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Inspecting Heat Pumps In Hot Weather


dtontarski
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Hi Fritz ,

Could you please explain to me why it is a bad idea to run the heat pump in the heating mode when it is over 75 deg. I once was told not to run the heat over a certain temp. with a heat pump. I then spoke with a heating contractor who went over my head with his explanation on why it would not hurt a thing by running the heat pump in heating mode no matter what.

If anyone can point me towards some documentation I would appreciate it.

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Brandon, I don't know the answer to that, hopefully one of our A/C experts will chime in. I once had one blow a fuse (old unit) in the condenser unit and the service tech told me it was due to high head pressure. I quit operating them in the heat after that. I don't really see the point anyway as long as you can run the A/C you get a good idea of the condition. The reversing valve could be bad but I think that is pretty rare.

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To clarify my initial inquiry - I inspected a 2002 built home with a 2002 Trane condensing unit (Model TWR036D100A0). Inside the home was a Trane electric furnace. (it gets cold up here) The thermostat was a basic design with either a heat or cool position. The outside temperature was in the 80's. I checked distribution with the unit in cooling mode, shut it down for a while, and then set the thermostat to heating mode to check to see if the electric furnace component of the set-up would come on. The buyer told me that he was told that this shouldn't be done. I interpreted the info I was able to get online for this particular Trane equipment as implying that it was quite state of the art and would handle this switch over safely. The good news is that I didn't seem to blow anything up, but I would like to hear from any of the HVAC experts out there. How should these units be tested when it is over 75 - or shouldn't they. I don't want to "interpret" myself into damaging an expensive piece of equipment. It seems others that use this forum could benefit from some advice on this as well. Thanks.

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A heat pump is nothing more than an air conditioner that is running in reverse. The compressor doesn't reverse flow, it is accomplished through the use of a reversing valve. So, in the heating mode the evaporator becomes the condenser and the condenser becomes the evaporator. Let's say it's 80 degrees outside and inside the house it's 80 degrees as well. You wouldn't hesitate to turn the air conditioning on when it 80 degrees inside and outside would you? So, if you put it in the heating mode the evaporator is outside and it 80 degrees, the condenser is inside and it's 80 degrees, same difference.

I run them just to make sure the reversing valve works (I have found a few that were defective). Once it starts blowing heat and I get a delta T I stop the test.

I have a call into a few of my contacts with the manufactures, that I still have from back in the day, for some official looking documentation. Once I get it I'll post it here.

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Terry

If in your above example of 80 degrees there is no difference in heating or cooling mode, why do I always have a different temperature split in different modes? My Delta T is always higher in heating mode. There must be a difference between the two modes.

I would highly recommend that everyone follow the manufacture's installation guidelines in order to stay out of trouble. Here are some examples:

Per Carrier Heat Pump Owner's Manual, "Do Not Operate Below 55 Degrees on Cooling Mode" or "Do Not Operate Above 66 Degrees on Heating Mode". Click here: http://www.croppmetcalfe.com/pdf/om25-2.pdf

Per Carrier Heat Pump Installation and Start-Up Instructions - The minimum outdoor operating ambient in cooling mode without additional accessories is 55°F, and the maximum outdoor operating ambient in cooling mode is 125°F. The maximum outdoor operating ambient in heating mode is 66°F.

http://www.xpedio.carrier.com/idc/group ... SESSION=NO

Per Goettl Heat Pump Installation & Operating Instructions - Do not operate this unit at outdoor temperatures

below 60°F on cooling nor above 75°F on heating.

http://www.goettlmanufacturing.com/manu ... 9-0505.pdf

Jeff Euriech

Peoria Arizona

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Temperature splits are different since you have to add the heat of compression into the mix and the air flow rates are different, not to mention the condenser (outdoor coil) is larger (more surface area) than the indoor coil. In otherwords it is apples and oranges.

I test them during summer, but only a few minutes and then stop. You can over heat the compressor, run a high head pressure, etc. When you have a high outdoor temperature, you are putting excessive heat into the system that it was not designed to handle, temperatures and pressures will run up quickly.

Jim

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I knew my statement would bring everyone out of the woodwork.

The constant is temperature and pressure. As most know; if you leave a can or refrigerant in a garage (out of the sun) you can tell what the ambient temperature it is by measuring the pressure of the can. In my 80 degree example it doesn't matter if the unit is running in a/c or heating. I wouldn't hesitate to run the a/c if it were 80 in the house and 80 outside, all the unit sees is temperatures and pressures (almost).

The true unknown is superheats and sub-cooling (at least to me). I have never put on a strap-on thermostat to measure these in the heating mode. While the temperatures and pressures will remain constant I'm not sure what the sub-cooling from the evaporator and the superheat from the condenser coil would be. This is why I wanted to talk to a my friend that works at Trane before commenting.

In the end it's always proper to defer to manufactures recommendation so, if they say don't do it, then don't.

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After great conversation with the retired owner of a Carrier dealership I stand corrected.

As Jim had mentioned, the condenser surface area is much larger than the evaporator so, in the heating mode it is capable of extracting more heat than the evaporator is able to reject. High head pressure is the result. I had thought that the evaporator, being multi pass, would be able to keep up. Wrong.

He said that while starting it to make sure the reversing valve worked wouldn’t do any harm running it for too long could result in damage.

Moral of the story: If you stray from manufactures recommendations you are on your own.

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I contacted a few name brand manufacturers and they recommended using the Carrier test parameters..

cool mode no lower than 55 degs and no heat mode above 65 degs. I may simply stick to the 62 degree rule of thumb for both for simplicity and make sure I clearly state in the report if I tested the heat or cool mode and why/why not.

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