Page 1 of 2 12 LastLast
Results 1 to 13 of 25
  1. #1
    Join Date
    Jan 2008
    Posts
    40

    Frost on heat pump

    Hi,

    Yesterday I noticed that my heat pump was running longer than usual and the aux heat strips kicked in at an outdoor temperature where the heat pump can usually handle the load itself. I checked the heat pump and there's a layer of frost/snow covering the entire coil. My service tech is coming to take a look next week (it's too cold now) and in the meantime I've shut off the heat pump and am running on emergency heat.

    What could I be looking at here? Just want to be informed/prepared.

    Thanks
    cinergi

  2. #2
    Join Date
    Sep 2010
    Location
    Wa.
    Posts
    119
    Could be a lot, bad relay?

  3. #3
    Join Date
    Sep 2010
    Location
    Wa.
    Posts
    119

    Hmm

    Where are you located?

  4. #4
    Join Date
    Jan 2008
    Posts
    40
    Montreal, Canada. It gets down as low as -20 Celsius (currently -11).

    -cinergi

  5. #5
    Join Date
    Sep 2006
    Location
    Montana
    Posts
    983
    There could be a lot of different things going on. Depnending on what temp. your compressor locks out, it may be set to run when it's too cold out. I usually lock out heat pumps at 30*F-35*F. There are a lot of people that will run them down to 15*F-20*F. At those low temps., I feel like the heat is not adequate enough to be comfortable. You could be having defrost problems, which could be a defrost board or defrost thermostat. It could be low on refrigerant. There are several things that could be going on. It surprises me a little bit that you'd have a heat pump in such a cold, northern climate though.

  6. #6
    Join Date
    Aug 2010
    Location
    Monroe County, PA
    Posts
    99
    Since the heat pump works by drawing outside air across the coils on the outside unit, and those coils are even colder than the outside air itself, frost and eventually ice can build up on them. Every so often, either by a timer or by other sensors, the system goes into a defrost mode, where it effectively runs in reverse to heat up the outside coils enough to melt off the frost and ice. You can usually tell because you can hear the outside compressor running but the its fan doesn't turn - and in most cases it starts producing steam for a few minutes. Frosting over is actually less common as it gets colder because there is less liquid moisture in the air and any snow that does fall is typically drier. A misty 30 degree morning is actually one of the worst for creating frost on a heat pump.

    When the system fails to go into defrost mode, for any number of reasons that only a tech can figure out, it can frost over. Once it gets to this state its effectiveness is substantially diminished, and continued use can actually harm it.

    You did the right thing by switching it over to strip heat for now. Unfortunately, there is nothing further you can do until the HVAC man comes to diagnose the fault and fix it. It may be something simple and you only get charged for a service call, it could be a bad board, or it could be that a leak has made the charge pressure off, which would require fixing the leak and refilling the system. Prices aren't allowed here, but at least you have an idea of the range of services you may be looking at to get it running again.

  7. #7
    Join Date
    Sep 2006
    Location
    Montana
    Posts
    983
    Quote Originally Posted by Tom Campbell View Post
    Since the heat pump works by drawing outside air across the coils on the outside unit, and those coils are even colder than the outside air itself, frost and eventually ice can build up on them. Every so often, either by a timer or by other sensors, the system goes into a defrost mode, where it effectively runs in reverse to heat up the outside coils enough to melt off the frost and ice. You can usually tell because you can hear the outside compressor running but the its fan doesn't turn - and in most cases it starts producing steam for a few minutes. Frosting over is actually less common as it gets colder because there is less liquid moisture in the air and any snow that does fall is typically drier. A misty 30 degree morning is actually one of the worst for creating frost on a heat pump.
    When the system fails to go into defrost mode or frosts over prematurely, for any number of reasons that only a tech can figure out, it can frost over. Once it gets to this state its effectiveness is substantially diminished, and continued use can actually harm it.

    You did the right thing by switching it over to strip heat for now. Unfortunately, there is nothing further you can do until the HVAC man comes to diagnose the fault and fix it. It may be something simple and you only get charged for a service call, it could be a bad board, or it could be that a leak has made the charge pressure off, which would require fixing the leak and refilling the system. Prices aren't allowed here, but at least you have an idea of the range of services you may be looking at to get it running again.
    I think you've got it backwards. When the ambient is colder, humidity is higher. The warmer the ambient, there is less humidity. It is currently 18*F where I'm at and the humidity is at 68%. Where my parents live, which is 220 miles from me, it's 5*F with the humidity at 82%.
    Last edited by big sky hvac; 01-20-2011 at 01:27 PM.

  8. #8
    Join Date
    Jan 2008
    Posts
    40
    Thanks to all who replied. The heat pump lock-out temperature is set to -8 C. I've never had a problem with frosting before, even during freezing rain.

    Heat pumps are very common in my area despite the cold climate. They handle the heating load very well down to approximately -8 to -12 C.

    I don't know if this would help to diagnose the issue, but when I inspected the heat pump yesterday, it happened to be in the defrost mode at the time with the compressor running but the fan stopped. As I stood there, it made a whooshing sound as it usually does at the beginning and end of the defrost cycle (due to the reversing valve switching over), but the fan didn't turn on - it seemed to continue its defrost cycle. Finally, a few minutes later, the reversing valve switched over again (whoosh!) and the fan started. Usually when the fan restarts after a defrost cycle, it blows out a big cloud of warm moist air, but this time it didn't happen.

