On 9/14/2010 9:47 PM, David C. wrote:
> We just had a heat pump system installed to replace our propane furnace.
> We had no A/C before, either. We kept the propane for emergency heat.
> My question is how low can you set the night setback and still have a
> reasonable recovery in the morning.
Get a modern thermostat. The best ones record the actual recovery time
and adjust the recovery start time based on the actual heating
performance in your house and the outside temperature.
> Right now I have it set for 69*
> heating and 65* on night setback in the heating mode.
Sounds reasonable to me. That should provide comfortable sleeping
conditions for most people.
> Night SB kicks in
> at midnight, back up at 8:00 AM.
Why don't you let it step down earlier? Unless your house is poorly
sealed, the thermal mass in your house will make sure that the
temperature will not decline quickly? You are not saving much by having
it down for 8 hours, minus the recovery time. You may only be cutting
out one or two heat cycles depending how well insulated your house is.
Get a recording thermometer that will show you a temperature history.
You may not even be getting to 65 in that time. If you want to get
really tricky but still comfortable in the evening, you can use a
4-period thermostat and have the temps step down in an orderly fashion.
> I'm not going to worry about the A/C
> part until next summer.
The guidelines for heat and AC turn-backs are pretty much the same since
the thermal mass is the same.
> The problem is my wife has been told by apparently "several people" that
> any more than a 4 degree night setback will cause the compressor to run
> excessively the next day to bring the heat back up, wiping out most of
> the energy savings from setting the temperature back. Frankly, this
> sounds like BS to me, but I thought I would get some other opinions from
> people who have BTDT. I would prefer about an 8 to 10 degree setback,
> but if 4 is the most recommended, I guess it's better than nothing.
I took a course on an overview of commercial HVAC issues. The
recommendation for commercial buildings with heat pumps was no more than
5 degrees setback for a 12-hour period, due to excessive power
consumption to recover the heat in the morning. Something to do with the
strengths and weakness of heat-pump efficiency. For 36-hour or longer
setbacks (weekends and extended shutdowns) it was acceptable to let the
buildings drop to 55, but no lower, to protect the pipes from freezing.
This was based on testing done at Penn State U to cut commercial
building costs. Dunno if it is applicable to a home though.
Jon
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