General Procedure for Sizing a Heating Plant

The basic procedure used in sizing a heating plant is as follows: We isolate the part of the structure to be heated. To estimate the amount of heat to be supplied to that part, we must first decide on the design indoor and outdoor temperatures. For if we maintain a temperature of, say, 70F inside the structure and the outside temperature is, say, 0F, then heat will be conducted and radiated to the outside at a rate that can be computed from this 70 temperature difference. If we are to maintain the design inside temperature, we must add heat to the interior by some means at the same rate that it is lost to the exterior.
Recommended design inside temperatures are given in Table 13.10. Recommended design outdoor temperatures for a few cities are given in Table 13.5, p.
13.27. (More extensive data are given in the ‘‘ASHRAE Handbook—Fundamentals,’’ American Society of Heating, Refrigerating and Air-Conditioning Engineers.)
Note that the recommended design outdoor winter temperatures are not the lowest temperatures ever attained in each region. For example, the lowest temperature on record in New York City is -14F, whereas the design temperature is 0F. If the design indoor temperature is 70F, we would be designing for (70 - 0) /(70 –  14), or 83.3%, of the capacity we would need for the short period that a record cold of -14F would last.
Once we have established for design purposes a temperature gradient (indoor design temperature minus outdoor design temperature) across the building exterior, we obtain the heat-transmission coefficients of the various building materials in the exterior construction for computation of the heat flow per square foot. These coefficients may be obtained from the manufacturers of the materials or from tables, such as those in the ‘‘ASHRAE Handbook—Fundamentals.’’ Next, we have to take off from the plans the areas of exposed walls, windows, roof, etc., to determine the total heat flow, which is obtained by adding the sum of the products of the area, temperature gradient, and heat-transmission coefficient for each item (see Art. 13.3).

Choosing Heating-Plant Capacity. Total heat load equals the heat loss through conduction, radiation, and infiltration.
If we provide a heating plant with a capacity equal to this calculated heat load, we shall be able to maintain design room temperature when the design outside temperature prevails, if the interior is already at design room temperature. However, in most buildings the temperature is allowed to drop to as low as 55F during the night. Thus, theoretically, it will require an infinite time to approach design room temperature. It, therefore, is considered good practice to add 20% to the heatingplant capacity for morning pickup.
The final figure obtained is the minimum heating-plant size required. Consult manufacturers’ ratings and pick a unit with a capacity no lower than that calculated by the above method.
On the other hand, it is not advisable to choose a unit too large, because then operating efficiency suffers, increasing fuel consumption.
With a plant of 20% greater capacity than required for calculated heat load, theoretically after the morning pickup, it will run only 100/120, or 831⁄3%, of the time. Furthermore, since the design outdoor temperature occurs only during a small percentage of the heating season, during the rest of the heating season the plant would operate intermittently, less than 831⁄3% of the time. Thus it is considered good practice to choose a heating unit no smaller than required but not much larger.
If the heating plant will be used to produce hot water for the premises, determine the added capacity required.