Frost on heat pump

[cinergi]

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

[HVAC dawg]

Could be a lot, bad relay?

[HVAC dawg]

Where are you located?

[cinergi]

Montreal, Canada. It gets down as low as -20 Celsius (currently -11).

-cinergi

[big sky hvac]

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.

[Tom Campbell]

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.

[big sky hvac]

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%.

[cinergi]

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

[badtlc]

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.

[cinergi]

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

[Tom Campbell]

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/moisture-holding-capacity-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%.

[Tom Campbell]

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.

[martyinlincoln]

Make sure to tell the tech about the fan shutting off in heat mode.

[leoxxl]

why using heat pump? when outdoor temp is as low as -20 C, the unit efficiency is very very low. Severe frosting also may damage the coil.

[beenthere]

Don't get much frost on a heat pump that is working correctly at -20C.

[cinergi]

The tech came over earlier today. The problem is a sticking reversing valve. He tried to change the coil but that didn't help, so he needs to change the valve - a much bigger job as I'm sure you all know.

-cinergi

[bzzline]

Just wondering do you have a txv or bullet type meter device on the outdoor coil. or what type of unit is it

[cinergi]

It's a TXV inside the house adjacent to the indoor coil. It actually had to be replaced 2 years ago because the old one was sticking, causing the heat pump to go out on high pressure cutoff.

-cinergi

[bzzline]

may have moisture issues

[cinergi]

Possible, but a dryer was installed after the TXV failed 2 years ago. It's still there.

cinergi

[snooked321]

I was thinking at first that it could be a low charge problem? You did say you heard the RV switching?

[bzzline]

hard to tell from here it may be iceing up due to a bad run capacitor If the fans dropping out in the heat mode but also could be a bunch of things

[cinergi]

The tech clearly showed me it's the reversing valve by manually jumping 24 VAC to its coil. I could hear the coil clicking, but there was no accompanying "whoosh" sound as usually happens due to the rapid change in pressure in the refrigerant lines when the RV is operating normally.

-cinergi

[Kevin O'Neill]

I believe your tech has it figured out right.

[cinergi]

Just wanted to post the final update. The reversing valve was replaced last week, and everything has been running fine since then. The technician found a lot of black sooty "gunk" in the lines and in the old reversing valve, which explains why it failed. It could also explain why my TXV failed 2 years ago.

He did a nitrogen purge of the system, used chemicals to equalize the pH, installed a new filter dryer, and put new oil and refrigerant into the system since the old one was contaminated. He said the compressor seems to be undamaged judging by its current draw, sound, and pressures.

-cinergi