Heat pump outdoor unit aspect
I live in northern North Dakota. We recently built a new SIP house with a Mitsubishi MXZ heat pump. We placed the outdoor unit on the North side of the house due to proximity to the utility room and for presentability as the south face is the main entrance. In my mind at installation, it was an air conditioner that would provide some heat in the shoulder season.
I have been surprised at how much heat we actually get from the unit, though. It provides almost all the heat in the house down to almost -10F. The result of this, however, is a great deal of ice building up underneath the unit from the defrost. For the most part the unit itself is able to remain clear, except for buildup where the heat exchanger curves around the left side of the unit. However, if I am not diligent about clearing icicles from the drain pan holes ( the unit is on a stand that holds it 18″ off the ground), it would back up and fill the drain pan in short order. While this isn’t a huge deal, it is a failure point. To add to the issue, it is near the boiler exhaust, which can add a fair bit of humidity to the area that the heat pump is in, particularly on cold nights with an inversion.
My question is would it make enough difference to move the unit to the south side of the house to offset the cost, effort, and potential decrease in cooling efficiency. I expect it would get more airflow in that location, though, which would limit the loss in cooling. The cost to relocate it would be in the $1,200-1,800 range.
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Replies
When it comes to vapor injection units, -10F is not that cold. People also tend to overestimate heat loss for a new build. A 3 ton unit like that can heat a good sized, reasonably well built house without issues in zone 6.
Moving it won't solve the icing issue. I would definitely move it up a bit, in snow country you want it about 2' off the ground. You can use a couple of stacked patio pavers to raise it. If you are in a windy area, you might need a wind baffle for it as well.
Boiler exhaust is very corrosive, you definitely don't want any of it getting on the coil. See if a you have enough length left on the install to add a couple of elbows or extension on it to move it up or a bit further away.
Thanks for the advice. So the extra solar gain won’t help to decrease the ice buildup or more thoroughly de-ice the unit during the day? Water will drip off our south facing roof even on a below zero day when the sun is out.
The solar gain may help keep ice out of the unit itself, but the meltwater is still likely to refreeze when it gets to the ground. You need to keep the unit up high enough to be clear of snow and ice buildup as Akos mentioned, and you want to make sure the ice buildup doesn't run out over any walkways.
Note that you can use unistrut and the associated fittings to build custom metal stands at low cost. Just be sure to brace the stands so that they don't wobble too much if you use long legs to get a high stand. Simple X bracing with strut between the legs is usually sufficient, and you often only need to do this on one side (not both) in each direction.
Bill