Thread: Why A/C can't be tested in the winter

As many times as i've explained to customers as to why i can't fully test the a/c when the temp is below 65, they still ask, "but why"? They're looking for an explanation more technical yet comprehensive. Can any of you guys explain it short yet technical so that I can plagiarize your explanation? I guess my explanations are too vague for them.

Thanks

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I simply tell them that it is possible to damage the compressor when it is so cool. If it is below 55 or so, I simply tell them that it is going to blow cold air if it is working or not working because it is so cool/cold outside!

Originally Posted by John Watson

As many times as i've explained to customers as to why i can't fully test the a/c when the temp is below 65, they still ask, "but why"? They're looking for an explanation more technical yet comprehensive. Can any of you guys explain it short yet technical so that I can plagiarize your explanation? I guess my explanations are too vague for them.

Thanks

JW: As per the chief engineer at Carrier, Corp., anything above 32° F. should be safe for all but the most ancient units. In older units with no crankcase heaters the refrigerant can be pulled into the compressor in liquid in stead of vapor form and damage the compressor.

To agree with and extend the remarks by AD and Scott,

Do Not Operate Below 55°F/12.78°C
Your outdoor unit is not designed to operate when outdoor
temperatures are lower than 55_F/12.78_C without modification.
If operation below this temperature is required, consult your
Carrier dealer.

This is a little blurb is from the first Carrier central A/C manual I could find on the internet.
So basically, we don't operate the unit due to liability of not following the manufacturer's instructions.
There are all sorts of exceptions to the rule, but as home inspectors, we don't know what modifications have been made to each unit.

The real reason that the manufacturers puts that in the manual is because under just the right (or wrong) conditions, liquid refrigerant can may it's way back to the compressor. The compressor is designed to compress gas, not liquid and it will break. The connecting rod, piston, valves, crank shaft can break meaning the compressor will have to be replaced. "your paying me $xxx hundred dollars and the repair will cost $xxx thousand" and the reward of operating the unit during cold weather does not justify the risk of breaking it. You don't really prove much by running the unit when it is cold outside anyway since it is pretty easy to get it to blow cold air in the winter!

If the unit has a Scroll compressor it is safe to operate in just about any temperature. It is the old piston type that have the problems.

The real problem is just as Jim said, liability. If compressors failed on a regular basis just because it is or has been below 55, every compressor in the south east would be broken. Women will turn on the A/C as soon as they feel hot, whether it is or not! With being able to find statements, as Jim did on the Internet, our butts are hanging out if we turn it on. If it's already broken we can take the fall, and this is how I explain it to my customers.

Ask them if they remember a little high school physics and postulate... can you compress a liquid?

No need to describe piston and scroll compressor operation and theory... this always suffices..

Oh, and it helps if you have State with some SOP's you can refer to...

"LIMITIATIONS
1. The Inspector is not required to:
A. Activate systems that have been "shut down" or otherwise deactivated.
B. Operate cooling equipment when the ambient temperature has been less
than 65 degrees Fahrenheit within the previous 24 hours.
C. Report on the efficiency of the equipment.


I'm just saying...

I do believe that JW's original post requested information regarding why he "can't" test an A/C below 65° F. He did not ask why he "should" or "shouldn't". Those are completely different questions.

Originally Posted by A.D. Miller

I do believe that JW's original post requested information regarding why he "can't" test an A/C below 65° F. He did not ask why he "should" or "shouldn't". Those are completely different questions.

If it is too cold the oil pools into the compressor. Liquids cannot be compressed, if started the compressor will hydralic and cause damage.

Like many others have said most AC systems can be safety run well below the 65°F mark. I think nowadays it boils down to a business descision if around or below the 65° mark.

Chuck

Originally Posted by A.D. Miller

I do believe that JW's original post requested information regarding why he "can't" test an A/C below 65° F. He did not ask why he "should" or "shouldn't". Those are completely different questions.

"but why"?

He asked for a good explanation for clients.

Originally Posted by Jim Luttrall

He asked for a good explanation for clients.

How many time and ways did you stand in the same room with your client trying to explain the whys and should nots of this before they said
( but why we just don't understand )

And then did you try another explanation and they still said
( but why we just don't understand )

Check and see if something is wrong with the client

Best

Ron

Thanks to all for the replies. You too A.D

Merry Christmas

Originally Posted by Mike Gault

"LIMITATIONS
1. The Inspector is not required to:
A. Activate systems that have been "shut down" or otherwise deactivated.
B. Operate cooling equipment when the ambient temperature has been less
than 65 degrees Fahrenheit within the previous 24 hours.
C. Report on the efficiency of the equipment.

If I ever had to abide by that SOP, I'd never ever operate the cooling system here because every day of the year it gets below 65°F at night. Here we use "current temperature" rather than "within the previous 24 hours."

Originally Posted by Russel Ray

If I ever had to abide by that SOP, I'd never ever operate the cooling system here because every day of the year it gets below 65°F at night. Here we use "current temperature" rather than "within the previous 24 hours."

Often the same here RR.

If it's been down to 60 overnight and gets up to 80 during the day (with power having been on at the condenser the whole time) I'll certainly run it... normally it's been 'running' for hours in AC mode anyway...

Running the system below 65 can give the false impression that the system is operating correctly. A system could have a low charge and not be noticable in cold temps, also a good chance the condenser fan won't run.

Others have already covered the potential to damage the equipment but I will try and add some technical theory about refrigerants for the sake of discussion.

In order for the HVAC system to be operationally evaluated in cooling mode the refrigerant is required to be in very distinct states of matter at certain locations within the refrigerant circuit (gas or liquid). These differing states of matter are easily obtained with refrigerants by carefully controlling temperatures and pressures within the coil components of the refrigerant circuit This is basically the refrigerant molecules repeatedly changing state from a gas to a liquid then back to gas as it travels through the circuit. At each phase transition point within the circuit pressures are mechanically changed dramatically and heat is either absorbed or rejected at a coil in order to facilitate the required phase transition.

Now here is a crucial point to be made…

There is a very distinct temperature and pressure relationship designed into refrigerants and satisfactory performance is only obtained within a specific range of conditions. If we attempt to operate the system outside of that range then system performance will begin to suffer. At some point the attempt to evaluate system performance during out of range conditions becomes pointless because no useful information can be gained.

Simply put, the required refrigerant phase transitions can not be completed successfully because the needed temperatures and pressures can not be obtained under the present conditions. We have therefore reached the physical limits of the refrigerant properties and/or the design limits of the equipment.

Keep in mind that the manufacturers of residential HVAC equipment design those systems to be operated under an anticipated range of operating conditions. They do not "anticipate" that a cooling system in a residential setting will be routinely operated when low outdoor ambient temperatures conditions are present.

This is not to say that the system will absolutely be damaged if it is operated at low outdoor temperatures just do not expect to be able to make any definitive determination concerning how the system will behave when ambient temperature are back within the "anticipated" range.

Now whether or not a "less than 65 degrees Fahrenheit" limit is a good place to set the bar for not operating the system in cooling mode is a subject that is open to debate.

I once had a client that insisted on a discount as I didn't check the A/C.

FK: I have heard that request several times.

I declined.

FK: Me too, each time. Now it is just a habit.

=============================
The outside temperature has been below 55 degrees in the last 24 hours so I didn't test the operation of the air conditioner.

Most manufacturers have this or a similar statement in their operating instructions: Do Not Operate Below 55°F/12.78°C. Your outdoor unit is not designed to operate when outdoor temperatures are lower than 55_F/12.78_C without modification".

When the temperature is below 55 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.)
=============================

That's my explanation.



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