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. 


Tags: climate, cold, heat, pump

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Hi Julie, as a point of clarification, in cooling mode a heat pump is essentially an air conditioner, and there's nothing new about A/C failures tied to high heat days. Everyone knows that air conditioners always seem to fail on the hottest day ;-)

Or are you referring to a different type of failure, i.e., system is operating but doesn't have enough capacity to handle the load?? (due to poor design or some type of degradation such as clogged filter, refrigerant leak, etc.)

Hi David - Thanks for replying. Yes I'm referring to the cooling mode of a heat pump, i.e. air conditioning. In all honesty I don't know what the specific failures are, but wanted to inquire with folks to see if there was any common knowledge about failures that we haven't heard about.

It sounds like in your opinion even a traditional AC would fail on the hottest day and given the way that heat pumps cool this would be no different for them?  

I think you took my 'hottest day' comment a bit too literally. In any case, there's no doubt that certain failure modes are more likely as outside temperatures climb (more stress on components).

And yes, a heat pump is nothing more than an air conditioner with an extra valve that allows it to operate in reverse in winter, plus some extra electronics. In that regard, a heat pump has a couple of failure modes that a straight A/C doesn't have -- in particular, failures related to the reversing valve (including the valve itself and the solenoid that controls it). In that case, a failure may leave the unit stuck in cooling mode (so it won't heat) or stuck in heat mode (so it won't cool). Which way a valve will fail depends on its design rather than the ambient temperature. But all other failure modes in cooling mode would be more or less the same as with straight air conditioners.

Keep in mind that while heat pumps may be relatively new in colder locations, they've been around for a long time (I had my first heat pump 40 years ago, in central North Carolina), and in fact dominate the HVAC market in many milder areas of the country.

This is extremely helpful, thank you David! I will circle back if I come across any future reports on more clearly defined issues with heat pump failures on high heat days. 

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°.  



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.


@Walt, I hesitate to respond since this is rather off-topic, but my previous comment apparently wasn't clear. My rebuttal was directed at your comment that a "...heat pump works much harder on the coldest days."  That's not the same as saying it does more work over the course of a winter vs a summer, which may or may not be the case depending on climate, and in any case isn't pertinent to the original question about failure rates associated with high heat days (or to your point, really cold days).

Head pressure is in fact what determines a compressor's workload. The number of BTU's transferred is a *consequence* of that but doesn't tell us how much work the compressor is doing. Through the magic of phase change, energy is absorbed and released in the refrigerant cycle. Phase change is responsible for BTU transfer.

Current draw (amps) is a good proxy for the amount of work being done by the compressor at any given moment. Current is directly proportional to head pressure. If you refer to your heat pump's expanded performance data, you'll see it draws MUCH less current at 9F than at 110F.

Here's a better way to explain: If an air conditioner has a serious maintenance issue that causes the refrigerant not to condense, there may literally be zero BTU's delivered, but the compressor is still doing work. It may not be useful work, but in physics, it's still considered work.

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.

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.

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.


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