Yesterday I noticed that my Carrier heat pump was struggling to keep up with my thermostat settings inside the house. It was around 35°F (2°C) outside, I had the thermostat set at 72°F (22°C), but the internal temperature wasn't rising above 70°F.

I went outside to take a look at the heat pump and found around 2" of ice/hard snow on all four sides, none on top. It's not snowing outside and hasn't snowed this winter; the ice/snow is all coming from the heat pump. The area around the heat pump is free of debris and the top is clear as well.

I hooked a hose up to our hot water supply and blasted the heat pump until all the ice/snow melted off (is this a bad idea?). All was functioning properly until a few hours later, I noticed ice was once again starting to build up, and the heat pump started falling behind.

I do have aux heat, though I'm not sure on the specs of it. I know it isn't gas/oil, so it must be electric.

The service tech isn't able to come out until February. He said to call him if it breaks in the meantime... which isn't encouraging.

So that's my question. How can I protect my heat pump from damaging itself in the meantime, and how can I keep my family warm as the temperature dips into the 20°F range next week? Do I turn the heat pump off at the breaker and hope my thermostat knows to use aux heat only? Or maybe I dig through the settings in the thermostat to tell it to use aux heat only? Will using aux heat only damage other systems outside the heat pump, or do people generally switch to aux heat only when it gets this cold?

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    It sounds like your heat pump is not going into defrost properly...you may be stuck with aux heat in the meantime :/ Dec 22, 2021 at 4:52
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    @ThreePhaseEel Yep, agreed completely, the heat pump should know / be able to go into defrost mode when needed. Some T-stats automatically go into aux heat mode when needed, others need to be set to EM (Emergency) heat....that totally turns off the outdoor unit. Dec 22, 2021 at 8:12
  • Re: hosing off with hot water. FWIW I've been told by a tech that spraying water into the unit risks damaging electronics inside.
    – Neal Young
    Dec 24, 2021 at 22:26

5 Answers 5


Running AUX heat continuously is fine. It's just expensive, compared to your heat pump.

You likely have one of these problems:

  • Low refrigerant: You should not be losing refrigerant. The refrigerant loop should be hermetically sealed. Topping up your refrigerant may fix the immediate problem, but it will come back. The leak needs to be fixed. You can not do this yourself.
  • Failed TXV: The TX Valve, or TXV, regulates how much refrigerant flows through your system (oversimplification). If it's stuck and not regulating, you can get the same symptoms as low refrigerant, among other things. You can not replace the TXV yourself.
  • Failed reversing valve: The reversing valve is what switches your heat pump between heating and cooling. If your heat pump provides NO heating, then a failed valve is a possibility. But since you say it does provide heat, but not enough, this is not likely your problem. You can not replace the reversing valve yourself.
  • Failed defroster: Heat pumps need defrost circuits because, during normal operation in very cold temperatures, ice will develop in the outdoor unit. If you're handy, you can pop the cover off the unit and identify the control board (with an integrated defrost circuit) or the dedicated defrost board. There is, usually, a set of defrost TEST pins that you can short together, or a switch. This will cause the defrost to kick on, and within a minute or two the ice should start melting. If the ice starts melting, then the defrost thermostat/sensor is likely to be bad. If the ice does not start melting, then the problem could be the defrost board, or maybe it's not getting power, etc. Either of which are possible for you to replace.
  • Failed fan motor: After you melt off the ice, get your heat pump to start heating the house. If the fan in the outdoor unit doesn't come on, then you might have a bad fan motor or a bad fan capacitor. This is also something you can do yourself.
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    "It's just expensive" probably hundreds of US$ for a whole winter I would expect. So it's not something I would just shrug off.
    – Nobody
    Dec 24, 2021 at 9:50

One more place the problem may be, besides those already mentioned.

The outside unit's fan has a relay that is supposed to de-energize the fan during the defrost cycle. If this relay sticks closed, then the unit will pull cold air in through the coils during the defrost cycle, and prevent the coils from defrosting.

I had this happen during a blizzard in 1996. I was out there with a brush cleaning off the outside coils during the snow, until I figured out what the root cause of the problem was. A $15 relay fixed the problem. Before I was able to perform the repair, I manually disabled the fan during the defrost cycle by pulling a wire off one side of the relay contacts until I was able to perform the repair. Need to be careful doing this as that relay controls 115 VAC for the fan.


When I considered a heat pump some years ago, the pump typically shut down around 40F and the unit used resistance heaters.

