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Below 65 degrees??


homnspector
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Like most home inspectors I have a clause in my contract that I will not operate the A/C unit at ambient temperatures below 65 degrees due to "possible compressor damage". Clients naturally want to know the condition of the system.

Will it in fact hurt the system? I have heard that the freon or refrigerant may be in a liquid state at low temps and valves can be damaged and have also heard lack of lubrication in cold weather. Are either of these correct?

Does the same hold true for a heat pump running in A/C mode at low temps? Is 65 degrees the correct cut off?

I would like to be able to give clients a reasonable explanation of why I won't test the unit. Thanks A/C experts![:-bigeyes

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Originally posted by homnspector

Like most home inspectors I have a clause in my contract that I will not operate the A/C unit at ambient temperatures below 65 degrees due to "possible compressor damage". Clients naturally want to know the condition of the system.

Will it in fact hurt the system?

I have heard that the freon or refrigerant may be in a liquid state at low temps and valves can be damaged and have also heard lack of lubrication in cold weather. Are either of these correct?

There are different types of compressors. A scroll compressor can slug liquid refrigerant all day and suffer no damage. If you try that with a piston compressor you can damage it very quickly. However the refrigerant in the system is still gaseous at 65 degrees, it condenses at closer to 40 degrees.

So, with piston compressors, 65 degrees is a reasonable cutoff point if you want about a 15 degree safety margin. (Unless the compressor is equipped with a crankcase heater, as many are.)

Does the same hold true for a heat pump running in A/C mode at low temps?

No. Heat pumps (and many air conditioners) have crankcase heaters that keep the compressor warm so concerns about liquid refrigerant shouldn't be an issue with them unless power to the outdoor unit has been off.

Is 65 degrees the correct cut off?

If you aren't certain about the type of compressor or the presence of a working crankcase heater then yes, it's a good, conservative, safe cutoff temperature. Also, at least some of the manufacturers caution against running the unit at temperatures below 65 degrees.

I would like to be able to give clients a reasonable explanation of why I won't test the unit. Thanks A/C experts![:-bigeyes

Just keep telling them what you've been telling them; running the AC when it's cold out might hurt the machine.

Also, in your first paragraph, you said that the clients want to know the condition of the system. Running the system when it's cold out will tell you only a little of that. To really judge the condition of the system in cold weather, someone's going to have to block off portions of that outdoor coil with pieces of plywood or blankets or the Sunday paper while reading the system pressures. I'm guessing that, as a home inspector, you don't want to go there.

- Jim Katen, Oregon

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Running a/c when it's cooler might give incorrect Delta T's (temperature drop) across the evaporator as well. This may lead to an incorrect conclusion that the temperature difference between return air and supply air is insufficient.

A/C needs a higher head pressure to operate correctly (providing a full column of liquid in the liquid line to operate the TEV or cap tube correctly). This is assuming that the a/c isn't equipped with low ambient controls which cycle or modulate the condenser fan speed to keep the head pressure up.

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Anytime you operate equipment outside of the manufactures guidelines you open yourself up to possible liability if something occurs today or at a later date. It is better to stay within all guidelines. As an example, Goettl Air Conditioning Installation Manual says, "Do not operate this unit at outdoor temperatures below 60°F." Goettl Heat Pump Installation Manual says, "Do not operate this unit at outdoor temperatures below 60°F on cooling nor above 75°F on heating." If need be you can always print out the installation manuals and show it to your customers. Some examples:

http://www.goettl.com/manuals/GoettlMan ... 9-0505.pdf

http://www.thermopride.com/Manuals/13_SEERAC_I&O.pdf

Most manufactures will require a low ambient kit to be installed if you are going to use the air conditioning unit during cooler months. (under 65 degrees) An example of what may be required:

1. There should be a crankcase heater installed on compressor.

2. A low ambient control (45° contact) added to the 24 volt control circuit.

3. Suction line accumulator.

4. Confirm compressor superheat.

These items will assist in preventing liquid floodback, flooded starts, and refrigerant migration during off cycle. These situations are detrimental to the life of a compressor.

