Central Heating Radiators

Most radiators nowadays are of the panel type. This means that they are almost flat but are grooved or dimpled or in some way wrinkled in order to increase the surface area within the total size which is called the ‘picture frame’ area. On this surface area their output depends.

The other traditional design, called the column radiator, is rarely good looking, being designed for high output in a given space, and often known as the hospital radiator. Although not likely to be considered now for domestic use, it is if met the only radiator normally equipped with feet for floor standing.

An imported style of radiator which is different in design is that which has a plain front face, e.g. of aluminium, with a heating coil closely soldered or brazed to the back. Radiators of this type offer a high resistance to flow and must be supplied through a two-pipe system or microbore, not a single-pipe system.

If other types and designs crop up from time to time they must be examined on their merits and it is up to the suppliers to say what those merits are – and to give some proof of what they claim, in particular the nominal heat output upon which all calculations must depend. The safest procedure is to insist upon buying only what has received MARC approval.

All radiators, being vessels which are capable of collecting air, should be equipped with an air release tapping, to be fitted with an air cock or automatic air vent. All radiators should be supplied with the proper means of support, which usually means wall brackets.

Radiators may be finished in enamel or left in primer to be finished by the customer. In the latter case, and for subsequent redecoration, it makes very little difference what type or colour of paint is used, with the one exception that so-called metallic paints, such as aluminium and bronze, must not be used since they reduce the heat output substantially. It is worth noting here that after repainting three or four times the thickness of paint film will become a barrier to heat movement and the paint should then be stripped before repainting again.

The output of a radiator is one aspect examined by MARC. It is measured as the heat emission from the panel when there exists a temperature difference, between the water inside the panel and the air just outside it, of 55 degC or 100 degF. Obviously the emission will be higher if the differential is greater, less if it is lower. Perhaps less obviously, a radiator will give of its best when first coming to work in a cold room. As the room warms the output rate will slow down in proportion, this occurring naturally, without the use of instruments or controllers. Though we must rely upon MARC for a definitive output it is often convenient to get a rough idea of how big a radiator is going to be needed before the design is finalised.

So, suppose a radiator surface is given as lm2, the side you see is only 0.5m2. For approximation this may well suffice, but it will be a trifle on the high side. The surface area of a panel, you will recall, is artificially increased by the use of profiling. Steel radiators are recommended for use with indirect systems only.

Where to Put the Radiator

We must start any such consideration by knowing how a radiator – any radiator, whether water or oil filled – works. It delivers heat as natural convection, a rising warm air current, and as radiation. Since the radiation content can be up to half the total, its importance cannot be overlooked. This leads to the first basic conclusion, that wherever the radiator is situated it must be able to ‘see’ the room. That is to say that it must not be hidden, either permanently behind a sideboard or similar, or effectively by having an easy chair usually pushed back against it. As well as shielding the radiant warmth from the room such treatment will result in overheating the adjacent furniture, with rapid deterioration of textiles and plastics.

We must then think about the radiation from the back side of the radiator. Radiators are always fitted to walls or panels, and usually by brackets supplied. Tests have shown that there is an appreciable loss of heat from the room, through the wall, for this reason. Further tests, interposing insulating slabs and reflective insulation between radiator and wall were not encouraging. The first showed no real change, while the second reduced heat loss but also reduced radiator output.

The logical conclusion is that the radiator would be best placed in the middle of the room. In practical terms it means that it should be fitted to an inside wall, and if this is done overall heat savings of the order of 5 – 10% can be expected. In addition there will usually be a significant saving in pipe runs. This location is in obvious conflict with what we have come to accept, that radiators should be fitted under windows. We must remember however that there was just one reason for that, namely to counter descending downdraughts off the cold glass, with reduction of condensation. That remains a valid reason, except that it ceases to apply where double glazing is fitted. The under-window position has always introduced contention with ladies whose attachment to their full length curtains has assumed greater importance than the technical refinements of heating practice.

