1. Cooling And Heating Load Calculation Principles
2. Ashrae Cooling And Heating Load Calculation Manual Pdf

Estim a tin g Fuel Requirements and Heating CostsEstimating fuel requirements and heating costs is not an exact science. It involves too many variables to be anything other than an estimation. Moreover, there are four different formulas used for making these calculations. Depending on which formula is used, it is possible to arrive at four different estimations of fuel requirements and heating costs for a specific situation.

It is no wonder, then, that two competent engineers can submit estimates that will differ as widely as 30 percent or more. These facts are not offered to discourage anyone but to present the true picture. Any attempt to calculate fuel requirements and heating costs will not produce precise figures––only an estimate. The problem is to make this estimate as close an approximation to the real situation as possible.The four formulas used in calculating fuel requirements and heating costs are:1.

Heat loss formula2. Corrected heat loss formula3. NEMA formula4. Degree-day formulaIf one formula is used to calculate the fuel requirements and heating costs with one type of fuel (e.g., oil) and another formula is used for a second type (e.g., natural gas), the results are practically worthless for comparison purposes.

For example, the NEMA formula (created by the National Electric Manufacturers Association) will present the use of electric energy much more favorably if the results are compared with calculations for oil or natural gas based on the heat loss formula. A true comparison is possible only when two fuels are both calculated with the same formula. Each of these formulas is described in the following sections.The Heat Loss FormulaThe heat loss formula results in higher percentages of total require ments 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.

Cooling And Heating Load Calculation Principles

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 Btu2. Total hours in the heating season3.

Average winter temperature difference4. Btu per unit of fuel5. Efficiency of utilization6. Difference between inside and outside design temperaturesThe 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 cli- mate.

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 experi- enced in a locality over a 3- to 4-year period. These outside design temperatures are available for a large number of localities through- out 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. Natural gas3. The Corrected Heat Loss FormulaThe corrected heat loss formula was devised in 1965 by Warren S. Harris and Calvin H.

Fitch of the University of Illinois. It takes into consideration internal heat gains, which have shown a marked increase over the past 40 to 50 years. These internal heat gains make a significant contribution to the total required heat of a house or building, a factor that makes the original 65°F heat base established some 40-odd years ago too low for making a correct estimate. Furthermore, an inside average temperature of 73°F is probably more correct than the 70°F temperature previ- ously used.According to Harris and Fitch, the degree-day base (for all areas except along the Pacific Coast) must be corrected by the percentage given in Table 4-7. The calculated heat loss should then be reduced by 3.5 percent for each 1000 ft. The house or building is located above sea level. If the heat loss is below 1000 Btu per degree tem- perature difference, the 65°F degree-day base should be further reduced to the figures given in Table 4-8.Table 4-9 illustrates the use of the corrected heat loss formula for determining comparative fuel requirements for No.

Ashrae Cooling And Heating Load Calculation Manual Pdf

2 oil and natu- ral gas.The Degree-Day FormulaThe degree-day formula was devised some 40-odd years ago by theAmerican Gas Association and other groups and has since been revised (see pages 627–28 of the 1970 ASHRAE Guide) to reflectinternal heat gains and the levels of insulation. The correction factors involved in the revision are given in Tables 4-10 and 4-11. These correction factors should also be applied to the corrected heat loss formula (see the previous section).The degree-day formula is based on the assumption that heat for the interior of a house or building will be obtained from sources other than the heating system (e.g., sunlight and body heat of the occupants) until the outside temperature declines to 65°F. At this point the heating system begins to operate. The con- sumption of fuel will be directly proportionate to the differencebetween the 65°F base temperature and the mean outdoor temperature. In other words, three times as much fuel will be used when the mean outdoor temperature is 35°F than when it is 55°F.

The mean outdoor temperature can be determined by taking the sum of the highest and lowest outside temperatures during a 24- hour period, beginning at midnight, and dividing it by 2. Each degree in temperature below 54°F is regarded as 1 degree day.The degree-day formula is applied by dividing the heat loss fig- ure by 1000 and multiplying the result by the figure for the unit fuel consumption per degree day per 1000 Btu design heat loss, which, in turn, is multiplied by the total number of degree days in the heating season (calculated on a 65°F base) and then by the correction factors given in Tables 4-10 and 4-11.

The application of this formula is illustrated in Table 4-12.The NEMA FormulaThe NEMA formula was created by the National Electric Manufacturers Association. A constant of 18.5 is commonly used in the formula and reflects the percentage of heat loss that must be replaced by the heating system during a 24-hour period.

In other words, the heating system will provide heat 77 percent of the time during a 24-hour period (24 X 18.5), while other heat sources will supply heat during the remaining 23 percent.The NEMA formula consists of dividing the heat loss by 3413 (the number of Btu per kilowatt-hour) and multiplying this figure by the total annual number of degree days, which figure is then multiplied by the constant (usually 18.5). The resulting product is then divided by the difference between the indoor and outdoor temperatures.