The Minisplit in Cold Weather

I tried an experiment this week during our cold snap. We've kept the door closed to the first floor ell (bedroom and bath) and let it run cold, because the Fujitsu wasn't sized to heat that space too. I opened the door early in the cold snap, and let the heat pump go, leaving it set on 70F. What I found was that overnight the main space went to 66F, and the upstairs and back bedroom were 3-4F lower.

My calculated heat loss in these conditions is about 24,000 BTU/hour, and the heat pump is rated at about 17,000 BTU/hour at about 10F. You'd think it would not be able to keep up. My heat loss number may well be too high, and the rated output of the unit may be quite conservative.

One other thing I wonder about is that even though the room was not at the setpoint it seemed that the unit didn't run on full output much. My system has the temperature sensing built into the wall cassette, so it may be sensing a higher temperature than out in the room. It may make sense in severe weather to set the thermostat up to 72F instead of our normal 70F. Unlike a boiler, these variable speed units taper off the output as the space approaches the setpoint instead of always running at full bore, so that may be a disadvantage of a smart unit - it's trying to stay at a more efficient operating point instead of making me as comfortable as possible.

Anyway, comfort trumped further experimentation and we closed the door again to the ell.

 

Comments

[Bob Lemaire]

We experienced a low performance condition a couple of weeks ago when it was about 36 deg outside with dense fog. The mini-split had to spend more than half of it's time defrosting, seriously reducing the net output. It was new to us, but I imagine that you've encountered those conditions more than once on MV.

[Marc]

That's interesting! I haven't noticed it, at least to the point that we lost capacity. Here, that outside temp is about halfway to design temp difference, so we likely wouldn't see it as a capacity issue. What product do you have there? Published data shows a multiplier of 80-85% of capacity on most minisplits, 50% sounds as though something might not be just right.

[Bob Lemaire]

Daikin Quaternity 12KBtu. In the shoulder months, it heats the house, but Dec-Feb it just adds what it can to the middle of the house and the woodstove, sun, and some oil take care of the rest. So it's not a capacity issue for us either But the defrost cycle is noticeable and was happening more than not during that particular day. When it's dense fog close to freezing, the outdoor unit freezes up pretty quickly and has to work hard to defrost - much more than in relatively dry 10 degree weather. Those are pretty rare conditions here in central NH but I'm surprised that you haven't experienced it in the maritime climate of Marthas Vinyard.

Other than that particular set of conditions, the unit has been putting out pretty well this week down to zero, and continues to run lukewarm at 5 below, so I think it's working OK.

[Eric]

How cold was your "cold snap," 10F (based on your mentioning the rated output at 10F, I guess that must be it?)

Around here we call that "warming up enough to snow" - my mini-split hasn't run much in the last couple months. ;)

One thing I do want to measure is what (if any) standby draw the mini-split has in cold weather, when it's not in use. Any idea if they keep themselves defrosted "just in case?" We have a Fujitsu 12RLS2

[Marc]

Yeah, that's why I moved from New Hamster!
I don't know how much my system draws. I looked at a Mitsubishi single zone ducted recently during the "cool snap", it's using about 40W when there is no draw for heat - there aren't defrost cycles during those hours because there isn't heat being pulled from the air. I need to check if some of those watts are keeping the fan going on low to sense temperature - I don't think so in this case.

[Bob Lemaire]

Some units have a crankcase heater in the compressor.

[Marc]

Yes, that's a possible draw as well.

[John Semmelhack]

I've had similar experiences to you in very cold weather (starting at ~20F for me). It seems I need to set the controller about 2F higher than my typical setpoint in order to "trick" my MItsubishi 12K wall mount to work a little harder. It certainly has the capacity. I suspect the Mitsubishi "I-See" just wants me to put on another sweater...

[Marc]

Thanks, John, for sharing your experience, helpful to hear.