The majority of air heaters are gas fired, and an estimated million are installed in this country. They take two forms, floor standing and wall hung. Unlike boilers they do not require or. use cast iron heat exchangers, since they do not have to resist water action. Although the air heater does not supply domestic hot water it is now usual for a small and independently operated water heater to be included, possibly as an optional item, within the case. It will share the fuel supply, flue etc.
Both the size and the independence of the water heater are very satisfactory features. It means that the water heater is just right for the job it has to do, unlike a large boiler which in summer is expected to work at low efficiency simply to supply hot water. The independence of both units means that water heating is not dependent upon space heating. More, it allows the air heater to be used for an opposite purpose, for circulating a stream of cooling air in summer, simply by allowing the fan to work while the burner is shut off. In this connection, it is quite common practice for the control system to include a reverse acting thermostat, which will not allow the fan to start until the heat exchanger is up to temperature. In order to promote ventilation without heating this device must be bypassed by means of a ventilating switch, which must of course be returned to ‘off before heating is required.
Another device which is standard equipment on most air heaters is an air filter. Air is no less surprising than water in its capacity for carrying impurities. Much of what is carried in air is invisible to the eye until it accumulates in bulk on a filter. But it is important both to keep the filter in place and to keep it clean.
There are two types of air borne matter which concern us. One is partly what we would call dust, inorganic matter, I.e. ash from a coal fire, plaster from a wall, etc. and partly organic but not living, e.g. lint, minute particles from textiles, plants and so on. The second class of substance is living matter, air borne bacteria and larger organisms. Both classes have a tendency to stick to the hot surface of a heat exchanger, the second class forming a plastic substance which adheres firmly to the surface. The worst result of this is to lower the efficiency of heat exchange progressively. A second result can be a slight tang in the ejected air from the ‘cooked’ living organisms. While the mechanism is not fully explained, any dust which passes through after heating seems to have an enhanced property for creating a dry feeling on the mucous membrane. Filtration of the air is therefore very desirable.
Cleaning the filter is necessary in order to keep the plant at work: a choked filter can shut it down. A device known as a filter flag is often used. It is a pressure operated indicator, which it is possible to connect to a light or buzzer, which shows when the resistance of the filter has reached a point demanding cleaning. Filters are of two types, washable or throwaway. Instructions are given for care of washable filters, and generally these, after washing, are shaken to remove excess water but put back in a wet condition. Attempts to dry them might cause damage.
All units of this type include the air fan which creates air flow over the heat exchanger and supplies the pressure necessary to cause flow through the duct system. This may lead to an arrangement which is up flow, down flow or even cross flow, referring to the direction of air flow across the heat exchanger. The purchaser cannot usefully become involved in such technicalities, all units of an approved type being capable of achieving a given standard of working efficiency. It might however be relevant to duct connections, and so be preferable for the air outlet to be above, or below, or at the side, in a given case.
It is worth enquiring about the extent and the needs of servicing. Time is money, and ready accessibility saves time. Such features as a bolt-on cover plate which comes away with burner complete, or with instruments, are worth more than a unit which has to be taken apart item by item. Another feature which must be kept in mind is the space needed. A unit may be fitted in a cupboard or under stairs, suitably lined with insulating material but leaving little room to spare. With this in mind, units are often designed to be dismantled from the front, and it is therefore necessary that the unit shall face the openable door, so that with the door open there is ample space in front of the unit. But it should also be ascertained whether side or back access would be required, which could only result in the complete removal of the unit every time it had to receive attention.
Although the pressures employed in a fan powered duct system are very low, it is necessary to take great care with jointing of the flow duct runs generally, and where both the flow and return ducts join the heater. Joints are usually of the socket/spigot type and after putting together are taped. Leakage on the flow side is wasteful, but leakage on the return side, around the heater, can be dangerous, since fan suction can deny combustion air to the burner. If a slide-in filter is fitted, it should be pushed firmly home after cleaning, to leave no air gaps.
Quiet operation has always been one of the makers’ aims, and in its way this becomes more important with air heaters than with boilers, because if a unit is noisy its noise can very easily be carried along the large ducts. Although a burner is capable of being noisy this is not often the real worry. It is the fan which has received most attention, and a look across the range of models now on offer shows that makers have abandoned the small fan working with frantic haste, in favour of the much larger, lazier fan, which works at lower speed to achieve the same result. This brings other benefits, such as lower wear, better pattern of air flow over the heat exchanger, and slower fouling rate on the fan itself. It is a down flow unit, the fan being fitted to a slide-out plate in the top compartment. The tall centre compartment houses the burner equipment and heat exchanger, and if required the small water heater which has a branch off the gas supply and one back into the air heater flue. The plenum base, though optional, is necessary, and is not supplied standard in this case because the unit might act as a replacement on an existing installation. This unit, concentrating upon width reduction, measures approximately 1200 mm high by 600 mm deep, by 300 mm wide. Any of the extras may be applied independently.
