Figuring out building heat-transmission loads

(The following is taken from the Sheet Metal and Air-Conditioning Contractors' National Association's Residential Comfort System Installation Standards Manual, seventh edition.)

Before sizing the heating-and-cooling system components, it is first necessary to determine the anticipated design loads for each conditioned room. After each room load has been calculated, it is then combined with all other loads to arrive at a total system capacity.

Although it is possible for individual rooms to experience the maximum calculated heating or cooling load at least some hours and days of the year, it is unlikely that all rooms will experience these peak loads at the same time. During early morning summer days, rooms with east-facing windows will usually experience high solar-heat gains, while the west- and north-facing spaces may have half or no system load.

When calculating a total system cooling demand, these time-of-day and room-orientation effects usually allow reducing the total system capacity; however, the heating capacity should not be down-sized, as it is still possible for all rooms to experience the same peak heating loads at the same time on cold and cloudy days or nights.

Architectural factors

There are many architectural factors related to the design of any residence that can have a significant impact on the heating-and-cooling system loads. Large window areas, which would normally have significant solar gains, may be partially or completely shaded by roof overhang or sidewall. An exterior entrance may include a separate interior door that creates an "air lock" that can reduce air infiltration. Architectural details may also require exterior glass with tinting to reduce solar gain, super-insulated skylights, (or) more efficient lighting systems.

The load-calculation requirements of this section are based on Standard 90.2 - Energy-Efficient Design for Residential Structures - as published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

This manual and most computerized load programs follow the calculating procedures described in this section, which were published in Manual J - Load Calculation for Residential Winter and Summer Air Conditioning, seventh edition, published by the Air Conditioning Contractors of America.

Refer to the complete Manual J for a more detailed description of heating-and-cooling load calculation procedure.

Today there are many easy-to-use computer programs available to speed the process of heating-and-cooling system sizing; however, it is important to understand how these loads are calculated before attempting to computerize the calculation process. Almost all computerized load programs follow the manual calculating procedures outlined in this text.

Heat loads

The design heat loss must be calculated for the winter outdoor-design temperature as described earlier. Because the maximum heat loss usually occurs during the early morning hours, before sunrise and at a time of occupant inactivity, the heat gains due to solar radiation and internal heat gains are not considered in the heat-loss calculation.

When central-heating equipment is installed, the equipment-sizing load (design-heating load) is equal to the heat loss for the entire house.

The heating load for each individual room must also be estimated to determine:

· The room-supply cfm (heating) if a "central" warm-air system is installed.

The size of the individual room-heating units (baseboard radiation, for example) if a "distributed" heating system is installed.

Outside-design conditions

The outside-design temperature is not the lowest temperature recorded. Temperature extremes occur only a few hours per year and do not represent the actual conditions experienced during average winter weather. A locality may have a record low of minus 20°F (minus 29°C), but the coldest weather normally experienced might be 0°F to 5°F (minus 18°C to minus 15°C), and the average winter temperature might be 35