Blower and Motor
Forced-warm-air furnaces are equipped with variable-speed blowers designed for continuous duty. Blower motors are available as either direct-drive or belt-driven units (Figures 11-24 and 11-25). Most modern furnaces are equipped with direct-drive motors. See the section Blowers and Motors in this chapter for a detailed description.
Air Filter
A forced-warm-air, gas-fired furnace is supplied with either a disposable air filter or a permanent (washable) one. Typical gas furnace air filters are illustrated in Figure 11-26. See the section Air Filters in this chapter for a detailed description.
The first step in planning a heating system is to calculate the maximum heat loss for the structure. This should be done in accordance with procedures described in the manuals published by the Air Conditioning Contractors of America (ACCA), especially their Manual J, Residential Load Calculations, or by a com- parable method. Correctly calculating the maximum heat loss for
the structure is very important because the data will be used to determine the size (capacity) of the furnace selected for the heating system. The ACCA’s Manual J contains the most accurate calculation methods for sizing heating and air conditioning equipment. Computer programs and worksheets designed to simplify the calculation procedures found in Manual J are also avail- able for purchase.
Note
High-efficiency (condensing) gas furnaces must be correctly sized. Oversizing will not lead to increased energy costs, as is the case with the traditional standing-pilot gas furnaces, but it will cause short firing cycles. If the furnace operates with short firing cycles, it will never get hot enough to dry out the condensate extracted from the combustion process. The condensate accumulates, becomes gradually more and more acidic, and begins to corrode parts of the furnace. Oversizing high-efficiency furnaces may also result in uncomfortable temperature swings in the living areas and excessive air flow from the room registers. Oversized furnaces require larger blowers than correctly sized ones.
Caution
A gas furnace must be properly set up and installed by a certified HVAC gas heat technician, a representative of the gas furnace manufacturer, or someone with equivalent experience.
Warning
If the heating or heating/cooling installation is to be approved by either the FHA or VA, heat loss and heat gain calculations must be made in accordance with the procedures described in the ACCA’s Manual J.