This unit has a heat output of 15.8 to 45 kW and is wholly accessible from the front.
This is the method which has to be adopted when the unit is situated in a cellar, or on a ground floor of solid construction. It is comparable with the type of unit long known as a basement heater, in which the external connections to both flow and return air are at the top of the unit. Upgrading to a comfort system is of course a special function of the unit shown here.
A unit fitted in a basement or cellar, whether it be an air heater or a boiler, can present a special kind of problem which deserves mention. It must be of the conventional flue pattern and so the burner draws its air from around the unit. A basement does not have a natural air supply in the way that above-ground rooms have, but the basic minimum need is no less important. It should not be left to chance, to the likelihood that enough air will find its way down via the stairs, even through a shut door. The way to handle this is to run a vertical duct, of 100 mm diameter or greater depending upon the heater. The upper end of the duct should be well clear of ground level, and fitted with a mesh cover. The lower end should terminate in the vicinity of the air inlet to the burner.
The free standing and wall mounted units broadly described so far are typical products of those firms which are members of the Warm Air Group of SBGI, the Society of British Gas Industries. Enquiry at any gas showrooms will produce more details of such units. It should be noted that each manufacturer makes a range of heaters, in size of rated output, and almost always offers some variation in physical size and shape, to fit varying installation conditions. For that same reason there will be a choice of connection positions, for air flow and return, perhaps for water connections to a water heater.
In many cases what are structurally the same heaters may be equipped for oil firing instead of gas, and such details may be obtained from the major oil companies or their local agents.
We will not take a great deal of space over units which are thoroughly catered for in readily accessible quarters. But perhaps we can devote more space to what might be called minority types, simply because they are less well known. There are two which deserve mention.
There is Afos, a combined air and water heater. It has two sections, boiler and air heater, usually allied within one casing but capable of being fitted separately. After the burner, which may be gas or oil, the heating medium is water, and a generous supply of domestic hot water is said to be available. The heat exchanger in the air heater differs from that in a direct fired unit, in being at much lower surface temperature. Consequently it avoids most of the charring of dust and microbe particles mentioned earlier. Heat into air is controlled by modulation with room thermostat operation, and this is a feature which compares well with the more usual on/off mode of operation. The fan, protected by a washable filter, is appropriate to a full duct system, and in addition to having variable fan speed control the unit may be given a programmer.
The other type of unit relies upon the existence of an independent boiler, and like the Afos it warms air in a water-to-air heat exchanger, with the same physiological benefit. The unit is the Heart warm of Massrealm Ltd. In dimensions it is an exact fit as a replacement unit into one of the original Sugg air heaters and is therefore tailor made to avoid having to make duct modifications. But more than that it is quite suitable to act as a unit in its own right, with enough fan power for a full duct system.
As a replacement unit it assumes that the old burner and heat exchanger are both in need of replacement – hence a new boiler, and a heat exchange section complete which takes far less room than that which it replaces. As a new unit it emphasises the principle that a heater needs a flue, and a warm air generator needs a good position relative to duct work, and the two are not necessarily coincidental. With Heart warm each may be given its own best situation, and simply connected by hot water pipework. The system may be regarded as a normal wet system in which the entire heating load is concentrated at one point, in the heat exchanger of the Heartwarm. Domestic water will be available in the usual way. In some circumstances it may be claimed that this is a variant of a wet system with no long circuits capable of suffering damage, or frost when idle.
Electric heating by warm air systems is dealt with in a preceding post. The principal unit concerned with duct systems is Electricaire, the large storage heater working on off-peak current. Individual storage radiators achieve much the same effect but not centrally and by duct.
The use of electricity at standard tariff to provide warm air, by oil filled radiator or by fan convector, cannot be included in a list of standard recommended methods. It is bound to be expensive and if adopted should be done in full awareness that the basis is convenience, not economy. The weekend cottage might be a suitable place for such a system, where the ‘on tap’ element is worth a lot.
Solid fuel is no longer a serious contender in this market, except in the following respects. Room heaters have a substantial part of their output in the form of warm air rising off the appliance, and kept in the room by strict control of the chimney opening. This is a valuable part of the total efficiency, but it is, as it were, freelance, not a part of a system. A solid fuel boiler may be used to supply the hot water fed to the Heartwarm unit just described, provided that the system includes a hot water cylinder or other buffer for excess heat.
It was not unknown for a solid fuel stove to form the heat generator in a brick central unit, and so long as all the safety precautions against fire and overheating are observed this remains valid. But the decline in brick centrals has almost eliminated that type of heat source.