It is interesting to note that the principle employed in electric storage heating is likely to be applied to a much greater extent in the future. The principle is that of converting ‘instant’ energy, which if not captured when generated will be lost, to a form which can be stored, mainly as heat. Sources of instant energy include solar heat, wind, wave and tidal power and some more obscure sources. These will become more important as the supplies of natural gas and oil are depleted.
The reasons differ a little, though. Natural sources occur naturally, for instance the sun shines by day. But electricity is the same by day or night, except in cost of production. Hence it is cheaper to make it at night to use by day, for those applications in which this can be done; space heating is one of them. Electricity can be stored as chemical energy, which is the principle of the battery, but that is in another context.
The conversion of electricity takes place by ‘burning’ it, by hot wire as in an electric fire, in a situation in which all the heat generated passes into the storage medium. There is every good reason why the storage material shall be as compact as possible, if only because storage heaters already have a reputation for being on the bulky side. The two factors which govern the heat capacity are the weight and a heat retention factor called specific heat. Water, which has a very good specific heat, has been used, and iron billets, also lime and other substances for which some virtue has been claimed. But in the end practically all models come back to a ceramic material, which is either firebrick or silica brick. The latter has more of a tendency to spall or break with continual heating and cooling. Ceramics are not the best materials thermally but they are very robust and without noticeable expansion/contraction problems.
Thermal storage in its earliest form did not employ ceramics, but quite ordinary cement mixtures, used in the construction of solid floors. If less total storage capacity were required the insulation could be raised to the top of the concrete section if necessary.
But of course such a decision could be made only once. Quite a lot of experience has been gained about the composition and application of screeds which resist the stresses of heating and cooling.
The more ingenious members of the public developed methods of operating which were very flexible but there is no doubt that underfloor heating has, in a changeable climate, the outstanding disadvantage of being inexorable. This combines badly with the anticipatory nature of the means of putting in the heat. If you decide that tomorrow will be cold you arrange for a greater charge to go in overnight. And if, as is by no means uncommon, tomorrow is not at all colder, the floor is still going to give off its stored heat, and there is nothing you can do about it, except leave all windows open or go out for the day. In converse conditions you are left to shiver.
This is not to decry underfloor heating for what it was – a pioneer effort which brought a lot of comfortable warmth to a nation shedding its spartan traditions.
The storage radiator is in many respects a notable advance upon the storage floor. It is not a constructional item, and so is available to anyone. It does not bring with it rules about avoiding the use of heavy carpet. Against it in small premises is that it is a piece of furniture, which the floor was not. But principally it is not, at least not necessarily, inexorable. There are in fact three classes of storage radiator. There is the common one, the cheapest, in which the heat leak is scientifically controlled by the amount of insulation given to it during construction, but is not otherwise controllable. This is comparable to the floor.
The other two classes of storage radiator have a common feature, that they are more heavily insulated so that random heat leakage is much reduced. Then, in the one type the heat output when required is damper controlled, usually manually. A damper is opened which allows air to pass from below through a passage or flue in the ceramic mass, where it becomes heated and passes out at the top. The second type of controlled unit is inactive until, under the influence of a room thermostat and probably a clock, a fan starts passing air through the heated core of the unit.
Either of the controlled types of unit is a great advance upon the first, in particular for quite common cases of a living room which is not much occupied until evening, by which time a natural unit is well past its best. The natural unit is quite well suited to halls, stores, libraries, and other situations in which bursts of warmth alternating with much cooler periods do not provide suitable conditions.
Another advantage to be claimed for controlled outlet models is that they relieve the user of a good deal of reliance upon a crystal ball. Whether tomorrow is going to be colder or warmer is less of a crisis if one may take the safe view and turn up the heat input, knowing that if it is not needed a lot of it will stay inside the unit until it is wanted, or it will form a substantial nucleus of heat so that the amount put in next night will be less. The maximum input is of course controlled automatically by a thermostat, adjustment of which is input control.
Inevitably the unit storage radiator grew up, into a central unit called Electricaire, big enough to provide warmth for several rooms. It is a large unit, and some are vertically disposed while others are horizontal. They are of the well insulated fan controlled type, the fan being powerful enough to propel the warm air through a duct system into the rooms to be warmed. In this connection it should be noted that standard units almost always have a stub duct system in mind, the fan being not powerful enough for a full duct system unless specifically stated.
Comparing an Electricaire unit with a stub duct unit using different fuel, the Electricaire has what might prove to be a decisive advantage in not needing a flue. As a heat unit it could suffer from its ability to become exhausted. That is, if given specially heavy duty during the day its heat charge could become used up before it is due to come on charge again. Instantaneous producers do not have this problem.
As a system it should be pointed out that it is unusual for an Electricaire unit to be connected to a return air duct, and return air is more usually allowed to find its own way back, if it wishes to do so. While this may work out quite well, it does tend to rely upon leaving certain doors open and is not therefore entirely automatic in its operation.
