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Heat pump COP and preheating

Has anyone ever seen an analysis that indicates the benefit of having an air source heatpump run more before the nightly temperature drop in order to obtain a higher COP than later in the night?

A similar pre-charge could be done for cooling before the outside temp rose a lot.

In the past with leaky poorly insulated houses I don't think this made much sense, but now with some very tight and well insulated houses I belive it would be beneficial.

The main trick is to mine weather report data and control the thermostat based on COP curve, energy loss rate, and heat schedule.

Also does anyone know where one can locate Trane heatpump COP's?


Asked by Ed Miska
Posted Wed, 03/05/2014 - 19:43


7 Answers

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Ed, I am a huge fan of the idea. Totally makes sense. This winter has been in the twenties days and at 0 nights. Install an oversized mini split and run the temp up days and coast. Opposite for summer cooling. Would work for me as I am not attached to any exact temperature. Too many today are. Adding lots of thermal mass or some other actual storage would be needed to stay right at a given set point. Energy misers passive solar lovers... choose a wider range of temperature to live in.

Answered by aj builder, Upstate NY Zone 6a
Posted Wed, 03/05/2014 - 19:57

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As A.J. pointed out, adding interior thermal mass to you house design will have this effect. I addressed the issue in my article, All About Thermal Mass. In that article, I wrote:

"A high-mass wall introduces a thermal lag or time delay in the flow of heat from the exterior to the interior. If the effects of strong afternoon sunlight only penetrate to the interior surface of the wall in the middle of the night, the air conditioner is more likely to be operating when outdoor temperatures are cooler. Since air conditioners operate more efficiently at night, when the outdoor temperature is low, than they do during the heat of the day, energy savings can result from the thermal lag introduced by a high-mass wall."

Answered by Martin Holladay, GBA Advisor
Posted Thu, 03/06/2014 - 05:41
Edited Thu, 03/06/2014 - 05:42.

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Internal mass + time schedule controller temp will also work toward that.
Just rise the set temp by 1-2c at the correct timing and lowring it by 1-2c lower than the regular set point during night time.

As discussed in the past, placing the exterior unit so that it is heated by the sun during late day time will also help with efficiency.

Answered by Jin Kazama
Posted Thu, 03/06/2014 - 11:51

Helpful? 0

If you are using modulating heat pumps like inverter-drive mini-splits the model isn't that simple. The modulating units get far higher efficiency at part load than when running full speed, which is often a greater effect than the outdoor temperature issues, so running it flat-out to pre-heat the house would result in LOWER net efficiency under some circumstance, break-even in others, and only see a benefit occasionally. "Set and forget" turns out to be a higher efficiency approach with many of them, unless you'll be gone for a signficant amount of time.

See the bench tested efficiency at different output levels across temperature for a couple of popular 1-tonners in this document:

Answered by Dana Dorsett
Posted Thu, 03/06/2014 - 13:07

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Dana, thank you for link. Jin Kazama I like your post.. And must say you have a great name.

Answered by aj builder, Upstate NY Zone 6a
Posted Thu, 03/06/2014 - 17:58

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Hmm. For those of us whose time-of-use peak metering rate runs
from noon to 7pm, this would be kinda self-defeating unless the
burst could be squeezed into the morning. Especially with a modern
heat pump whose COP doesn't drop off as significantly until it gets
*really* cold...

In general I shut down over the peak period and any afternoon warmth
simply helps the house flywheel over it. Hasn't gotten below 60F
from a 67-ish setpoint all winter...


Answered by Hobbit _
Posted Thu, 03/06/2014 - 21:11

Helpful? 0

AJ: ah yeah ? what do you like about my post? the white background color perhaps? :p

I quickly sketched an idea about a year ago for a factory i am going to build in the coming year,
where multiple RLS2H units from Fujitsu will be providing most of the heat .
I thought about installing some kind of baffles to control front and rear input/output of the exterior unit, to use some kind of solar heat through thermal mass panels go boost out efficiency during day time .
Would probably have to be very "cheap" to be worth it though.

Also could use the principle of the canadian "solar wall" to provide warmer air to the units during very cold days ( which was just every days from november up to now this year !! )

Answered by Jin Kazama
Posted Thu, 03/06/2014 - 23:03

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