How can it damage an air-conditioner to run it when it is cold outside?

[sevensages]

TL;DR Summary

How can it damage an air-conditioner to run it when it is cold outside?

When I was inspecting the house that I later bought about ten years ago, it was the winter time. It was cold outside. I wanted to run the air-conditioner to test that it would cool the house off effectively, but my realtor asked me not to. She said that it would damage the air-conditioner to run it when it is cold outside. I asked her "how would it damage the air-conditioner to run it when it is cold outside?" She just told me "the air-conditioner is not made to run when it is cold outside." Well, duh. I knew that. But that does not answer the question.

Perhaps a residential air-conditioner would not cool the house as effectively in cold weather as in hot weather. But I cannot figure out why it would damage a residential air-conditioner to run it in cold weather. Maybe my realtor was misinformed.

How can it damage a residential air-conditioner to run it when it is cold outside?

I'm looking for a specific answer that tells me what parts of the air-conditioner would be damaged by running it in cold weather, and I am looking for information that tells me how exactly those parts would be damaged by running the air-conditioner in cold weather.

 

[sophiecentaur]

TL;DR Summary: How can it damage an air-conditioner to run it when it is cold outside?

But I cannot figure out why it would damage a residential air-conditioner to run it in cold weather. Maybe my realtor was misinformed.

Condensing water vapour can freeze solid under frost conditions and block the air circulation. I bought a dehumidifier and it stopped working in an unheated house. No big deal because the ice melted when I pointed a fan heater at it and I was back in business. I didn't realise that I could have bought a better device with a cut out at low temperatures.
Every day's a school day.

 

[sevensages]

Condensing water vapour can freeze solid under frost conditions and block the air circulation. I bought a dehumidifier and it stopped working in an unheated house. No big deal because the ice melted when I pointed a fan heater at it and I was back in business. I didn't realise that I could have bought a better device with a cut out at low temperatures.
Every day's a school day.

Do you mean that condensing water vapor on the evaporator can freeze solid under frost conditions and block the air circulation?

If so, how would this damage the air-conditioner?

The question of the OP has not been answered yet.

 

[sophiecentaur]

If so, how would this damage the air-conditioner?

It would stop working.That would make a non-engineer suspect it was broken. Realtors don't have Physics degrees.

The question of the OP has not been answered yet.

Which one hasn't been answered? The answer is freezing condensation blocks air flow. This can happen when the air in the house is already pretty chilly.

 

[sevensages]

It would stop working.That would make a non-engineer suspect it was broken. Realtors don't have Physics degrees.

Which one hasn't been answered? The answer is freezing condensation blocks air flow. This can happen when the air in the house is already pretty chilly.

I think my realtor believed that if an air-conditioner ran in cool mode, it would actually physically damage the air-conditioner, not just not run effectively. What part of the air-conditioner would be damaged?

 

[sophiecentaur]

What part of the air-conditioner would be damaged?

My point was that it would block and temporarily stop circulating air - the job it's supposed to do - and waste several hours whilst the heat exchanging matrix clears.. There's no point in beating up a realtor for knowing no Physics. You are now better informed than she.

 

[sevensages]

My point was that it would block and temporarily stop circulating air - the job it's supposed to do - and waste several hours whilst the heat exchanging matrix clears.. There's no point in beating up a realtor for knowing no Physics. You are now better informed than she.

Are you saying that you don't think that running an air-conditioner when it is cold outside will actually physically damage the air-conditioner at all?

 

[DaveC426913]

Are you saying that you don't think that running an air-conditioner when it is cold outside will actually physically damage the air-conditioner at all?

I think, the thing is, it could.

• Freezing parts that are not supposed to freeze can burst pipes. (Does AC coolant expand when it freezes, like water does? I'm not sure.)
• AC units also dehumidify - a byproduct of cooling the air. That causes water to condense and collect and usually run off. But if the water freezes, it can gum up the works and overflow its mechanics, generally wreaking havoc in unpredictable places.
• An AC unit that can't circulate and cool itself may overheat.

I think @russ_watters is our resident 'spert.

