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Set comfort targets and find your design temperatures

Every sizing calculation starts with the indoor temperature you want and the extreme outdoor temperature you design for. Using the right design temperature stops you buying equipment that is too small on the coldest night or too big the rest of the year.

How

1. Choose an indoor winter target (many textbook examples use 20 C) and a summer upper limit you can live with.
2. Find your local winter design temperature. In the US this is the ASHRAE 99% or 99.6% value, meaning the outdoor temperature is colder only about 1% of hours. Other countries publish their own design tables, so check your national building guidance.
3. Find your annual heating degree days (and cooling degree days if you will cool) from your national weather or energy agency.
4. Note your summer humidity: it decides later whether evaporative cooling is an option.
5. Record the site microclimate notes from your site survey (wind exposure, frost pockets, shade).

Do these first

• site-1

Sources

1. HVAC heat loss calculations (course M319). PDH Online, accessed 2026-11
2. Energy in buildings: heat loss calculation. The Open University, OpenLearn, accessed 2026-11

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