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    Heat pumps have gotten much better over the years, and can run much colder. They solve the icing problem by detecting it, and going into reverse (without the interior blower running) and heat the coil to melt the ice. Their coils are designed to shed ice efficiently. Dec 24, 2021 at 1:09
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    And, without the fan in the outside unit running. See my answer.
    – SteveSh
    Dec 24, 2021 at 11:55

After you switch on Emergency Heat turn up the thermostat so the heat comes on. Check outdoor unit to make sure it is not running. On more than one occasion we have seen this happen. If so there should be a disconnect or breaker on the outside wall that sources the unit.

(Duh on me. I almost gave everyone a laugh by calling it the condensing unit.)


For a heat pump to pull heat from the outside air the temperature of the coils have to get colder than the outside air. With the air so close to freezing that means it is highly probable for the moisture in the air to condense and freeze to the coils. The thermostat should be smart enough to figure out that the outdoor temperature is too low for the heat pump and switch to auxiliary heat and/or know to run through a defrost cycle to prevent ice buildup. It sounds like you need a new thermostat.

When the thermostat in my house died I found out that a thermostat that could properly manage defrost and/or automatic switching to aux heat were rare and expensive. By "expensive" I mean something around $500 when a typical mechanical thermostat is $30 and an electronic thermostat with Wi-Fi and a phone app costs $100. I called some HVAC people about installing a new thermostat like my old one and most didn't even know what I was talking about (I guess air sourced heat pumps are not popular around here) and the one HAVC guy that did know what I was talking about had only one model of thermostat and quoted an install (parts and labor) in the $500 range.

I'm guessing the people that installed this was not aware icing could be a problem, or they knew it was possible but gambled on the cheaper thermostat. It is quite possible that there is some other problem which makes this icing problem worse, and @longneck discussed them in another answer. Whatever the case this should not happen because a thermostat designed for air sourced heat pumps would prevent this.

A long term fix will almost certainly involve a new thermostat. In the short term you should be able to set your thermostat to use only aux heat. On my old thermostat it had different modes, one mode called either "aux heat" or "emg heat" would run only the auxiliary heat source for heat. The other options were "cool" (run heat pump in cooling only), "heat" (heat only but switch between heat pump and aux based on outdoor temperature), "auto" (heat pump, aux heat, or cooling, based on outdoor temp and desired indoor temp range), and "off".

My heat pump is rated to operate down to at least 20F, and from your comments I assume yours is as well. If the heat pump ran before in temperatures this low and not ice up then either you had an unusually humid weather event or something broke. I could not find a replacement for my thermostat at a price I was willing to pay, you may have better luck in your area as it sounds like air sourced heat pumps are the norm in your neck of the woods.

I ended up bypassing the heating function of my heat pump and used a "cheaper" (about $100) thermostat. I haven't decided if this will be a permanent fix. I have natural gas heat and the season where an air sourced heat pump makes sense where I live was already quite short when I bought it, with natural gas prices getting lower since that time I'm not facing any extra costs by using only the natural gas heat.

I realize that running on electric resistance heat would cost you money so consider ways to minimize the time it's needed. If the outdoor temperature is above 40F then it may be safe to use the heat pump. Perhaps you can use the heat pump during the day and aux heat at night. If you have some electric space heaters then consider using them, if you will be using electric resistance anyway until this is fixed then heat only the rooms you need until then.

I stressed the need for a proper thermostat because it seems that even many HVAC installers don't know air sourced heat pumps need thermostats designed for them. Ground source heat pumps can't ice up like air source heat pumps and so there's no need for frost protection on those. When you can get a tech out to look at your heat pump then it may be wise to ask about the thermostat if the tech doesn't bring it up first.

  • While this is certainly possible, it doesn't make a lot of sense to me, because there are plenty of thermostats that are designed to handle heat pumps. If it was the thermostat's job, rather than the heat pump control board's job, to decide when to run aux/emergency heat then it would need to know the outside temperature, which a normal thermostat simply doesn't have. In theory that could be an easy add-on. But more importantly, proper detection of when to run the defrost cycle is not going to be time-based or temperature based but rather based on internal sensors of the heat pump to Dec 25, 2021 at 23:12
  • determine based on outside temperature, coolant pressure/temperature or other sensors in the outdoor unit determining when a defrost cycle is needed. So all that could be thrown into a $500 thermostat, but it would make a lot more sense to be part of the heat pump's standard circuitry. Dec 25, 2021 at 23:13
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    Whether it makes sense or not is debatable, I just know in my HVAC the switchover to aux heat was done by the thermostat and I have no reason to expect my HVAC system to be unusual. This does mean the thermostat has to know the outdoor temperature, and that is a rare feature. Air sourced heat pumps are still rare which makes thermostats built for them rare. People going cheap and not putting in the right thermostat makes them rarer still, if they don't sell then shops stop carrying them.
    – MacGuffin
    Dec 26, 2021 at 2:56

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