    Obviously there's something wrong with the defrost cycle. Judging from the above symptoms, seems like it could be the defrost board not controlling the sequence properly:

    Normal defrost sequence: Fan stops and reversing valve switches > compressor continues running for ~5 minutes in defrost > fan restarts and reversing valve switches again.

    Possible faulty sequence (based on my observations): Fan stops but reversing valve doesn't switch > compressor continues to run in heating mode > reversing valve finally switches halfway through the cycle > compressor continues to run in defrost, but for insufficient time > fan restarts and reversing valve switches.

    Does this seem reasonable?

    By the way, I'm an Electrical Engineer who takes an interest in HVAC and understands how the systems work, but not an HVAC professional. No DYI for me in this case :-)

    -cinergi

  9. #9
    Join Date
    Jul 2008
    Posts
    551
    Did you see any ice melting during the defrost cycle? I don't think the sequence is going to randomly change. Seems more like a reversing valve issue.

  10. #10
    Join Date
    Jan 2008
    Posts
    40
    No, I didn't see any ice melting during the defrost cycle, nor did I feel any warmth coming out of the top of the heat pump. After the "whoosh" sound midway through the cycle, however, I did hear metal expansion clicking noises as if parts within the heat pump were finally beginning to heat up (as one would expect during defrost).

    Based on this, the issue seems to be that the reversing valve is only switching to defrost midway through the defrost cycle, either because it's defective/partially stuck or because its coil is not getting the correct voltage signal from the control board (which would mean the board is faulty).

    -cinergi

  11. #11
    Join Date
    Aug 2010
    Location
    Monroe County, PA
    Posts
    99
    Quote Originally Posted by big sky hvac View Post
    I think you've got it backwards. When the ambient is colder, humidity is higher. The warmer the ambient, there is less humidity. It is currently 18*F where I'm at and the humidity is at 68%. Where my parents live, which is 220 miles from me, it's 5*F with the humidity at 82%.
    I don't want to thread-jack the OP's thread, but % humidity and moisture content in the air are not the same. When it's 100% humidity it does not mean that we are breathing H20 instead of air, it means that the air is at 100% of its holding capacity.

    As air gets colder, it's capacity to hold water decreases (http://www.engineeringtoolbox.com/mo...air-d_281.html) so while the outside humidity may shoot up to 70% when it gets very cold, there is still significantly less water in the air than at 40% on a hot day.

    This is why air inside a heated house gets so dry in the winter. The overall atmospheric water content is low (even though it is 70% humidity outside). That same water volume, inside a heated house, is far below the warmer air's capacity, so your indoor, heated air humidity is 20%.

  12. #12
    Join Date
    Aug 2010
    Location
    Monroe County, PA
    Posts
    99
    Quote Originally Posted by cinergi View Post
    Thanks to all who replied. The heat pump lock-out temperature is set to -8 C. I've never had a problem with frosting before, even during freezing rain.

    Heat pumps are very common in my area despite the cold climate. They handle the heating load very well down to approximately -8 to -12 C.

    I don't know if this would help to diagnose the issue, but when I inspected the heat pump yesterday, it happened to be in the defrost mode at the time with the compressor running but the fan stopped. As I stood there, it made a whooshing sound as it usually does at the beginning and end of the defrost cycle (due to the reversing valve switching over), but the fan didn't turn on - it seemed to continue its defrost cycle. Finally, a few minutes later, the reversing valve switched over again (whoosh!) and the fan started. Usually when the fan restarts after a defrost cycle, it blows out a big cloud of warm moist air, but this time it didn't happen.

    Obviously there's something wrong with the defrost cycle. Judging from the above symptoms, seems like it could be the defrost board not controlling the sequence properly:

    Normal defrost sequence: Fan stops and reversing valve switches > compressor continues running for ~5 minutes in defrost > fan restarts and reversing valve switches again.

    Possible faulty sequence (based on my observations): Fan stops but reversing valve doesn't switch > compressor continues to run in heating mode > reversing valve finally switches halfway through the cycle > compressor continues to run in defrost, but for insufficient time > fan restarts and reversing valve switches.

    Does this seem reasonable?

    By the way, I'm an Electrical Engineer who takes an interest in HVAC and understands how the systems work, but not an HVAC professional. No DYI for me in this case :-)

    -cinergi
    If it only ran a few minutes, it may be that it is simply cutting out of defrost cycle too early. Most of the simpler systems are designed to enter defrost after every XX minutes of use (typically 30,60,or 90) and then run in defrost until the coils reach a specific temperature (varies by brand), OR until a certain period of time has elapsed. If it is slow to heat up and/or the timeout is kicking in too early, it may be able to get up to a temp needed to melt everything, but never gets a chance to run long enough to do so. And as the ice builds thicker and thicker, it needs an even longer period of time to melt it off - and so it just gets worse and worse.

    It's all conjecture until someone with the knowledge and the tools to diagnose it come out.

  13. #13
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,629
    Make sure to tell the tech about the fan shutting off in heat mode.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Comfortech Show Promo Image

Related Forums

Plumbing Talks | Contractor Magazine
Forums | Electrical Construction & Maintenance (EC&M) Magazine
Comfortech365 Virtual Event