Choosing domestic warm air heating system: a place for the furnace

Press, Volume CXI, Issue 32587, 22 April 1971, Page 23

Choosing domestic warm air heating system: a place for the furnace

(By. a heating engineer)

Warm air heating contractors are sometimes asked by prospective buyers, or architects, which type of warm air heating system is best—one which introduces 100 per cent fresh air or one which operates mainly with recirculated air. The choice of a system will depend on the amount of money to be spent and the type of furnace which is installed, but climatic conditions will have a bearing on the design and the more severe these conditions are, the more extensive the system must be if satisfactory performance is to be achieved. Comparatively simple, indeed one might almost say primitive, systems work with satisfaction to their owners in milder climates such as obtained in Auckland, but to apply such systems in colder areas, such at the central part of the North Island and much of the South Island, would certainly result in poor or even completely unacceptable performance.

The advantages claimed for warm air heating over hotwater systems are as follows: They are cheaper to install because the ducting system requires less skilled labour than pipework, the air outlets are cheaper than heat-emitting devices such as radiators and convectors, and so on, and no cold feedexpansion tank is necessary. Heat is quickly available once the furnace is turned on. This is of particular con cem to business people who are out of the house for long periods and wish to have heat available quickly once they return. They can be used to introduce a controlled amount of fresh air during the heating season and the fall I may be run without the I burner to provide venti- 1 lation only in hot wea-l 1 ther. Where better than 1 average ventilation is re- 1 quired, consideration 1 should be given to using < a two speed fan which < can be operated at low i speed for heating. and’<

sped up for use in Summer. Ducts will, of course, need to be sized for the larger air quantity. If floor outlets are used, these can be located at source of greatest heat loss, such as window walls, sliding glass doors, and so on where it would be impossible to fit radiators or even skirting heaters.

If the above advantages are to be used, the system must be correctly engineered and designed to suit climatic conditions of the locality. Air volumes should, always be such that the temperature of the air leaving the outlets is not so high that distribution is impaired; outlets should be placed to blanket areas of greatest heat loss with warm air and if recirculation is used, care should be taken that air supplied to a room can, in fact, find its way to the return intake. Ideally, a return should be placed in each room but economics usually demand that one centrally located return be

installed, in which case relief grilles should be placed in intervening walls or doors, or an inch or two cut off the bottom of the latter.

i Some recirculation is i essential if economy is to be > achieved and some fresh air is desirable in order to keep the occupied space fresh and ; maintain leakage outwards, thus minimising draughts around windows, and so on.

To admit all the air handled by the furnace from outside will add substantially to the running costs. If we assume a typical house to have a heat loss of 60,000 B.t.u. an hour when the outside temperature is 30 deg. F lower than that inside, and a furnace fan handling 900 cubic feet a minute of air, the amount of heat required for bringing the fresh air up to room temperature will be:

900 cfm x 60 x 0.018 8.t.u./ cu. ft x 30 deg. F rise equals 29,160 8.t.u./hr. This indicates an increase in fuel consumption of 29,160 x 100 or 48.4 per cent. If the annual fuel bill, assuming all air recirculated, were say $6O, this would rise to $89.05 if all air were brought from outside. Generally, the practice is to use about 25 per cent fresh air and 75 per cent recirculation

and this will add only 12 per t cent to the fuel bill, i.e. $7.25 a to the $5O mentioned above. F The attached sketches show three popular methods of installing two models of furn- a ace, and the notes describe r the features of each method. c It should be borne in mind that cost of the initial installation is not the Anal cost of owning a warm air heating system and that a more expensive type of system may well prove more economical in the final analysis, when running costs have been considered. SKETCH k The furnace is installed in a basement or garage or crawl-space with a fan draw--1 ing 100 per cent outdoor air from below the house. The ; garage door must'be open or g evenbs must be provided in* walls so that fresh air can enter. If the basement is closed to the outside and a generously sizfed grille is provided in the stairwell (or the door to the stairwel) the unit will handle mainly recirculated air. Some ingress of fresh air is almost inevitable around

the garage door and if the unit has no filter, dust will be drawn into the fan and circulated through the house.

SKETCH B The furnace is installed in separate furnace room with an air intake from either outside or the occupied space. The intake must be large enough to supply air for the fan, for burner and draught regulator in chimney. If the intake is too small, an erratic burner working will result and the fan can draw fumes from relief door or draught regulator and circulate them through the house. The air intake can be located so that the unit draws either fresh air or recirculated air; intakes to both inside and I outside can be used to admit t both, but precise control of s amounts would be difficult. < Remarks regarding filter in t (A) are applicable here. i < Filters placed in the intake < are not effective as the. restrictions they . impose cause air to be drawn in t

through cracks round doors and so on. If these are sealed, the fan causes a depression ih the room which upsets the burner performance and allows the fan to draw fumes from the relief door and draught regulator. This layout can also be fused for a downcast furnace

; delivering air to ducting be- : low the floor of a singlei storey type of house. 'SKETCH C i The furnace is installed in ' a separate room with com- . bustion air intake; fresh air and recirculation ducts taken ! from outside the room with ! a simple damper for propor-

tioning; an air filter at the ■ unit filters both fresh air and recirculated air. The unit is fitted with a centrifugal fan which produces more presi sure than a propeller type - and can thus handle the r greater resistance of filter i and larger duct runs. The i system can be extended at - some future date and a cooling coil added to the supply duct if required. This type of furnace can be used with arrangements (A) and (B) also. Arrangement (A) is lowest in first cost but unless arranged to handle 100 per cent recirculation, will cost the most to run and has no provision for air filtration. Arrangement (B) is similar to (A), but with the added cost of a furnace room. Arrangement (C), is the most expensive in first cost, but provides air filtration, better economy of operation, ventilation in summer, is suitable for greater system resistance (longer duct runs, cooling oil, and so on) and is essential in areas where climate is extreme.