Convectors form a class of appliance whose heat output is almost entirely in the form of a warm air current. There are two forms of convector, natural and fan assisted, and they deserve to be considered separately. The natural convector resembles the radiator in size and in being wall hung. It is particularly useful in situations where a high radiant factor is not required, or where a high convected output is best. A library or an art gallery is an example, requiring a good level of ambient warmth but absence of high spots caused by direct radiation.
The convector is capable of giving an output well above its rated value, since it can be connected to a pressurised system working at a temperature in excess of that which an atmospheric system can achieve. The front face remains at a safe temperature.
The detailed design of convectors varies considerably between makes, but all those worth considering have in common that they should be able to claim MARC approval.
The low level of radiant heat output of a convector brings another difference from radiators. We have been at pains to stress that radiators must not be crowded by furniture, which would intercept the radiant heat, with loss of useful efficiency and possible damage to the furniture. The only prohibited area around a convector is above it. For instance one would not hang a picture or a mirror over a convector.
The fan convector is very different, because of its powered output. It is usual to fit this item within about 0.3 m of the floor, allowing space for the circulating air to return to the lower half of the unit. Then the discharge, which in some cases has some directional adjustment, is angled downwards at the floor some 2m distant. In this way it effectively prevents natural layering, the stratification which allows colder air to collect at floor level. The fan convector correctly installed can promote the lowest room temperature gradient of all heat emitters, no more than 2 or 3 degC difference between head and foot level.
To be able to do so it requires two conditions. First, it must have an uninterrupted distance in front of it of at least 3 m and preferably more, so that the ‘throw’ pattern can develop along with its secondary entrainment pattern. Then, it should be in a central position on a wall in order to obtain full advantage from the secondary entrainments which occur at both sides of the main air stream. A unit fitted near the end of a wall will lose that effect on one side.
The heat exchanger in a fan convector is a concentrated one. This means that a fan convector has an output equivalent to a natural radiator of at least six times its wall or picture frame area. It means too that the unit offers a quite high resistance to flow and must be fitted in a two-pipe circuit.
A fan convector is almost entirely dependent upon the fan for its output. Consequently its on/off control is very easily carried out, by means of an electric switch. Additional controls include a speed changing switch, to adjust the output to the range of temperature required, for example high speed for cold weather or quick warm-up, with medium or low speed for milder times or when the room is just about up to temperature. The final adjustment, to achieve a close control of room temperature, is brought about automatically by a built-in thermostat which senses the temperature of the air returning to the unit. Best running results are obtained by using the speed change switch so that the thermostat keeps the unit running most of the time. It is for example better for room temperature control to have the unit running 20 minutes out of every 30, than 10 in 30.
The aim in this post has been to present a broad picture of what goes to make up a wet system. Certain patterns emerge. For instance:
A two-pipe system is better than a single-pipe system, but either is vastly better than a gravity system.
Except in a couple of small areas in Britain where the water is relatively harmless, no one should be allowed to fit a direct water system for heating.
Radiators, convectors and fan convectors can be installed relatively cheaply in many properties, using the close-coupled type of system.
The best temperature gradient results come from skirting heaters and fan convectors.
Fan convectors use very little wall space, are recommended for living rooms, not bedrooms.
Radiators remain simple, easy to clean; their radiant output of some benefit.
There is room for all types of heat emitter, not only in the market but in any one house. For example, matching unit to duty, one could choose a fan convector for the living room, radiators for bedrooms, a natural convector for the hall.