As an example of the method described for sizing a heating plant, let us take the building shown in Fig. 13.3.
A design outdoor temperature of 0F and an indoor temperature of 70F are assumed. The wall is to be constructed of 4-in brick with 8-in cinder-block backup.
Interior finish is metal lath and plaster (wall heat-transmission coefficient U 0.25).
The method of determining the heat load is shown in Table 13.11.
Losses from the cellar include 4 Btu/ (hr)(ft2) through the walls [column (4)] and 2 Btu/ (hr)(ft2) through the floors. Multiplied by the corresponding areas, they yield the total heat loss in column (6). In addition, some heat is lost because of infiltration of cold air. One-half an air change per hour is assumed, or 71.2 ft3 /min [column (3)]. This causes a heat loss, according to Eq. (13.28), of
To the total for the cellar, 20% is added to obtain the heat load in column (7).
Similarly, heat losses are obtained for the various areas on the first and second floors. Heat-transmission coefficients [column (4)] were obtained from the ‘‘ASHRAE Handbook—Fundamentals,’’ American Society of Heating, Refrigerating and Air-Conditioning Engineers. These were multiplied by the temperature gradient (70 0) to obtain the heat losses in column (6).
The total for the building, plus 20%, amounts to 144,475 Btu/hr. A heating plant with approximately this capacity should be selected.