On Tue, Sep 14, 2010 at 9:47 PM, David C. <cavanadd@frontier.com> wrote:
> We just had a heat pump system installed to replace our propane furnace.
> B We had no A/C before, either. B We kept the propane for emergency heat.
> B My question is how low can you set the night setback and still have a
> reasonable recovery in the morning. B Right now I have it set for 69*
> heating and 65* on night setback in the heating mode. B Night SB kicks in
> at midnight, back up at 8:00 AM. B I'm not going to worry about the A/C
> part until next summer.
>
> 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. B Frankly, this
> sounds like BS to me, but I thought I would get some other opinions from
> people who have BTDT. B I would prefer about an 8 to 10 degree setback,
> but if 4 is the most recommended, I guess it's better than nothing.
>
It's nonsense, as long as you don't cause the emergency heat to kick
in. The magic of heat pumps is that they're more than 100%
efficient[1]. Turning on the backup heat costs, with electric
resistance backup, three or four times as much as heat from the heat
pump; propane will be different numbers, depending on how efficient
your furnace is, and what the relative cost of propane and electricity
is. But the number of joules required to keep a house at 68F is
higher than the number of joules required to let the house cool down
overnight, and heat back to 68F.
How low you can go without the backup heat depends on a whole lot of
things. Probably biggest of them is how well insulated your horse is.
If it's well insulated, without lots of air intrusion, the amount of
heat required to raise the temperature will be less, as will be the
temperature drop overnight, without heat. It'll also depend on what
the outside temperature is, as the colder it is outside, the more heat
you'll lose, and the less effective your heat pump will be (assuming
you've got an air-source pump, and not a ground-source one).
Depending on how fancy your thermostat is, there may be a way to
lockout the backup heat. If you can do that, you can experiment with
how big a setback you can put up with. Part of that will be whether
you're willing to put up with a house that's chilly for a couple hours
after the heat starts coming back.
Depending on your house, and climate, and heat pump, you may well
need the backup heat for the coldest part of the year. Air-source
heat pumps don't work well below an air temperature of about 40
degrees, though modern ones are someone better, some working with a
COP[1] of better than two down to 20F. Ground source pumps work
better in cold weather, as the ground tends to be warmer.
[1] They're not, of course. The energy comes from the air or the
ground, depending on the heat source of your pump, but not somewhere
you have to pay for.
[2] Coefficient of performance; a measure of how efficient the system
is. COP of 1 is 1 watt consumed for 1 watt of heat; CoP of 2 is 1
watt spent for 2 watts of heat, and so on. See point [1],
--
David Scheidt
dmscheidt@gmail.com
_______________________________________________
Shop-talk@autox.team.net
Donate: http://www.team.net/donate.html
Suggested annual donation $12.96
Archive: http://www.team.net/archive
Forums: http://www.team.net/forums
|