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Old 01-25-12, 11:00 AM   #1
abogart
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Lightbulb How cold is TOO cold for an ASHP?

*TEASER WARNING*

I have a couple old window AC units sitting in the garage, and this forum has really got me thinking about putting them to use.

When I get a chance (among the many projects I intend to tackle) there is a spot in the garage wall that I can easily knock out and put one of the units in backwards. I'm just curious how much heat a little 6,000 BTU window AC unit can actually put out when the outside temp here in Michigan is in the 20's, or if it will even work at all. I figure the garage is a good enough place to start, as I do go out there and work from time to time. At least I can experiment without cutting holes in the house or ductwork.

I'll give it a shot as soon as I can and keep you all posted.

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Old 01-25-12, 04:59 PM   #2
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My Sanyos start working badly when it gets down near 5 or 6 degrees F..
But above that, they keep the house nice and warm..

A few years ago, I started wondering about ASHPs and messed around with backwards ACs.

I actually found the posts on the web!!

Window AC used as a Heat Pump.?. - DoItYourself.com Community Forums
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Old 01-26-12, 09:22 AM   #3
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It depends a lot on the system. My old central heat pump could handle freezing but not much lower. The new central heat pump is good at least to 25F, the coldest we've had this year.
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Old 01-27-12, 01:21 PM   #4
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My little ASHP (Sanyo) has worked well down to 12F. I can see that the power used per hour has increased but it just keeps the warm air wafting about in my home.

But I think that the lower limit is influenced by the type of refrigerant that is used.
  • Boiling Point of R-22 = -40.7 C (-41 F)
  • Boiling Point of R-290 = -42 C (-44 F)
  • Boiling Point of R-12 = -29.8 C (-22 F)
  • Boiling Point of R-600a = -11 C (12 F)
  • Boiling Point of R-410a = -48.5 C (-55 F)

These boiling points are for atmospheric pressure. When the refrigerants are in a system, they are under pressure, and the boiling point will be higher. This is where a pressure/temperature chart (AKA: PT Chart) comes in handy.

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Old 01-27-12, 04:22 PM   #5
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I'm not sure how well the actual unit handles cold but at -4C my controller switches to the electric backup. I doubt the controller has any knowledge at all of the heat pumps ability to work at this temp and is just programmed to switch over.
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Old 01-27-12, 04:51 PM   #6
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Quote:
Originally Posted by strider3700 View Post
I'm not sure how well the actual unit handles cold but at -4C my controller switches to the electric backup. I doubt the controller has any knowledge at all of the heat pumps ability to work at this temp and is just programmed to switch over.
That's about 24.8 deg F. Not super cold. At least not for a modern Mini-Split (R410A).
The Sanyos do a good job down in that range. They really start using more power
when it gets down in the teens, but can still keep the house nice and warm,
until we get single digits.. At about 8 deg F (-13.33C) power use maxes out,
but is still not too bad.
When it hits 5 deg F (-15C), that's when I just turn them off and go to backup.

We've been lucky this season, since it's only dropped to 4 F, once or twice..
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Old 02-11-12, 07:46 AM   #7
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I feel the residential heating by ASHP is not specially challenging in N-America,if the widgets ,supplied locally, could only somehow manage -15degC with reasonable COP (>>1,5).

In N-Europe practically all AC (R410) at heating mode cope easily with that
minus15C ... some even "well" with -25C.

Below one collection of test reports (note, defrosting included in the results) in Finnish:

Ilmalmppumppuvertailu - VTT:n testiraportit | Scanoffice

and further one sample of ME`s MUZ/MSZ FD35VABH -model:


http://www.scanoffice.fi/uploads/vtt...S-08820-10.pdf


Look at pdf.page 9/11 (POWER W vs. TEMPERATURE C(elsius)

The corresponding COP vs.temp. (defrosting incl.!) shown at .pdf 7/11

It is for sure the pumps do not enjoy the conditions -20C ... -30C.
The deep-cold POE-oils should be returned back to the compressor at random basis (forced defrosting mode,trigged like egg-clock by software) ,eventhough there is no frost at the evaporator due to extra low humidity in the outside air (RH <30%)!

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