over-ridden by a push button or similar. Next evening the heating will come on as usual without further action by the user since over-ride changes are self-cancelling.

Next on the list of instruments which are wired back to the boiler is the low limit thermostat, often called the frost stat. This is of great value to anyone in the habit of leaving the house untenanted for weekends or longer in mid-winter. One can of course keep the heating wholly at work, with thermostats set low. Over a week or two this is not the most economical of measures. The frost stat arranges that only when danger threatens will the system come on to work. The sensor is fitted at the most vulnerable place, where the first freezing might be expected, and the thermostat is usually set to a safe margin, such as 5°C or 40°F. When this temperature is reached it cuts in, bypassing all other controls such as room-stat and clock, until the house is warmed sufficiently to satisfy it. A boiler which is fitted in an outhouse is an example of one which warrants a frost stat.

If each radiator has a thermostat each room can be positively controlled. There are those who claim that this automatically leads to fuel economy, but there is an element of nonsense in that. It certainly gives the opportunity, but from that point on it is entirely up to the user. Since one result of intending to use a radiator thermostat is that radiators may safely be oversized the user has the power to use more than normal heat. A pleasant feature of thermostatic radiator operation is that it is modulating. The water flow is progressively cut down to control the temperature, and this compares with the on/off operation in which a radiator alternates between hot and cold.

Radiator thermostats may have integral or remote sensing, but in either case it is necessary to keep this as far as possible from the influence of untypical temperature conditions.

The influence on the rest of the system must be taken into account. The use of radstats goes with a continuously running pump, stopped only by the clock during ‘off periods. One cannot after all have two controllers trying to do the same job. But there are installers who believe in putting a roomstat into the system as well, getting over the objection by setting it just above the highest temperature which would be expected from the system working normally on radstats. Presumably the roomstat then acts as a watchdog or longstop. Running experience of this method is hard to come by, and against it is the fact that radstats are not cheap, and the roomstat is an extra cost on top. Otherwise there seems to be no objection.

We must refer to the modulating or mixing valve which attracts a very mixed reception. The essential feature of this device is that it takes the boiler water, at boiler temperature, and allows it to pass to the heating system after mixing with a proportion of the cooler return water from the heating system. To put this another way, only a proportion of the water in the heating circuit is passed to the boiler for reheating at any given time.

In principle this is quite good, since if the proportions are right it is a means of adjusting the warmth input gradually to suit changes in ambient temperature. It is in effect a continuous operation of the kind we have suggested might take place seasonally, adapting the circulating water temperature to suit the prevailing weather.

It is plain that the mixing valve is not suited to being adjusted manually. That would be a full time job for someone. It must therefore be automatically controlled, by a thermostat. Where will the thermostat get its routine orders from? The logical answer is that since it is controlling the indoor temperature it will have its sensor indoors. But, say the objectors, this is too slow. The sensed change has already taken place, the corrective action will take time to workits way around andbecome effective. By that time it may not be needed and an opposite movement must be made, resulting in continuing oscillation of temperature, which is most certainly not the object of the system.

So those who favour the device now claim that the proper place for the sensor is outdoors, where its function is anticipatory. It feels a change in the outdoor temperature, and passes the information indoors so that the system picks up in anticipation of the effects being felt indoors. There is some initial difficulty in finding the best place for the sensor. But suppose the house has a predominantly southern facing aspect, as it might do with a high pitched single pitch roof? Wind is as disturbing in its way as sunlight, so there must be some protection afforded — which the house cannot share though almost equally affected.

But the greatest objection, ignored by those who favour mixing valves, is a matter of timing. The indoor thermostat may be too slow in responding. But the whole of modern practice, as outlined in a preceding post0 is aimed at preventing outdoor conditions from reaching indoors – ever. What it succeeds in doing is to prolong the time taken and to modify the result when it does happen. Meantime the delay between the system receiving a signal and its effect becoming apparent is unchanged.

So it amounts to this: that a man may have a mixing valve at work which gives every satisfaction, bringing up the indoor heat release just in time to meet the oncoming cold spell. He then undertakes a big programme of insulation, which doubles the time of cold penetration and halves its intensity. And still the system reacts as before in response to the outdoor sensor. The result is bound to be at least as much overcompensating, and temperature oscillation, as the indoor sensor might give.

It might be possible to introduce a form of calibration into the system to compensate, though no one has offered to do this. The safest conclusion is that mixing valves seem a good idea but are perhaps best left out because of practical problems.

We do not include such items as automatic air vents in controls, since these are as fundamental as, say, the pump. Most of the useful forms of control are included in the above list, and if we may mention one other device which deserves to be omitted, it is the thermometer to measure air temperature. The only criterion of heating should be comfort, not a mathematical symbol unrelated to health, time of day or any personal factor.

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