Having established that there is an equal case for fitting radiators on inside walls or under windows, we come in the former case to ask where. Within broad limits this is not important. It calls for a small amount of imagination, a visualisation of how the warm air will flow as it rises off the radiator. For example, given only one radiator in a room which is long and narrow one would not fit the radiator at one end, but roughly central — unless occupation were to be confined to one end. The situation chosen for the radiator must allow not only for the radiator to ‘see’ but for a free air flow to have access to it. Radiators must not be boxed in, for appearance or any other reason, unless it is fully appreciated that this can result in a serious fall in heat output.

A more widespread move to take radiators away from the window position will draw new attention to the radiator shelf. This device has a strictly limited purpose. It prevents or mitigates wall staining above the radiator, which is justification enough. Its effect upon radiator output is usually to bring about a slight decrease, and care must be taken to fit it in the right relative position, not for instance sitting down tightly upon the radiator.

The conclusions about situation reached for radiators do not apply to convectors, fan convectors or unidirectional radiators. All appliances which are predominantly convectors will have positions which suit their warm air output whether natural or forced. It is a pity that warm air cannot be coloured, since this would greatly assist the imagination when choosing the best sites.

Connections to Radiators

Having examined the general form of the system, and expressed a preference for the two-pipe version, we come to the detail of radiator connection to any type of system.

The wall brackets must be securely fastened to the wall, in positions so that the radiator is suspended true and level. The final connections should be made between circulating pipes and radiator so as to impose no strain upon the brackets or the radiator.

Each radiator should have two valves fitted. It is bad practice to skimp in this respect, for even removing a radiator to paint it will involve the cost or inconvenience of draining down the whole system. One of the valves should be of the lockshield pattern, I.e. to be turned only with a special key. This valve is used for balancing when commissioning the system, as we shall explain later. The amount of opening should be recorded in case it is closed for any reason.

It is a good rule to fit the lockshield valve on the outlet end, the other valve for on/off control being on the inlet. The only time that order becomes obligatory is when the control valve is of the thermostatic type, since this type is almost always made for unidirectional flow. The on/off control is there to be used. It is the means of achieving warmth only when and where needed. The use of thermostatic radiator valves is explained in a preceding post. In general the angle pattern valve is preferable to the straight pattern. Not only does it make for a neater job, with less pipe showing, but the radiator may be removed from service without having to spring the pipes apart.

The Close-coupled System

The growing practice of double glazing has greatly reduced the significance of radiators under windows. It opens the way for considerable reduction in installation cost, particularly in premises which tend to be long and narrow, in either a vertical or a horizontal direction. In some cases radiators may be fitted back to back, the ultimate in pipe economy. A system may combine this type of run with longer runs made inevitable by the house plan. Skirting Heaters

The radiator, however much it is styled, is an alien object in any home. If it were not wanted for heating no one would have it. But everyone has skirting boards, and heaters which closely resemble skirting boards must be widely acceptable. That is just one of the advantages of skirting heating. Others are that it does not take up wall space, that it encourages a better vertical temperature gradient. The reason that we do not see more of it in the UK is that it costs more than radiators, but another answer might be that we sometimes run out of wall. Walls are often used to back sideboards, bookcases, cupboards, thus effectively making those skirtings unusable.

For a rough check, once the heat requirement of the room has been calculated, an average skirting heater will give 450 watt/metre or Btu/ft length of heater. This is based upon a mean temperature difference of 55 degC or 100 degF. In marginal cases a search of the market might show that one make offers significantly more output per unit length.

The skirting heater is, in effect, a tube whose heat emission characteristic has been greatly increased by a number of fins brazed to it. This is encased in a panel structure fitted with a damper. The damper controls the amount of air able to flow over the fins, hence the heat output of the unit. A closed damper will generally bring the output down to about 30% of the rated value. A more satisfactory method of construc tion is that which employs a bypass pipe with valves on the finned tube, so that when not wanted in circuit it may be isolated. __

These notes, on skirting heaters and similar apparatus, are necessarily general in character, and aimed to explain the nature of the items. Each manufacturer introduces his special feature, which may add something to ease of operation, or appearance, or ease of maintenance. Once the reader is aware of the broad facts he or she must judge the various special claims on their merits in relation to the job in hand.

Skirting heaters emit a certain amount of radiant heat, though the proportion is much lower than in the case of radiators. Both because of this and because of the rising convection, it is not advisable to stand solid objects permanently in front of the heaters at close range.