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Old 01-01-20, 10:14 AM   #9
MN Renovator
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Originally Posted by JRMichler View Post
Your guesses for sizes of heat pumps are about right for a poorly insulated house with reasonable (less than 10%) window area.

A well insulated house would have at least R60 ceiling, R30 walls, and R3 windows. My last house was insulated like that and had a one ton central air conditioner. In a stretch of hot weather where the daily highs were 100 to 104 deg F, the AC ran only 58% of the time. We normally heat to about 70 deg F and cool to about 78 deg F. It takes two or three days of 90 deg F or hotter to get the house from 70 to 78 deg F. A well insulated house is nice in an area where power outages happen.

Heating/cooling systems do not heat/cool by square feet floor area, but by heat loss or gain. The heat loss or gain is calculated from square feet of wall, window, and ceiling, plus internal heat gain. An example heat gain calculation for a 400 square foot corner bedroom on the top floor:

Wall area (20 + 20) X 9 feet high / R30 = 12.0
Ceiling 400 square feet / R60 = 6.7
Windows (3) at 3 X 4 feet / R3 = 12.0
Total = 30.7
Heat gain at 105 F outside 75 F inside = (105 - 75) X 30.7 = 920 BTUH
Heat gain from two people at 400 BTUH each = 800 BTUH
Heat gain from a big TV set at 100 watts = 340 BTUH

Total heat gain = 2060 BTUH. This is the load that should be used to size the AC.
You seem to have calculated the gain/loss related to the difference between the interior and exterior temperatures correctly but you haven't factored in solar gain through the windows. This is the reason my house uses 15000BTUhr to cool on a 83f when it's sunny when 15000BTUhr can heat my house to 70f when it's 8f outside at night without solar gain.

The rest of the math checks out. I think I'd aim for windows with a U-value of 0.2 or better(R5) if I was going to go with R30 walls and an R60 ceiling. Heat rejecting glass especially on any Southern and Western exposures in sunny California.
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