 

[AlexB23]

I think, the thing is, it could.

Freezing parts that are not supposed to freeze can burst pipes. Does AC coolant expand when it freezes, like water does?

Regardless, an AC unit that can't circulate and cool itself may overheat.

AC refrigerants do not expand when cooled off. It expands when it warms up though.

Source: https://c03.apogee.net/mvc/home/hes/land/el?utilityname=grayson&spc=hel&id=1804

 

[PeterDonis]

Does AC coolant expand when it freezes, like water does? Im not sure.

The refrigerant inside the AC unit doesn't freeze at the freezing point of water. It freezes at a much lower temperature. The phase transition in the refrigerant inside an AC unit is between gas and liquid.

The danger of a freeze on the evaporator, apart from the blockage of airflow, is that the ice on the outside of the evaporator might damage the coils and cause a leak.

 

[PeterDonis]

I wanted to run the air-conditioner to test that it would cool the house off effectively

Quite apart from any possible risk of physical damage, this is a pointless test to run if it's not hot outside (where "hot" means "outside temperature significantly above desired inside temperature).

In mild weather (outside temperature roughly equal to desired inside temperature), even with the AC unit "on", the compressor will be cycled off almost all the time, because as soon as it turns on, the low side pressure will quickly drop below the shutoff setting and the compressor will turn off again. And it will take a long time for the low side pressure to get high enough to be above the turn-on setting and turn the compressor on. All you will really be doing is exercising the compressor on/off mechanism occasionally. You won't be doing any meaningful test of how well the system can cool the house.

In cold weather (outside temperature roughly at, or below, the desired air outlet temperature when the AC is on to cool the house), even with the AC unit "on", the compressor will never turn on at all, because the low side pressure will never get high enough to be above the turn-on setting. So all you will be doing is running the fan to circulate air through the house.

what parts of the air-conditioner would be damaged by running it in cold weather

Aside from the possible effect of an evaporator freeze that I mentioned in post #10 just now, one other risk I can think of off the top of my head is that running the compressor under cold conditions might damage it due to insufficient lubrication.

 

[sevensages]

The refrigerant inside the AC unit doesn't freeze at the freezing point of water. It freezes at a much lower temperature. The phase transition in the refrigerant inside an AC unit is between gas and liquid.

The danger of a freeze on the evaporator, apart from the blockage of airflow, is that the ice on the outside of the evaporator might damage the coils and cause a leak.

If the water on the outside of the evaporator freezes, blocking airflow over the evaporator, do you think that that could result in liquid floodback into the compressor?

 

[sevensages]

Quite apart from any possible risk of physical damage, this is a pointless test to run if it's not hot outside (where "hot" means "outside temperature significantly above desired inside temperature).

In mild weather (outside temperature roughly equal to desired inside temperature), even with the AC unit "on", the compressor will be cycled off almost all the time, because as soon as it turns on, the low side pressure will quickly drop below the shutoff setting and the compressor will turn off again. And it will take a long time for the low side pressure to get high enough to be above the turn-on setting and turn the compressor on. All you will really be doing is exercising the compressor on/off mechanism occasionally. You won't be doing any meaningful test of how well the system can cool the house.

In cold weather (outside temperature roughly at, or below, the desired air outlet temperature when the AC is on to cool the house), even with the AC unit "on", the compressor will never turn on at all, because the low side pressure will never get high enough to be above the turn-on setting. So all you will be doing is running the fan to circulate air through the house.

I'm not an expert on air-conditioners.

To me, it sounds like that you are saying that most or all residential air-conditioners have some sort of "low pressure sensor" that will prevent the compressor from turning on if the refrigerant pressure is too low. I have never heard of this.

Do most residential air-conditioners have low pressure sensors that will prevent the compressor from turning on if the refrigerant pressure is too low? If so, where are these sensors?

I remember that you worked in automotive air-conditioning, not residential air-conditioning. I wonder if automobiles have these low pressure sensors, and I wonder if you are possibly (erroneously) thinking that residential air-conditioners have these low pressure sensors because automobiles do.

