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re: auto thermostats
7 nov 2002
tono wrote:
>important! if you have a heat pump, turning the heat down more than
>about 2 degrees can end up costing you more money so it's probably
>not worth it.
only 2 degrees?
>the reason is because when it's time to make it warm again, the backup
>or emergency heat kicks on, and that is as expensive as running regular
>electric heat.
say it's 40 f outdoors, and the house has 500 btu/h-f of thermal
conductance and 5k btu/f of capacitance, and the heat pump can supply
2(70-40)500 = 30k btu/h. say it follows this rule: "if the house temp
drops 0.5 degrees below the setpoint, turn on the heat pump. if it ever
drops 1.0 degrees below the setpoint, turn on the resistance heat," which
costs 10 vs 3 cents/kwh. how large can an 8 hour once-a-day setback be
before it stops saving money? is the breakeven dt only 2 degrees?
perhaps dt can be larger with a different rule during setback times, like
"only turn on the resistance heat if the heat pump has been running for
over 30 minutes without meeting the setpoint."
nick
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