Compressor Failure

Some one told me to not operate a compressor (compressed gas cooling system) if the electricity has been turned on for less than 24hours. They said it was because the oil in the compressor mixes with the refrigerant.

Is this due to the refrig. vapors condensing and mixing? IF so, what does the unit do to return system’s refrig. back in to vapor??

Thanks for the help!

Don’t believe everything that you hear…so, no that is not correct.


Reference below from InterNACHI How to Inspect HVAC Systems course; Air Conditioning; Inspection Restrictions.

"Inspection Restrictions

Compressors should not be operated when it’s below 65° F outside. Compressors should not be operated when the electricity has been turned on for less than 24 hours. Under these conditions, it is possible to damage the compressor. Oil may be mixed with the refrigerant in the base of the compressor. "

Most modern condensers have a low-wattage warmer inside the compressor to prevent the refrigerant from condensing and mixing with the oil at low temperatures. So, if it has been below 65F for several days AND the power was shutoff to the AC unit, it is not advised to power up and run the AC even if is warmer than 65F degrees outside. The little warmer likely has not had enough time to get the compressor up to a safe operating temperature.

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William- I am glad to see the responses to your question. Back when that course was made (2007) I got in a bunch of dog fights over my replays. As did guys like Charley Bottger and others long retired taking up fishing…

Here is one of the threads you can find in a search under “Inspecting HVAC Systems”. Here I am explaining why this procedure of inspecting an A/C Unit in the winter is not a good idea as the inspection SOP’s are there to protect you, a Non-HVAC Expert.

It’s 2023 and there are fewer reciprocal compressors in use out there, so this risk factor is about extinct.

The literature that you are looking for would be from the equipment manufacturer.

The reasoning of the 65° in home inspection SOP/state laws is that at this point, if you don’t understand HVAC principles there is a “potential” to damage the equipment. Home inspection practices never jeopardize a potential to damage the property we are inspecting. Therefore, they are not supposed to the operated.

A heat pump can be operated in any temperature at any time. It is designed differently than an air conditioner and the system operates normally in both heating and cooling below 65°. This occurs almost every 45 minutes during a normal run cycle.

Crankcase heaters have two extra wires that go into the bottom of the compressor or have a metal band wrapped around the compressor with two wires coming from them. Heat pumps have these devices. Air conditioners normally will not (this would be an aftermarket modification). There are compressor designs that have internal heaters as well. There is a small resistance circuit within the compressor that takes 120 V away from the 240 V supply current and feeds this current through the start winding of the motor that keeps the compressor warm during the off cycle. This is the reason why you have single poll magnetic contactors on some compressors. One leg of power remains on in the compressor providing a heat source to prevent the migration of refrigerant. These compressors are frequently found on air conditioners and there is no way of visually determining their existence.

Concerning the issues of compressor damage from liquid slugging, it is very unlikely but possible to damage a reciprocal compressor. Seeing as it is “possible” for damage, it is outside the scope of home inspection SOP.

Under normal low ambient temperature operation, liquid refrigerant enters the compressor at the top and is dumped on top of the mechanical and electric motor within the compressor. These components are become warm during operation and will vaporize the liquid to a vapor in most cases.

Scroll compressors (versus reciprocal compressors) can handle liquid slugging due to their inherent design. There are no pistons or valves to be damaged.

The major concern is what Charlie is describing as “liquid migration”. Even without the lower temperature, refrigerant will migrate into the refrigerant oil in the compressor. When the compressor starts, the refrigerant will violently boil and take the oil with it as it is being pumped out of the compressor, draining the oil out of the compressor. If it does not return a reasonable period time, the compressor will be damaged due to the lack of oil. Another concern is that the liquid refrigerant dumping into a compressor will also wash out the oil if this is excessive. Liquid refrigerant causes the crankcase oil to boil and reduces its lubrication qualities. This condition generally occurs from long-term operation when the evaporator coil or air flow is restricted, it is not generally a startup occurrence when the temperature is below 65.

The issue on liquid compression (whether it can be compressed or not) is a matter of semantics when you study the reality of it all. It is not whether the liquid can be compressed or not, it has to do with the extremely small clearance between the piston and the head of a compressor. These tolerances are extremely small to maintain compressor pump efficiency and any object that is not in gaseous form can damage the working components. When you pump a liquid you use a water pump, when you pump air use an air compressor. They have different design and you wouldn’t pump water with an air compressor?! And to broaden things a little further, there are non-condensable gasses which can do just as much damage to the compressor if the refrigerant gas is contaminated.

The 24 hour issue is based upon first time startup of the equipment after a seasonal change. If the equipment has never experienced a 90° day in the past three months and/or electric power has been shut off to a compressor that has a crankcase heater, the compressor should not be operated for 24 hours after these heaters have been energized or the weather temperature has exceeded the 65° (or any other arbitrary number) for an extended period. This practice is for HVAC contractors, not for home inspectors. So, Russel Ray and others that live in these temperate climates can safely operate their equipment as they normally would.

I expounded on these issues not to condone or reprimand testing practices below 65°, rather for the purpose of expanding your understanding of the operation and for your reading enjoyment. Air conditioners can be operated in any environment if designed to do so.

To reiterate, we are not required to operate equipment below 65°. If it has been below freezing for an extended period of time prior to you doing an inspection and the outdoor air temperature has just reached an all time high for the week of 67°, I would not recommend starting that equipment. We get wrapped up in trying to do more than the other guy, and adding these liabilities only increases your risks of doing business. If you are a risk taker, go for it. The point is, it’s unecessary.


