Heat pump failure during high heat days?

Are there studies or have you had experiences where heat pumps have failed to cool on high heat days? We've known that cold climate heat pumps are needed in areas with colder climates (like Denver) however news of failure rates tied to high heat days was new. Looking for input to inform whether or not this is common or still being tested in different climate zones. 


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  • This is awesome! This is a great source to get started in the industry and this will be helpful to us since we also cater to the needs regarding HVAC material. Thanks for sharing!

  • Eric and David you may find this laboratory study of real world dirty AC coils brought in to the lab and studied how they worked. They tested the dirty air conditioners, then washed them thoroughly with a garden hose and tested again. They also used a commercial coil cleaning fluid and tested them for a third time.

    On average cleaning did not improve performance, because somewhat dirty coils turned out to work 7% better than clean coils and clogged coils worked 7% worse


    You Can Skip This Chore
    Cleaning your air conditioner's condenser probably won't make it work any better
    • Dig into this a little more and you find this - 


      He makes the point repeatedly that that study was misquoted, misunderstood or flawed in some way. I agree there is little to no value in cleaning a relatively clean coil. But if the coil is dirty enough to restrict the air flow - there are major issues like loss of cooling, high energy consumption and part failure.

      I am not slamming that study referenced on GBA. If I understand what it said, they removed condensers from the house and took them to a lab to be measured. Or did they remove the whole system? They are trying to to do laboratory quality measurement on real world problems. That is always tough.

      Back to the original question - I don't see that heat pump vs straight cooling is more failure prone by this. Heat pumps run a longer portion of the year so I expect them to need more maintenance. But I don't see heat pumps to be extremely failure prone. YMMV

  • I find most heat pumps need cleaning 2- 3-4 times a year.  When dirty coil do not transfer heat well.  High heat kills compressors and motors.  With lack of air flow or water vapor in HVAC system just rotts.  I have air and water Heat PUmps now 40 yr old in daily use - ya motors fail, coils leak, but if you keep coils clean every thing just has potential to work well.   


    • Hi Eric, excellent point. Keeping those coils clean is paramount to both short and long term system performance!  But I think multiple cleanings per year by a professional (I see you're a mechanical contractor) is a bit excessive for most folks. This topic warrants further discussion.

      Coil soiling is highly location dependent. Fortunately, most situations that call for frequent cleaning fall under 'prescribed homeowner maintenance'... requiring only a garden hose and/or leaf blower. For example, I advise clients with homes near the ocean to rinse their AC or heat pump coil several times a year to mitigate corrosive effects of salt. Likewise, units in close proximity to heavy vegetation require seasonal removal of leaves and other organic debris. And heat pumps located in areas prone to heavy snow drifts require extra attention in winter (greatly mitigated with raised platform!)

      Much less obvious is the hard-to-clean film that accumulates on coil fins, which can only be removed with a foaming cleaner designed for this purpose -- preferably by a pro. Otherwise performance will eventually suffer. But in my experience, if the homeowner keeps the more obvious contaminants under control, professional cleaning need only be done every other year in most cases. I recommend annual maintenance to those clients willing to pay so they "don't have to worry about it."

      Indoor coils are more difficult to clean (due to accessibility), which is why good filtration is so important.

  • This might be a little off topic for the question you asked -

    The industry is seeing an extremely high failure rate of capacitors for the last 5 to 10 years. My personal (anecdotal) experience is the capacitors fail more frequently as the outdoor temperature rises. As an attempt to address the capacitor failures, most supply houses are stocking higher voltage rating capacitors and 'Made in America' capacitors. Most residential (240 Volt single phase) capacitors were rated for 330V until recently and now most are rated at 440V. I'm not sure this has reduced the failure rate because I still see most service calls are due to a failed capacitor.

    Heat pumps and air conditioners use the same capacitors and the appear to me to fail at about the same rate. So my comment is that both air conditioners and heat pumps appear to be failing more frequently due to bad capacitors. If a study is looking for why "heat pumps are failing more frequently" - the study has to separate the 'more frequent failures' due to the capacitors from the impact of heat pumps vs AC.

  • Please share this “news of failure rates tied to high heat days” Where did you get this “news” from is this data anecdotal or statistically?

    The cold climate heat pumps IE hyper heat models are almost two stages systems with an extra port in the compressor. This is new technology to the industry so some problems are to be expected. I have not seen any documented cases.

    In truth my heat pump works much harder on the coldest days. If it is 9° outside and 68° inside a difference 59° but if it is 110° outside and 72° inside a difference of only38°.  



    • Hi Walt - A partner city said that there is some research that heat pumps have higher rates of failure in the heat. I am trying to find out if a report has been completed on this topic, or where this came from as well and will share as soon as I can find it.

      My intent for asking the forum was to find out if anyone else had heard this, as it was news to me.

    • Walter wrote: "In truth my heat pump works much harder on the coldest days."

      This is a common misconception. The magnitude of the outside-inside delta-T has no bearing on the amount of work done by a heat pump over a given period of time. Assuming continuous operation at full capacity in either case, the biggest difference is head pressure, which defines how much work the compressor is doing. Head pressure is MUCH lower at 9F than at 110F. (Consequently, current draw is also a lot lower at 9F than at 110F.) 

      In terms of wear-and-tear (different from work), the biggest strain on motors is starting and stopping, although less so for variable speed designs. In any case, there's not much difference at 110F versus 9F since compressor would likely be running continuously at full capacity in either case. Beyond that, certain components such as electronics and bearings are adversely affected by high heat, so it would be fair to say that a heat pump not only works harder, but is subject to more wear and tear at 110F than at 9F.

      • “The magnitude of the outside-inside delta-T has no bearing on the amount of work done by a heat pump over a given period of time. “ I disagree the inside outside delta is directly proportional to the number of BTUs required to maintain that delta, IE work required.

        “The biggest difference is head pressure, which defines how much work the compressor is doing”. Again I disagree, the amount of work the compressor cannot be measured with a single number. The amount of work the compressor is doing is proportional to the pressure differential the compressor is generating. When the outdoor temp is -10° the compressor has a larger differential than when it is 110°.

        Even in the relatively southern state of Missouri my heat pump logs twice as many run hours in the heating mode than the cooling mode. It is rare for me to find the AC mode running above it lowest speed and it is not uncommon to see the higher speeds in the heating mode. Again the unit is working harder in the heating mode.


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