A warm-air heating system supplies heat to a room by bringing in a quantity of air above room temperature, the amount of heat added by the air being at least equal to that required to counteract heat losses.
A gravity system (without a blower) is rarely installed because it depends on the difference in density of the warm-air supply and the colder room air for the working pressure. Airflow resistance must be kept at a minimum with large ducts and very few elbows. The result usually is an unsightly duct arrangement.
A forced-warm-air system can maintain higher velocities, thus requires smaller ducts, and provides much more sensitive control. For this type of system,
Equation (13.32) indicates that the higher the temperature of the discharge air Th , the less air need be handled. In cheaper installations, the discharge air may be as high as 170F and ducts are small. In better systems, more air is handled with a discharge temperature as low as 135 to 140F. With a room temperature of 70F, we shall need (170 70)/(135 70) 1.54 times as much air with the 135F system as with a 170F system.
It is not advisable to go much below 135F with discharge air, because drafts will result. With body temperature at 98F, air at about 100F will hardly seem warm. If we stand a few feet away from the supply diffuser (register), 70F room air will be entrained with the warm supply air and the mixture will be less than 98F when it reaches us. We probably would complain about the draft.
Supply-air diffusers should be arranged so that they blow a curtain of warm air across the cold, or exposed, walls and windows. (See Fig. 13.4 for a suggested arrangement.) These grilles should be placed near the floor, since the lower-density warm air will rise and accumulate at the ceiling.
Return-air grilles should be arranged in the interior near unexposed walls—in foyers, closets, etc.—and preferably at the ceiling. This is done for two reasons:
1. The warm air in all heating systems tends to rise to the ceiling. This creates a large temperature gradient between floor and ceiling, sometimes as high as 10F.
Taking the return air from the ceiling reduces this gradient.
2. Returning the warmer air to the heating plant is more economical in operation than using cold air from the floor.
Duct sizes can be determined as described in Duct Design, Art. 13.6. To illustrate the procedure, duct sizes will be computed for the structure with floor plans shown in Figs. 13.3 and 13.4. The latter shows the locations chosen for the discharge grilles and indicates that the boiler room, which contains the heating unit, is in the basement.
Table 13.11 showed that a heating unit with 144,475 Btu/hr capacity is required.
After checking manufacturers’ ratings of forced-warm-air heaters, we choose a unit rated at 160,000 Btu/hr and 2010 ft3 /min.
If we utilized the full fan capacity and supply for the rated 160,000 Btu/hr and applied Eq. (13.32), the temperature rise through the heater would be
In actual practice, we do not tamper with the flame adjustment in order to maintain the manufacturer’s design balance. Instead, the amount of air supplied to each room is in proportion to its load. (See Table 13.12 which is a continuation of Table 13.11 in the design of a forced-air heating system.) Duct sizes can then be determined for the flow indicated in the table (see Fig. 13.5). In this example, minimum duct size for practical purposes is 12 3 in. Other duct sizes were obtained from Table 13.7, for a friction loss per 100 ft of 0.15, and equivalent rectangular sizes were obtained from Table 13.6 and shown in Fig. 13.5.
Humidification in Warm-Air Heating.
A warm-air heating system lends itself readily to humidification. Most warm-air furnace manufacturers provide a humidifier that can be placed in the discharge bonnet of the heater.
Theoretically, a building needs more moisture when the outside temperature drops. During these colder periods, the heater runs more often, thus vaporizing more water. During warmer periods less moisture is required and less moisture is added because the heater runs less frequently.
Some manufacturers provide a woven asbestos-cloth frame placed in the pan to materially increase the contact surface between air and water. These humidifiers have no control.
Gas-fired heaters and boilers are usually provided with a draft hood approved by building officials. This should be installed in accordance with the manufacturer’s recommendations. Oil-fired heaters and boilers should be provided with an approved draft stabilizer in the vent pipe. The hoods and stabilizers are used to prevent snuffing out of the flame in extreme cases and pulling of excessive air through the combustion chamber when the chimney draft is above normal, as in extremely cold weather.