Also, a Heat Pump is further protected from refrigerant slugging by an accumulator at the compressor suction line which traps liquid refrigerant and oil before it gets into the compressor.

When Lennox first came out with the scroll compressor, they thought they didn’t need an accumulator. They thought their scroll could take it. They were wrong. We had to replace a boatload of locked up heat pump compressors, and they would not let us install an accumulator as they were under warranty.

Even the engineers from the company that invented the air conditioner had problems. We don’t need to go down that road do we?


I agree with Larry said.


David, could you simplify a bit? Is it a modern concern?


Thanks for this David!

Honestly, I’m so glad the SOPs are in place because it helps to gauge how in-depth inspectors should reasonably inspect and convey.

Though, I would REALLY like to understand what I’m looking at and how it works so that I can better explain a system or component to myself as well as clients.

Thanks again!


Exactly! Glad you perceive it this way. Not the majorities perception.

Again, Excellent perception of how to proceed in this field. Experience is how you get there. I recommend you look up every question you have before coming here for opinions (not answers). You will gain more insight than just the answer to one question. You will learn the “Why” to your questions. :+1:


Well, you never know what you may find out there.
I thought about it, but there is no definitive way to know if it is safe to start up a shut down system in the cold.

  1. Heat Pumps; no concern. They go into A/C every hour in the winter.
  2. A/C w/gas or electric furnace; is the compressor reciprocal? You can see this under the fan right?
    You can also see wires going into the bottom of the compressor, or a belly band heater from there.
  3. 65F is not a definitive temperature. Just because it got below 65 for a few hours does not count.
    If it’s 65 indoors and not much hotter indoors, the refrigerant will not migrate significantly, so it must be much colder outdoors.
  4. For the idiot states that require Delta-T, never run the A/C in the winter.
    First of all, you will never get the delta because the A/C starves for refrigerant because it does not have head pressure for it to work.
    Bump the compressor and turn it off. Test the heat before this if it wasn’t running before you arrived.
  5. If your not comfortable with any of this, don’t test it. Write in the report that you didn’t, and why. “A/C not tested due to potential damage to the equipment”. This should be in your SOP already.
    If your state demands otherwise, :fu:

There is a potential that the crankcase heater does not work, or not hooked up during a compressor change. How do you know?
A flashlight and screwdriver is not sufficient for this testing, so this testing is a roll of the dice at best…


Thanks David. This is not really directed at you but for others to chime in as well.

I use NACHI SoP and I do not perform any Delta-T. I operate or don’t operate depending on the season (outside air temps) and report what I did or didn’t do and why. I report visual deficiencies (corrosion, condensation issues etc.) and if a system is considered past its expected useful life.

I verify conditioned air is supplied to each living space or supply vent. I will rely on my senses at times, such as the conditioned air feels substandard (not warm or not cold). At that point I will use an IR device to give some confirmation of my concern. I typically report this concern as an opinion, I do not report my IR readings, and then make recommendations accordingly.

I would be interested in other inspectors processes or critical feedback.


I always write my replies for everyone, not just the OP.
I was just chiming in… :wink:


I will comfortably speak for many, we have come to truly appreciate your contribution here. It is nearly impossible to “pick someone’s brain” in the real world on these issues, much less find someone with your experience willing to do so. Valuable asset.


Truthfully Modern AC compressors/systems (Those 30 years or newer) have accumulators and will not be damaged if you operate them below 60- 65 Deg. However if you do not follow that SOP, of not operating below that magical temp. and call out a bad compressor/ system, when colder. You run the risk of it coming back on you that You Broke it because you test ran the system when it was too cold, clearly not following an SOP that is designed to CY (our) Assets.


And many manufacturer owner’s manuals continue to include these operating temperature parameters.


Actually, straight A/C vs Heat Pump do not generally have accumulators. So always look for it to be sure. Another fact about the 30 (+/-) old A/C’s, they did not come with the same compressors that Heat Pumps now use. They were the susceptible compressors where all the 65F stuff came from. When I was in HVAC school back then, we were taught the best practice was that the A/C should be “pumped down” for the winter, which pulled all the refrigerant out of the circuit and stored it in the outdoor condenser. This removed all refrigerant from the compressor where the oil is. The condenser unit was disabled to prevent it from running in this condition. Being 30(+) years old, they have most likely been changed out with a HP Compressor. Just a history lesson.

Another fun fact; there are straight A/C units that do run in the winter!
There are several ways to modify a normal A/C to run in the winter in buildings where the heat load inside the house is excessive (lots of glass facing the south and west, high heat loads from equipment operation, indoor swimming pools etc). Condenser fan controllers, variable speed fans, hot gas bypass etc. These modifications raise and hold the refrigerant head pressure up to what it was designed for during summertime conditions (95F).

So we can not assume anything in this business! If you find an A/C running in the cold, it might just be what it was designed for. Mostly commercial applications, but I have seen them in a few residential applications where the heat was running through most of the house, but one zone of the building had to be cooled to control constant temperatures. Solar loads from the sun are very powerful heat sources that must be managed when they change wavelength from UV to IR passing through glass. Ie. the green house effect inside your car in the winter.

Rooftop HVAC units can use cold outdoor air through an economizer to accomplish this, but split systems can not. In the case of indoor pools, exercise rooms etc, moisture (which is latent heat) must be controlled, which an economizer can not accomplish.

This all adds to the confusion than can/should be avoided.

(e) The home inspector is not required to:

  1. Operate heating systems when weather conditions or other circumstances may
    cause equipment damage

What an excellent discussion! I somehow missed this one.
Thanks David.