Aside from the possible effect of an evaporator freeze that I mentioned in post #10 just now, one other risk I can think of off the top of my head is that running the compressor under cold conditions might damage it due to insufficient lubrication.

Yeah, I thought that might be a possibility.

 

[PeterDonis]

If the water on the outside of the evaporator freezes, blocking airflow over the evaporator, do you think that that could result in liquid floodback into the compressor?

Yes, that could happen (it's usually called "liquid slugging"), and yes, it could damage the compressor. Most systems will have interlocks that should shut the system off if this happens, but no such interlock can absolutely guarantee to prevent damage.

 

[PeterDonis]

I remember that you worked in automotive air-conditioning, not residential air-conditioning. I wonder if automobiles have these low pressure sensors, and I wonder if you are possibly (erroneously) thinking that residential air-conditioners have these low pressure sensors because automobiles do.

Automotive systems certainly have them, and that's where most of my experience of them is, yes.

However, residential compressors have them too. Usually it's in the outside unit (at least for split units, which is the only kind I've ever dealt with as a homeowner), either in the low side line close to the compressor, or actually inside the compressor housing.

 

[sevensages]

Automotive systems certainly have them, and that's where most of my experience of them is, yes.

However, residential compressors have them too. Usually it's in the outside unit (at least for split units, which is the only kind I've ever dealt with as a homeowner), either in the low side line close to the compressor, or actually inside the compressor housing.

If the low pressure sensor is in the suction line, obviously the low pressure sensor would be sensing the refrigerant pressure in the suction line. If the low pressure sensor is inside the compressor housing, would the low pressure sensor be sensing the refrigerant pressure on the high pressure side of the air-conditioner or the low pressure side of the air-conditioner?

 

[berkeman]

While reading this thread I was wondering how home inspectors test air conditioners during wintertime (home inspections are often required for home sales). I did a Google search and found out that indeed there is a minimum outside air temperature that home inspectors will not do a functional test below:

https://www.brickkicker.com/why-shouldnt-we-test-a-cs-in-the-winter/

For an AC test in winter, you may be thinking about what can be the lowest outside temp to run AC? Generally, most professionals advise against operating your central air conditioner unless the daytime temperatures are consistently above 65°F

 

[PeterDonis]

If the low pressure sensor is inside the compressor housing, would the low pressure sensor be sensing the refrigerant pressure on the high pressure side of the air-conditioner or the low pressure side of the air-conditioner?

The low side, obviously. The compressor housing doesn't just house the high side of the compressor.

 

[russ_watters]

The above answers are correct, to my knowledge, and here's a link:
https://kaiserac.com/running-air-conditioner-when-it-is-cold-outside/

It mentions evaporator coil freezing and lubricant thickening, if the unit even turns on due to pressure safeties (which most if not all have). I think it would also theoretically be possible to have the refrigerant cool enough for liquid to get to the compressor, but I've never seen that happen.

And as said, coil freezing shouldn't cause permanent damage.

Also, I'd tend to question the usage of a dehumidifier in winter, since cold air is also much drier than warm air. If your dew point is freezing and your relative humidity is still high that means the house is near freezing and at risk of bursting pipes. I'd much rather add a little bit of heat to keep the air temperature safely above freezing (45-50F perhaps).

Quick story: my girlfriend bought a condo with a water source heat pump, in December. We turned it on (heating) at the walkthrough and it was fine. Since it was water source, we could test both heating and cooling modes. But come summer it stopped working. It would come on for a few minutes and then stop. Problem? Clogged condensate drain. In winter, there was no condensate, so even the air conditioning mode test wasn't good enough to find the problem.

 

[berkeman]

While reading this thread I was wondering how home inspectors test air conditioners during wintertime (home inspections are often required for home sales). I did a Google search and found out that indeed there is a minimum outside air temperature that home inspectors will not do a functional test below:

https://www.brickkicker.com/why-shouldnt-we-test-a-cs-in-the-winter/

I talked with my wife about this just now. She has worked in the home warranty industry for the last 10 years, and they rely a lot on home inspections done for pending home sales (obviously if some system is not working at the home sale, the home warranty going forward will not cover that pre-existing condition).

