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What happens if you run a central air conditioner when it's below freezing outside? Is this expected to work?

I know, it sounds crazy, but that's because you don't live in an overheated New York apartment building built in 1904 where the whole building is a single heat zone.

Our building has a big boiler in the basement that goes on for a certain number of minutes every hour, depending on the outside temperature, sending steam to every radiator in the building. In order to make sure that the coldest room in the whole building is adequately heated, that means lots of rooms are way overheated, and the temperature can reach the upper 80s (F of course).

The usual solution is to open and close windows manually. Apparently this is "by design", many of our windows actually have little panels in them you can open to get cool air. Then the temperature goes down to the lower 60s near the window, while it's still 80s elsewhere in the apartment, and it's drafty, too.

We're looking into installing thermostats on the radiators, but that is not the question.

The question is, what happens if we run the central air conditioner while it's freezing outside but over 80 inside?

The central air conditioner consists of an Aerosys THDC condenser (which is mounted through an "exterior wall" to an air shaft) connected through refrigerant lines to an indoors air-handling unit which has typical ductwork going to each room.

I don't think the people who made these condensers ever expected that they would be operating when it's well below freezing outside, so I am wondering if it should just work fine or if this is really a morbidly bad idea for some reason.

Empirically, it sort of worked for a couple of months, but when it got really cold, the condenser conked out, and now I have a repair call in to the AC company, but I don't know if that's because it was so cold outside or for some other reason.

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  • Do you have a ductless air conditioning system? Also what is the make and model of he system.
    – Mnc123
    Commented Feb 22, 2015 at 21:36
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    Have you tried just running the fan on the central air? That may help even out the temperature between your cooler areas near windows and the rest of the apartment. The moving air will also make it feel slightly cooler.
    – TomG
    Commented Feb 22, 2015 at 22:07
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    Do the radiators lack the usual mechanical valves you can tighten to reduce or eliminate water flow to them? That's what I always used to use back in university when we had a central boiler for the whole campus and horribly over-heated buildings.
    – R.. GitHub STOP HELPING ICE
    Commented Feb 23, 2015 at 10:15
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    OmG! I almost can't believe my eyes. Just to get this straight: You are not kidding and you are in what I believed was a developed country? No offence intended, but I can't seem to wrap my mind around this. Environmental aspects aside: Is energy free in New York? Or am I missing something?
    – I'm with Monica
    Commented Feb 23, 2015 at 16:02
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    Looks like your question was already answered but you do you have valves at the base of each radiator to throttle the steam flow? Depending on their shape and size a styled cover like a old school cabinet over the radiator would cut down on heat transfer. Or become a nudist LOL. Seems a shame to run the air to remove boiler heat.
    – user34099
    Commented Feb 25, 2015 at 13:40

2 Answers 2

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After reviewing the spec sheet for the unit i have found that the LX series has a variable speed blower that will allow the unit to operate under low ambient conditions. Is specifies the system can operate down to 20 degrees. Anything below this and the system will not operate. If you do not have the LX unit and do have the standard, the. The unit is not permitted for low ambient use.

Most central air conditioning systems do not work under low ambient operation and doing so can damage the system. Residential condensers will very rarely come with a crank case heater for the compressor as well as a low ambient control that can modulate the condenser fan speed to maintain proper operation of the system. This is however common practice on heat pumps and ductless units. Most will only allow operation down to 32 degrees. A low ambient kit is normally required to lower the minimum operating temperature of the system.

The issue on running an air conditioner in cold weather is that refrigerant will migrate underneath the oil in the compressor crank case. When the compressor is then energized the refrigerant will rapidly change state as it is agitated and absorbing heat. This will cause the oil to be forced out of the crank case and will cause the compressor to run dry on oil for a period of time until the crank case has refilled with oil in the system. This can cause the compressor to fail. A crank case heater is utilized to prevent this by keeping the compressor pre heated.

So in conclusion if your system is the LX model it will stop operating when the outdoor ambient is below 20 degrees. The fact that the system is advertised for low ambient operation means that it is ok to operate down to 20 degrees. If it is the standard model then you may have damaged the system from running it in low ambient conditions.

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  • Wow, this is super helpful! Indeed it looks like I have the standard model but I'm going to look into upgrading to the LX-series.
    – Joel Spolsky
    Commented Feb 23, 2015 at 0:54
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    Joel, if you are really looking for solutions involving $ for only part of your apartment, I'd suggest you investigate a Mitsubishi M-Series single head mini split heat pump. You can get one that is (model dependent) rated to operate to -13 F and I have heard that the real limit is -20 F. Not all mini splits operate this low so if you go with a different manufacture, confirm the limit. Additionally, the unit will offer you the option to heat if necessary. The Mitsubishi M-series efficiency ratings are amazing. mitsubishicomfort.com/products/product-listing/wall-mount
    – Keith Hoffman
    Commented Feb 23, 2015 at 8:23
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    I wouldn't bother getting a heat pump. The apartment already has sufficient heating. I would look into getting a slim side discharge condenser that allows for low ambient operation and that also has a compressor crankcase heater. You can also have an aftermarket crankcase heater installed on any compressor. It would just need to be wired in properly.
    – Mnc123
    Commented Feb 23, 2015 at 16:15
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It is not expected to work, but it might.

Many modern compressors have interlocks that prevent them coming on in too-low temperatures. If there's no interlock or heating element, you could also damage or destroy your compressor if you turn it on at too-low temperatures. This is because it is possible to pull liquid rather than gaseous refrigerant into the compressor, and that would damage it.

The same principle is why many dehumidifiers don't work in cold temperatures. Also, if the ambient temperature (of the compressor and the condenser, note!) is too low, you'll get icing, and the system won't actually work.

Of course some systems are designed to work fine under external freezing temperatures. Most modern cars run the compressor during the winter when you turn on the defroster. And of course data-centre HVAC systems would not keep the machine rooms cool if they didn't work in cold weather, but building-sized units often work on slightly different principles than a residential unit. So, your system could work fine, in which case you might get some effect, but in a residential unit I wouldn't expect it to do so.

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