Why're the air conditioners in my apartment bldg running in a snowstorm?
February 12, 2014 1:11 PM   Subscribe

HVAC question: I live on the top floor of a large apartment building that has all the air conditioning heat exchangers on the roof. It is currently snowstorming. Why can I hear multiple heat exchangers running right now?

Each unit in our building has an electric heat pump in the closet that runs the central air conditioning and central heating. Each unit also has its own heat exchanger box on the roof. By heat exchanger (maybe I'm using that term incorrectly), I mean the box with the fins and the fan that allows the air conditioning to vent heat outside. Our building does not have natural gas service, so I don't think they are fume exhaust fans.

I thought the boxes on the roof only served to exhaust heat for AC. So, why are they running right now in the winter? Do a bunch of crazy people have their AC running?
posted by wondercow to Technology (6 answers total)
Because it's a reverse flow heat pump, and they're heating. The inside coil is hot, the outside coil is cold. Instead of the usual reverse.

Id be curious if they're rated to run that way below freezing. Some aren't...
posted by emptythought at 1:17 PM on February 12, 2014 [4 favorites]

To dehumidify? When I see droplets on the insides of the windows of my car, I turn on the AC for a little while (in addition to leaving the heat on if it's cold out).
posted by Bentobox Humperdinck at 1:19 PM on February 12, 2014

Yes, sounds like they are heat pumps and they're running in reverse of the usual to heat your building. I'm not surprised they're running constantly -- heat pumps aren't great at temps below 40 or thereabouts.
posted by pie ninja at 1:35 PM on February 12, 2014 [1 favorite]

Interesting, thanks for the answers! I didn't realize a fully reversible heat pump system was a thing that existed. So, I'm effectively air conditioning the outdoors right now?

pie ninja, you are correct that these don't seem to be working too well. Some of the units sound like they are preparing for launch. I was debating asking the management company to check on all the air conditioners struggling through the winter, but now it sounds like they are operating as expected.

Now that I read up on it, this link I found through Google says that some heat pumps carry a secondary resistive heating element for situations like this (which is what I had assumed the primary method of electric heating was). That explains why our thermostat has an "emergency" mode. Every time it gets pretty cold (this is Atlanta, so...relatively cold), the leasing office sends an email telling us to put our thermostats in emergency mode. As a northerner transplanted to Atlanta, I've always scoffed at the idea of 20F being an emergency and ignored it, but...now it all makes sense. I'll listen from now on!
posted by wondercow at 1:53 PM on February 12, 2014

The previous commenters are correct. It is a reversible heat pump.

A more technical discussion can be found in the article on Heat pumps in Wikipedia.
posted by tckma at 3:23 PM on February 12, 2014

Heat pumps are more common in the south because while they are fairly weak at doing the job of heating, they are good enough for most weather. The resistive heater strips kick in when you get the more extreme cold, and consume a lot of electricity.

Natural gas is so cheap, though, that heat pumps have been less attractive for a couple years now. That can't last.
posted by intermod at 9:32 AM on February 13, 2014

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