She said that they are aware of this limitation for testing home air conditioner systems, and they extend their warranties from the winter to the summer for air conditioner systems after home sales.

 

[Rive]

...air-conditioner to test that it would cool the house...

=> If the house is cooled then the outdoor unit is heating. No freezing is expected there (that problem belongs to the kind of ACs which can be used for heating).

Still, the common AC outdoor unit is built to be operated when it's hot out there, and so the oil in the compressor is selected according to that. Can thicken up pretty well in cold => not recommended to start it up.

Heating it up is also not recommended

 

[PeterDonis]

=> If the house is cooled then the outdoor unit is heating. No freezing is expected there (that problem belongs to the kind of ACs which can be used for heating).

Yes, the freeze issue is at the evaporator, not the condenser. If the house is being cooled, the outside heat exchanger is the condenser. As you say, the outside unit is the evaporator for a heat pump unit that is heating the house. For that usage, ice on the outside unit should be extremely rare since there will not be enough moisture in the cold outside air to make any.

 

[sophiecentaur]

Yes, the freeze issue is at the evaporator, not the condenser.

Ahh- the evaporator is inside and it's the refrigerant that 'evaporates'. Water from the room air will then condense*. Terminolgy rules.

I have the feeling that I am having a proxy argument with a Realtor and the OP can't imagine a realtor can get technical things wrong.

That link is about home AC systems. Units in cars could be expected to operate well below freezing because drivers don't check that sort of thing when they start off. Maybe there is a safety circuit to inhibit the compressor from running till the temperature is safe. Perhaps that goes without saying

*This can also happen in condensing CH boilers which overcome the problem by using a holding tank which periodically siphons out with a lot of warm water to keep the pipe clear.

 

[Rive]

ice on the outside unit should be extremely rare since there will not be enough moisture in the cold outside air to make any.

I'm a bit skeptical about that. Morning frost at winter is regular (well, at many countries) even without an AC siphoning heat with significant temperature drop at the outdoor unit.
Also, mind the difference of absolute and relative moisture level. Low temperature air can easily be saturated even with really low absolute water content (=> you don't need much to get frost).

A 'defrost mode' is kind of mandatory feature these days.

 

[russ_watters]

As you say, the outside unit is the evaporator for a heat pump unit that is heating the house. For that usage, ice on the outside unit should be extremely rare since there will not be enough moisture in the cold outside air to make any.

All it takes for frost is an evaporator temp below the dew point and freezing temp. If you see dew or frost on the ground, there will be frost on the heat pump. All heat pumps have a defrost mode and they use it relatively frequently (enough that it noticeably impacts the efficiency).

 

[jack action]

Units in cars could be expected to operate well below freezing because drivers don't check that sort of thing when they start off. Maybe there is a safety circuit to inhibit the compressor from running till the temperature is safe. Perhaps that goes without saying

I know more about car A/C than home A/C and I've been biting my lips since the beginning of this thread.

Not only car A/C can work in the winter, but it is recommended to use them (It probably does it automatically anyway):

3. Maintaining the AC System

Regularly running the AC during winter can help maintain the entire air conditioning system, including the auto AC repair needs. The AC system contains lubricants, including oils, that keep the seals, valves, and hoses from drying out. If the AC isn’t used for long periods, those components can deteriorate, potentially causing leaks or requiring expensive AC repairs. By using the AC periodically, you can prevent the need for costly repairs and ensure the system continues to function efficiently year-round.

Running the AC for at least 10 minutes a few times a week ensures that the system stays in good working order, extending its lifespan and helping you avoid unnecessary maintenance costs.

How and why air conditioning can fail at winter

It’s not uncommon for car air conditioners to break down over the winter months. Clearly, they’re not breaking down due to overuse, since the A/C is rarely even on during the cold winter months.

Instead, the problem is usually caused by lack of use. Without refrigerant and oil flowing through the system, rubber seals or hoses can become dry and brittle. When that happens, the refrigerant can leak out, leaving you with no cooling power when the summer rolls around again.

