I’m going to devote the next several blogs to a discussion of heat-loss and heat-gain calculations. These calculations are the first step in the design of a home’s heating and cooling system.
In order to address this big topic in little bites, I’ll start by discussing heat-loss calculations. I’ll get around to heat-gain calculations and cooling equipment in a future blog.
Before digging in to the topic, however, it’s worth answering the question, “Why do I need to know how to perform these calculations?” If you are a homeowner or carpenter, you may not need to know anything about calculating heating loads. However, if you are a designer, architect, or builder, this knowledge will prove useful — even if you never perform the calculations yourself, but instead depend on computer software or consultants to perform the calculations.
Understanding the principles behind heat-loss calculations will help you understand how buildings work and how heating and cooling systems perform. With this knowledge, the quality of your dialogue with window suppliers, HVAC contractors, and engineers will definitely improve.
Outdoor design temperature
If you’re performing a heat-loss calculation to size heating equipment, you need to perform the calculation for the worst-case condition: namely, the coldest night of the year. (Because the coldest condition usually occurs at night, a heat-loss calculation does not consider solar gain through windows.) The temperature on that night is referred to as the outdoor design temperature. (To be precise, the outdoor design temperature is usually defined as the temperature that is equaled or exceeded for 97.5% of the time during the three coldest months of the year. Other sources define the outdoor design temperature as the temperature that is equaled or exceeded for 99% of the year. As it turns out, most homes will remain comfortable…
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