Most home inspectors will not be able to tell if a low ambient kit has been installed on a unit. It is better to assume it has not been installed and only use the unit at above 60 or 65 degrees.

Now in real life, we all know that you can run a unit under 65 degrees. It is done all the time. Last Thanksgiving, the news had said it was 55 degrees outside in Phoenix. As I went to take the dog to the back yard, I was amazed at how many A/C units were running in my neighborhood. It was a result of all the cooking that was being done that day. Yet I did not see a single air conditioning truck in my area the next day. Amazing.

How far under the recommend temperatures can you safely operate a unit? I go based on three factors:

A. What is the ambient temperature? The lower the outside temperature is, the greater the chance of having problems.

B. What is the indoor temperature? The lower the inside temperature is, the greater the chance of having problems.

C. How long are you going to run the unit? The longer you run the unit the greater the chance of having problems.

Example:

If the outside temperature is 55 degrees and the inside temperature is 75 degrees, I will run the unit. There is more than enough heat in the house to prevent liquid refrigerant from going back to the compressor and causing damage. There again, I only run the unit for about 5-10 minutes.

If the outside temperature is 55 degrees and the inside temperature is 60 degrees, I will not run the unit. There may not be enough heat in the house to prevent liquid refrigerant from going back to the compressor.

Other factors for your consideration:

1. All manufactures build in safety factors for their equipment. They know that if you tell a homeowner not to go below a certain temperature, they will operate their unit under that temperature anyway. So they move their temperatures up accordingly.

2. On the Thermo Pride Installation PDF you will see were they tell the installer to not operate the air conditioner on a day of 45° F or cooler. Interesting.

3. Liquid refrigerant will not compress in the compressor. Only gas will. If a large enough slug of liquid made it to the compressor you may damage it. However, for the most part, liquid refrigerant will mix in with the gas refrigerant and pass on through the compressor.

4. If you ask 20 home inspectors or A/C repairmen the minimum temperature for operating a unit, you will get 20 different answers. Interesting.

5. I agree with Terry. Running A/C when it's cooler will give incorrect Delta T's (temperature drop) across the evaporator. I do not test the splits. I tell my customers that the compressor started, the outdoor fan is running, the indoor blower is working, and I have cold air. I only test heat pumps in one mode only, not both modes.

6. I tend to run the air conditioning units under the recommended temperatures as a result of being trained in this field a long time ago. How does that saying go? Do as I say and not as I do. Follow all guidelines.

Jeff Euriech

Peoria, Arizona

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Jeff et al, very helpful information. I pretty much suspected 65 was not a hard and fast cutoff as I have gotten varying temps from different A/C persons. I learned the hard way on the heat pumps, turned one on when the ext temp was about 85 and blew an internal fuse.

Often is Az (as you know), the ambient temp may be low (say 40 degrees) but the unit on the roof in the direct sun is maybe 80-90 degrees. I would assume it is OK to operate in these conditions.

I wonder sometimes when I recommend "full evaluation by a licensed A/C contractor prior to close" as the unit could not be operated due to low temps that the A/c guy is able to tell them that it is working OK. I was thinking maybe there was some evaluation they are able to do at low temps.

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If it is too cold outside, I normally state in my report that the air conditioning was not tested and the reason why. I do not recommend a full evaluation by a licensed A/C contractor prior to closing. Maybe I should be doing that however if it's too cold, they will not start it up either.

Maybe some of our cold weather friends will tell us what they put in their reports when it's too cold to test a unit.

Jeff Euriech

Peoria, Arizona

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======

The outside temperature is below 60 degrees so I didn't test the operation of the air conditioner. When the temperature is below 60 degrees the gases in the air conditioning system can turn to liquid. Turning on the Air Conditioner, when the gas is a liquid, can cause the liquid to "slug the compressor". (Think of throwing a bird into a jet engine.)

I recommend that you get the service records from the owner or the company servicing the air conditioner. I recommend that you have a qualified air conditioning contractor test the unit IF the weather gets above 60 degrees for a couple of days sometime before closing. I also recommend that you purchase a home warranty that covers the air conditioner. Some don't.

You can search the internet for information and providers of home warranties.

In any case, don't wait until you need it before making sure it will work.

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