The Heat Loss Formula

The heat loss formula results in higher percentages of total require­ments because it does not take into consideration internal heat gains obtained from appliances, sunlight, the body heat of the occu­pants, electric lights, and other sources. The corrected heat loss for­mula includes these factors (see the following section).

The heat loss formula will include the following data:

1. Heat loss expressed in Btu

2. Total hours in the heating season

3. Average winter temperature difference

4. Btu per unit of fuel

5. Efficiency of utilization

6. Difference between inside and outside design temperatures

The product of the first three items (1-3) is divided by the prod­uct of the last three (4-6). In other words, heat loss X total hours X average winter temperature difference/Btu per unit of fuel X effi­ciency of utilization X (inside design temperature — outside design temperature).

The method for calculating heat loss is described in Chapter 3 and is expressed in Btu per square foot per hour per degree Fahrenheit design temperature difference.

The total hours in the heating season will depend on the location of the house or building. If it is located in a southern state, the heat­ing season will be much shorter than if it is located in a colder cli­mate. Assuming that the heating season begins October 1 and ends May 1, the total number of days for which heat may be required is 212. This figure is multiplied by 24 (hours) to obtain the total hours in the heating season (5088).

The average winter temperature difference is found by subtracting the average low temperature from the average high temperature for the location in which the house or building is situated.

The Btu per unit of fuel is determined for each type, and this infor­mation can usually be obtained from the local distributor of the fuel.

Most heating equipment will burn oil or gas at an 80 percent combustion efficiency. Electric energy is generally considered to be used at 100 percent efficiency.

The outside design temperature is the lowest temperature experi­enced in a locality over a 3- to 4-year period. These outside design temperatures are available for a large number of localities through­out the United States. If the house or building is located in a small town or rural area, the nearest known outside design temperature is used. This inside design temperature is the temperature to be main­tained on the interior of the house or building. The difference between the two design temperatures is used in the heat loss formula.

Table 4-6 illustrates the use of the heat loss formula for three types of fuel/energy:

1. Oil

2. Natural gas

3. Electricity

Table 4-6 Applying Heat Loss Formula to Three Types of Fuels

(1) No. 2 oil, 140,000 Btu/gal, 80% efficiency, 35,000 Btu/h loss, 73°F inside temperature:

35.0 TOC o "1-5" h z X 5088 73 — 41

140.0 X 0.80 X "70—0 = 727 gallonS

(2) Natural gas, 100,000 Btu per therm, 80% efficiency:

35.0 X 5088 73 — 41

100.0 X 0.80 X = 1018 therms

(3) Electricity, 3413 Btu/Wh, 100% efficiency:

35.0 X 5088 73 — 41

= 23,852 kWh

3413 70 — 0

Courtesy National Oil Fuel Institute

Posted in Audel HVAC Fundamentals Volume 1 Heating Systems, Furnaces, and Boilers