Ice Formation with Air-Source Heat Pump in Cold Climate
This is a home I helped design and build in 2019 in Northern Minnesota, climate zone 7. The main heat source is a Mitsubishi PVA-A24AA7-1 which is a central ducted multi-position air handler and a PUZ-A24NHA7 outdoor unit. The ASHP is working great, maintaining a constant temperature of 68°F inside, even when the outdoor temperatures drop well below zero. There is an electric plenum heater that will take over when the heat pump can no longer maintain the desired temperature. The electric/heating bills are very low, both the homeowner and myself are pleased with how the ASHP is performing with regards to heating the home.
We are having a problem with ice forming below the outdoor unit. The photo shows just how much ice has formed over the past couple months, we still have another 2 to 3 months of the heating season to go. The outdoor unit’s condensate has accumulated under the unit and is now up against the home and starting to flow over the driveway. Anyone else in a cold or very cold climate having this issue and how did you solve the problem?
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It's nice to see Instagram converge with GBA. :-)
Curious to see what folks think about the amount of defrost/melt going on here and if it's indicative of anything or just pretty normal given the climate.
Also, it's funny that FHB's Twitter post of a few minutes ago highlighted this house. Nice coincidence! For those who haven't seen, check it out: https://www.finehomebuilding.com/2019/12/18/a-concrete-free-slab-on-grade-foundation
Pretty cool house.
Thanks Patrick!
Would think a bucket of ice melt would get you through the winter and in spring a little landscaping to change the direction of the ice floe. Maybe if there is enough service loop to move the outside unit just a little further out and maybe away from the driveway it would help also. One could build a little flashing maybe to keep the ice off of the house
Thanks for the reply Keith, we actually had this problem to a lesser extent last year. We raised the heat pump and moved it a little farther from the house. Planning a different strategy but we have to wait until next summer after all that ice melts.
What is the temperature? I don't get a lot of ice build up at low temperatures (which are 10-15 deg F at my location) - because the water content of the air is so low. The 1-2 day a year we have freezing rain is when the ice build up comes - and it's mostly when I haven't cleaned my gutters and the water drips over the split. I question whether that is condensate or of you have another water source.
Hi Jay, we've been mostly in the teens and twenties most of the winter with a few days where we were sub-zero. Very mild winter for us so far but we have had more frosty trees this year than I can ever remember. Higher than normal humidity is causing the condensation.
Try sticking one of these on the bottom.
https://www.amazon.com/ABN-Automotive-Electric-Silicone-Heating/dp/B079G7P27S
It should be enough to keep the ice out. I suspect you need a 100 watt version. It does stick well even when you press it on to the frozen metal.
Very exciting to hear about the performance of the air source heat pump in upper Minnesota. Please tell us more about the numbers when you get them. We will be upgrading the heating system at our farm (Clay County, MN) next summer and would sure like to consider the heat pump.
We have had mild temperatures ( in MN) this winter with high relative humidity, ice fog as they are calling it. Could be the cause of so much ice. Some inventive mind will come up with a solution for this. How about heat cable or something like it. At some point it is liquid water, if you can, divert it to an area away from the house and drive. Better yet an earth tube. Next spring bore a vertical hole beneath the condensation outlet to a point below the frost line. Line with an ABS pipe or the like.
Hi Doug, your idea is exactly what our plan is. We have a foundation drain with crushed rock and a pipe leading away from the home five feet below the outdoor unit. Plan is to bury a 4 inch vertical pipe next to the outdoor unit. Unfortunately we cannot get it right under so we will have to somehow channel the condensate to the pipe. We will use a low wattage heat tape to keep the condensate liquid. I did hear back from Mitsubishi, they said the icing under the unit is not typical and suggested something similar.
What side of the house does the unit sit on? This can make a difference:
https://www.greenbuildingadvisor.com/question/mini-split-condenser-location-in-zone-5a
Not much help for this unit, but something to consider for future projects.
Attached photo shows our unit on the north side at the beginning of the Polar Vortex (-23º F) back in early 2019. A couple of days later, once the worst of it was over, there was a couple more inches of ice directly below the unit, but still nothing like what your unit is showing.
We're in climate zone 5, but even so, that looks like a lot of ice on and around your unit. I'd be surprised if that's considered normal, even for climate zone 7.
I'm sure Dana and others will offer more climate specific info for you...