How to prevent air conditioning failure

Luckily, the main way to prevent this problem is simple: run your air conditioner during the winter.

Sure, that might seem counter-intuitive. However, we’re not talking about having it on all winter, just a couple of minutes every few weeks will circulate the system and help keep it from drying out.

The A/C does more than just cool the air. It also serves to dehumidify the air, which can be handy when your windows fog up with moisture on a cold winter day. In fact, many cars automatically run the A/C system when you turn on the defroster for that very reason—which means you may already be running the A/C periodically without realizing it.

If your car doesn’t do this automatically, you can still manually turn the A/C on when defrosting to accomplish the same purpose, without freezing yourself in the process.

But the A/C unit is next to a hot engine, so that may make a difference in the recommended usage.

 

[sophiecentaur]

Not only car A/C can work in the winter, but it is recommended to use them

perhaps not all the time; AC will decrease your mpg. And, whilst on the subject, AC in an electric vehicle needs extra careful use too (same as the electric heating).

 

[Rive]

But the A/C unit is next to a hot engine

If you are familiar with them - how are the lubrication works for such an A/C unit in a car?
Is it hooked to the main oil pump?

Common outdoor A/C units has just that common 'sitting in a bucket with oil' kind of compressor.

 

[sophiecentaur]

Is it hooked to the main oil pump?

No. I has a closed system like all other refrigeration devices.

'sitting in a bucket with oil' kind of compressor.

This is why they tell you to leave the unit to settle when it's been moved around or you could get oil blocking the heat matrix.

 

[russ_watters]

While reading this thread I was wondering how home inspectors test air conditioners during wintertime (home inspections are often required for home sales). I did a Google search and found out that indeed there is a minimum outside air temperature that home inspectors will not do a functional test below:

https://www.brickkicker.com/why-shouldnt-we-test-a-cs-in-the-winter/

A bit more on low ambient operation: older residential units have fixed operation/limited controls; if the condenser is on, the condenser fan is on. Commercial units and probably newer residential units* have more active control of the cycle, turning off or modulating the condenser fan to maintain a set pressure.

You can also get a low ambient kit which includes a movable flap to further reduce airflow, and a compressor heater for the oil issue mentioned earlier.

*Condenser fan modulation also can save energy.

 

[PeterDonis]

Maybe there is a safety circuit to inhibit the compressor from running till the temperature is safe.

In cars, I don't think so; the low pressure cutoff is enough. When it's cold enough outside, the pressure of the refrigerant never gets higher than the low pressure cutoff, so the compressor won't run even if the A/C switch inside the car is turned on.

The problem with having an outside temperature sensor controlling this in a car is that such sensors can be skewed by the heat sources in the car itself, the main one being the engine, and by not having a good place available to sense undisturbed outside air.

 

[PeterDonis]

All heat pumps have a defrost mode

Doesn't that just use electric heat to warm the coils?

 

[PeterDonis]

Not only car A/C can work in the winter, but it is recommended to use them (It probably does it automatically anyway)

The compressor can't run if the low pressure cutoff stops it from running. In cold winter conditions, the refrigerant pressure inside the system will never get above the low pressure cutoff. Under those conditions, you can turn the "A/C" switch on inside the car, and have the light come on, and it won't make any difference; the compressor still won't run. If you know the sound of the compressor clutch engaging, try starting the car in cold weather, turning the A/C switch off to make sure it's disengaged, and then turning the A/C switch on again and listening for the sound of the clutch engaging. Under cold enough conditions, you won't hear it.

 

[Averagesupernova]

Concerning automotive A/C working in the winter, when the defrost is turned on it's not uncommon for the A/C compressor to run. Part of defrosting is to remove the moisture. Drive a vehicle long enough to get used to how well it defrosts and then when the A/C no longer works you will notice it does not defrost as well.

 

[russ_watters]

Doesn't that just use electric heat to warm the coils?

I think the typical method is to switch to AC mode briefly, but with the outdoor fan off:

https://www.hvac.com/expert-advice/heat-pump-defrost/