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 deerhunter 11-17-2010 01:28 AM

A question about heat loss rate

I did a little experiment with setting my thermostat at different temperatures.

The two different setting are 68 and 72. The result of the experiment greatly perplexed me. At 72 degree, the heat pump went through short cycles like every 8 minutes. When setting at 68 degree, the cycles were much longer, like 30 minutes or longer. I understand the the higher you set the temperature, the faster the heat loss. But I have the impression that heat loss is proportional to temperature difference. The outside temperature is below 40 degree. So the difference between inside and outside is about 30. At this much baseline difference, why a fluctuation of 4 degree matters so much?

 beenthere 11-17-2010 04:43 PM

Are you saying setting the stat temp to 68, with the outdoor temp the same as when the stat was at 72, it ran longer set at 68?

 deerhunter 11-19-2010 01:49 PM

I'm saying that when t-stat set at 68, the heat pump off time is longer. Yes it's expected to be longer, I'm only wondering why it's so much longer for just a difference of 4 degrees. The outdoor temperature is around 40. So the difference between indoor and outdoor is about 30. isn't the rate of heat loss supposed to be proportional to temperature difference? So a change from 28 to 32 is like less than 15%. But the HP down time decreased from about 30 minutes to 4 minutes (with HP up time about the same, 4 minutes).

The only thing I can think of is the thermal mass of the house exterior wall (bricks). It's at about 70 degree at day time. So, perhaps a few hours into the night, the interior side of the brick wall is still at around 68 degree, so the temp difference when t-stat at 68 is about 0. And when set at 72, there is some difference.

Still, it's very perplexing that at 68 degree, the inside temperature almost stand still but a mere 4 degree higher, it's loosing heat like crazy.

The thermal mass of the house also doesn't explain that the attic temp is about a little over 40 degree and the large windows that I have lots of. The temp difference at these areas is still around 20-30.

Quote:
 Originally Posted by beenthere (Post 535986) Are you saying setting the stat temp to 68, with the outdoor temp the same as when the stat was at 72, it ran longer set at 68?

 beenthere 11-19-2010 02:07 PM

It will depend on what time of day your doing this.

During the day. Solar gain plays a role in how long your heat pump will be off.

Lowering the indoor temp 4 degrees. Can easily be enough that the solar gain makes up enough of your heat loss that it will stay off much longer.

Next. Stack effect can play a big role in it also. Since you decrease the amount of heat loss in your ceiling by a much greater percentage then just the temp difference between the attic and conditioned areas.

 deerhunter 11-19-2010 04:10 PM

I did the observation at night, about from 11 pm to 12:xx AM.

I think it may still be the thermal mass of the house. I'm not sure stack effect can work so differently with a difference of 4 degrees. At day time, the temperature almost stays at 68-70 degree, so that's about the temperature of the structure at day time. This stored heat will help the house to stay at 68 degree with ease during the night, but if I want to go any higher, the thermal mass of the house will only work against it.

Quote:
 Originally Posted by beenthere (Post 536976) It will depend on what time of day your doing this. During the day. Solar gain plays a role in how long your heat pump will be off. Lowering the indoor temp 4 degrees. Can easily be enough that the solar gain makes up enough of your heat loss that it will stay off much longer. Next. Stack effect can play a big role in it also. Since you decrease the amount of heat loss in your ceiling by a much greater percentage then just the temp difference between the attic and conditioned areas.

 beenthere 11-19-2010 04:40 PM

Stack effect changes drastically with just a 2°F change in thermostat set temp.

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