Thanks for the reply Eric, the outdoor unit is on the north side of the home and unfortunately was the only place we could put it. We did hear back from Mitsubishi, they said this is not normal and gave a few suggestions. Our plan is to install a pipe to our foundation drainage system which is piped away from the home.
Looks like support for "3'+ above ground, away from everything" vs "low, near walkways or driveways" mounting. For now, a large amount of deicer.
Would be nice to see raw numbers for heat pump electrical use vs outdoor temperature.
Hi Jon, the only numbers I have as of now are the homeowner's monthly electric bills. My best estimate is the home heating cost for the month of December was around $80. We did install Leviton smart breakers but have yet to get them set up, struggling with connecting with the Leviton Hub.
$80 for December in Minnesota for a detached house is great!
Given that you will go several months without getting above freezing an ice sheet seen inevitable, and it become a matter of managing the ice.
I think you could tilt the unit so the water comes out the other side of the unit further from the drive and change the grade so the water moves away from the drive.
Does the unit have a heated pan?
Is the heat pump a hyper heat type?
Walta
Hi Walter, the unit is a light commercial, centrally ducted system. Not hyper heat as far as I can tell. SEER of 20.05. I'm sure the outdoor unit has a heated pan or the condensate would freeze before it drained. We've got some ideas on how to eliminate the ice, unfortunately they need to wait until next summer.
Be careful with salt around your heat pump; even a little splash of saline melt could cause corrosion of the coils. What I'm seeing in your photo could easily reflect normal operation, especially under high humidity conditions. As long the the coils are clearing the frost on every defrost cycle and the drain pan isn't filing up with ice, the system is doing what it should. The owners may need to periodically sprinkle some sand to mitigate the slip hazard and perhaps take a hammer and knock down the stalagmite before it reaches the unit itself.
Thanks Jon!
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I am looking at placing a tracer tape on line to drain into the ground...down here in zone6 msp
We are looking into the same, thanks William.
Absolutely don't use salt-based deicer around the unit. Salt will quickly corrode aluminum. This is so much of an issue that antennas near the ocean are often spray with a laquer prior to installation to protect the aluminum elements from degradation.
I would recommend you use one of the self-regulating type of heat tapes on that condensate drain line. I've used the one made by Raychem in the past. These are quite a bit more expensive than the regular heat tapes with a thermostat on one end, but they are FAR FAR more reliable and a lot better quality. They also make sure you don't have hot and cold spots along the cable because the way they operate ensures an even temperature along the entire length of the cable.
Bill
Thanks Bill, we are leaning towards a drain and heat tape as a solution. Raychem was also suggested by Mitsubishi.
This is why I think South facing exposures have an advantage, even with sustained below freezing temps, you get sublimation occurring from solar radiation. It's boosted by reflected radiation off the house wall. The sun might not change the BTU delivery or efficiency in a meaningful way, but it certainly helps with ice management.
$80 in Dec is great, but I'm curious why you didn't go with a hyper heat unit in Minnesota though?
Hi Josh, I had a discussion with Mitsubishi at IBS in 2019, this is the model they recommended. I don't recall the reasons they suggested the light commercial P-series over the M-series.
I agree with the south exposure, unfortunately we couldn't place the outdoor unit on either the south or west sides. Thanks for the comment!
You can't argue with the operating cost! Hyper heat or not, it's fantastic so far.
I live in southern MN Zone 6, and have a similar issue with a Daikin unit. I love the heat pump, hate the ice management.
Did you get your solution implemented? How is it working?
The post is old but ice buildup issues likely remain for some. A couple tips:
1) Make or buy a flexible plastic drip plate/pan. Check it periodically. Remove, bend tray to empty it and replace. Simple as an ice cube tray.
I use a piece of puck board (HDPE) at a small angle to direct water a preferred direction. The plastic board slides into and is supported by plastic rails (eg, snowmobile sliders mounted sideways as guides for a 3/8" HDPE plate... ice won't stick to either. )
Although easily removed, a good tap on the underside of the plate will typically clear it. When left too long, pull out the plastic board and smash the remaining ice bridge with a firm rap in the middle.
2) Too late now? Already a lot of ice built up? Chip what you can off the top and drape the remaining ice with a crazy carpet. Tie around its width closer near the discharge end and tighten it enough to form a broad u shape (trough) to direct water... keep an eye on buildup. Remove, clean and replace the crazy trough periodically.
A plastic garbage bag would probably work short term in a pinch.
Hope this helps someone out there ; )
Ken