Flues for the products of combustion are usually connected to a masonry type of chimney. A chimney may have more than one vertical flue. Where flue-gas temperatures do not exceed 600F, the chimney should extend vertically 3 ft above the high point of the roof or roof ridge when within 10 ft of the chimney. When chimneys will be used for higher-temperature flue gases, many codes require that the chimney terminate not less than 10 ft higher than any portion of the building within 25 ft.
Many codes call for masonry construction of chimneys for both low- and hightemperature flue gases for low- and high-heat appliances. These codes also often call for fire-clay flue linings that will resist corrosion, softening, or cracking from flue gases at temperatures up to 1800F.
Flue-pipe construction must be of heat-resistant materials. The cross-sectional area should be not less than that of the outlet on the heating unit. The flue or vent pipe should be as short as possible and have a slope upward of not less than 1⁄4 in/ ft. If the flue pipe extends a long distance to the chimney, it should be insulated to prevent heat loss and the formation of corrosive acids by condensation of the combustion products.
All combustion-type heating units require air for combustion, and it must be provided in adequate amounts. Combustion air is usually furnished directly from the outside. This air may be forced through ductwork by a fan or by gravity through an outdoor-air louver or special fresh-air intakes. If outside air is not provided for the heating unit, unsatisfactory results can be expected. The opening should have at least twice the cross-sectional area of the vent pipe leaving the boiler.
(H. E. Bovay, Jr., ‘‘Handbook of Mechanical and Electrical Systems for Buildings,’’ and D. L. Grumman, ‘‘Air-Handling Systems Ready Reference Manual,’’ McGraw-Hill Publishing Company, New York.)
Where some humidity control is desired, a spray nozzle connected to the hotwater system with an electric solenoid valve in the line may be actuated by a humidistat.
With humidification, well-fitted storm or double pane windows must be used, for with indoor conditions of 70F and 30% relative humidity and an outdoor temperature of 0F, condensation will occur on the windows. Storm windows will cut down the loss of room moisture.
Control of Warm-Air Heating. The sequence of operation of a warm-air heating system is usually as follows:
When the thermostat calls for heat, the heat source is started. When the air chamber in the warm-air heater reaches about 120F, the fan is started by a sensitive element. This is done so as not to allow cold air to issue from the supply grilles and create drafts.
If the flame size and air quantity are theoretically balanced, the discharge air will climb to the design value of, say, 150F and remain there during the operation of the heater. However, manual shutoff of grilles by residents, dirty filters, etc., will cause a reduction of airflow and a rise in air temperature above design. A sensitive safety element in the air chamber will shut off the heat source when the discharge temperature reaches a value higher than about 180F. The heat source will again be turned on when the air temperature drops a given amount.
When the indoor temperature reaches the value for which the thermostat is set, the heat source only is shut off. The fan, controlled by the sensitive element in the air chamber, will be shut off after the air cools to below 120F. Thus, most of the usable heat is transmitted into the living quarters instead of escaping up the chimney.
With commercial installations, the fan usually should be operated constantly, to maintain proper air circulation in windowless areas during periods when the thermostat is satisfied. If residential duct systems have been poorly designed, often some spaces may be too cool, while others may be too warm. Constant fan operation in such cases will tend to equalize temperatures when the thermostat is satisfied.
Duct systems should be sized for the design air quantity of the heater. Insufficient air will cause the heat source to cycle on and off too often. Too much air may cool the flue gases so low as to cause condensation of the water in the products of combustion. This may lead to corrosion, because of dissolved flue gases.
Warm-Air Perimeter Heating. This type of heating is often used in basementless structures, where the concrete slab is laid directly on the ground. The general arrangement is as follows: The heater discharges warm air to two or more underfloor radial ducts feeding a perimeter duct. Floor grilles or baseboard grilles are located as in a conventional warm-air heating system, with collars connected to the perimeter duct.
To prevent excessive heat loss to the outside, it is advisable to provide a rigid waterproof insulation between the perimeter duct and the outside wall.
Air Supply and Exhaust for Heaters. Special types of packaged, or preassembled, units are available for heating that include a direct oil- or gas-fired heat exchanger complete with operating controls. These units must have a flue to convey the products of combustion to the outdoors. The flue pipe must be incombustible and capable of withstanding high temperatures without losing strength from corrosion.
Corrosion is usually caused by sulfuric and sulfurous acids, which are formed during combustion and caused by the presence of sulfur in the fuel.