Electricaire units incorporate an air filter, usually on the inlet to the fan, and this should be kept clean. If it becomes choked, or even partly choked, it will affect performance and could be bad for the fan.
If we compare the warming action of a storage radiator with that of a hot water radiator, the nearest is the storage unit with uncontrolled output. Having less insulation it develops a generally higher surface temperature, and so is able to give off a higher proportion of invisible radiant heat. While this is an advantage it cannot outweigh the disadvantage of the uncon-trollability of output; but when we speak of surface temperature we are speaking always of temperatures within safe limits. It should be noted, though, that some build-up can occur locally if normal heat dissipation is prevented, as it might be if say a pile of clothing were to be placed on the unit. Such units should always be kept clear.
We need to know what to do about the place where the unit stands. As a general rule nothing has to be done unless there are quite clear instructions given to the contrary. It is usual for all kinds of heating appliances to be tested for their effect upon the floor on which they stand, and in particular for the temperature which they will create below the unit. A storage radiator needs no special floor preparation. But while it is no doubt safe there is still the possibility that more or less continuous warmth will have some effect upon the floor covering. This may be most marked when the material is in the plastics or man made category, which includes floor tiles and many carpets. The way to avoid a tendency to softening of tiles, or embrittlement of fibres, is to use a protective sheet of asbestos millboard or similar under the unit.
The case of an Electricaire unit might be somewhat different, in that its gross weight might need support. We can only advise that in any given case the electricity people or the vendor must say what is needed, since larger units have greater weight. A suspended floor at ground floor level could for instance be given local support by means of brick piers taken off the under-floor: or the flooring could be cut out and a concrete base put in instead. In any case we recommend the use of an insulating plate under any such unit on a wooden floor, but stress that this is not for safety but to avoid any deterioration of the underfabric.
It is quite common for units of the Electricaire type to have an air duct between fan and heating elements at the bottom of the unit, thus forming a good insulator.
Boilers are a class of appliance which require some base protection, unless otherwise stated, and even then if there is a need to protect floor coverings. But the whole matter of boiler standing is covered by Building Regulations and to be on the safe side these should be consulted. The Regulations draw a distinction between solid fuel and oil fired appliances on the one hand, and gas fired appliances on the other.
The operation of storage units is, as already mentioned, anticipatory. Instead of using a crystal ball one may rely upon the weather forecasts, or if thought more reliable one’s corns or piece of seaweed. But whatever means are used to predict the following day’s temperature pattern, today is the day to do something about it. The input controller of a storage unit is usually marked in numbers, say 1 to 8. It takes a little while to discover, by trial and possibly error, whether a forecast of fairly cold justifies an input of number 5, or of 6. But this is what it amounts to, and any practical difficulties which may be met are rarely associated with that aspect. The input setting must be made before retiring for the night, since it controls what happens during the night.
The method is the same whether the unit is an individual one or a large Electricaire. The total reliance upon forecasting is greatly relaxed, as we have pointed out, by having units which do not rely upon natural heat discharge but to a very large extent upon extracting the heat required. The real responsibility then rests upon the owner to make sure that he programmes sufficient storage. To err on the generous side does not imply waste and loss since most of the surplus heat is retained by the insulation until it really is wanted. Prudence, economy and a proper regard for the conservation of energy require one not to be extravagant in the production of heat. But the logic of having a heating system in preference to putting up with the cold leads to the supposition that one will do what is needed for comfort. The guide line through this apparent dilemma is that which is set out in the Introduction, namely to have as much warmth as is needed, when and where needed; and outside those limits, nothing.
The service or maintenance on a storage unit may fall into more than one category, from ridiculously simple to the other extreme. Cleaning of the air filter on all fan controlled units should be done as regularly as inspection shows to be necessary; this will vary from unit to unit depending upon external conditions and the amount of dust which is in the air.
Beyond that an annual inspection may be thought desirable. Shut the main switch even though the current is off-peak and the time is on-peak. Remove sections of the casing and look in particular at the controller. Make sure that it is free from dust and that the points or contacts are not burnt. Check that electrical terminals are tightly screwed down.
In the extreme case that a heating element has broken, this can always be renewed in the case of a storage unit, whether single or Electricaire. In many cases it can be done without dismantling the core, though much of the casing must be removed. In the case of floor heating replacement is dependent upon construction. A floor may be made with a solidly embedded heating element, in which case nothing can be done. Or it may have a withdrawable element, with points at which withdrawal may take place. If taking over a house with this form of heating it is desirable to find out which type of element is fitted. Breakage of a heating element is an uncommon occurrence, but if it should occur with an embedded one there is little point in considering whether to dig up the floor. The easier course would be to change the type of heating, if only to storage units or Electricaire, retaining the off-peak meter and tariff.