Devil’s Advocate: Air-source mini-split heat pumps

First of all, we heat our house with air-to-air mini-splits, so I am just playing devil’s advocate, which I love to do. We know the pros of air-source heat pumps right? (consistent electricity prices, often green options for electricity, including onsite PVs, low installation cost, AC “for free”, etc) But let’s hear some potential cons:

1. Fan running all the time on outside compressor (it’s just like an AC compressor… the fan runs hardest when it’s coldest trying to eke out something minimal like 1.2 COP) . Ours happens to be slightly mis-balanced for some reason and they haven’t been able to fix it yet so guess what, it’s NOISY outside when it’s 0F outside. How annoying is that? (Edit: I think we are just unlucky and that air-source heat pumps are almost always pretty much silent. That’s what the specs seem to say… I’m sure ours will get fixed and I’ll post an update here!)

2. Defrost coil. I forget what it’s called exactly (need to refer to the service manual), but our Mitsubishi model has something running all the time (there is a dip switch) which I believe is preventing freeze up when the system happens to be off in the cold. And it’s 50W-100W (I’m not exactly sure… I will report back). So add that up for 7 months. Heating the outdoors as they say. Assuming 50W * 24hr * 30 days * 7 months = 252KWh. That’s what we use for an ENTIRE month in the summer. Ouch. (I’ll update here when I have final measured numbers. SEE ** below)

3. Snow happens. We’ve had a VERY snowy January and 3 feet of snow on the ground. This means I have had to do a little shoveling around the outside compressor (which sits maybe 8 inches off the ground — pad plus little legs) to keep the air flow free for the fan. The fan does a very good job of this itself (causing a huge drift in front of it — photo coming) but it still seems like a good idea to do a little preventive shoveling right around the unit where a little snow piles up. In other words, it is not totally “set it and forget it” like a typical furnace or boiler might be (assuming you don’t forget to schedule a propane or oil delivery!) That said, one needs to make sure the exhaust vents for a fossil fuel system in your house are clear, so I’m not the only one checking/clearing snow. I believe there are new building regs in MA which require vents to be 8 feet off the ground.

4. No central control with multiple heads (for us). I think there are some more expensive models which can control multiple interior heads with one controller (like from Daikin) but ours is not like this. Not a big deal, and it can be seen as a plus, like radiators which typically have a few zones with individual thermostats. Just sayin… it’s not like “forced hot air” systems which are normally just one big zone. Unless you have a very small house and/or mild climate and/or passivhaus-esque envelope and use only a single head.

5. Slow to pick up the slack. It’s maybe silly (I think there are articles out there showing that for a super-efficient house like ours, setbacks don’t end up mattering much at all), but I can’t bring myself to keep the temp pegged at 68F at night. So we do set back the temps a bit. It’s been my impression that in the AM one needs to set the temp very high (like in the 70s) to get the room temp up to snuff relatively quickly, especially if it’s very cold outside and the output of the unit is low. And that’s annoying cause then you need to remember to set it back to 68F again. (The flip side of this is no temp swings… with a heat pump, it is always running (quietly) and so the temp remains constant. Vs our last house with forced-hot air which would crank heat LOUD then turn off then ON again and repeat.)

6. Stuff breaks. And when it does, it doesn’t seem like things are not necessarily in stock and ready to repair THAT DAY like it maybe is for a oil-fired boiler. (Feedback?) It’s been my experience that it means a special order from a regional supplier or sometimes the US distributor and both take time. Now, maybe there is evidence that these things break less-frequently, but… just sayin. I don’t think this is going to happen with a wood stove, is it?

7. Speaking of wood stoves…. even if you are operating with a good COP (of say 3 on a mild day) it is still using electricity, as opposed to solar or wood, which I think are better for the planet!

8. Source Energy…. And speaking of COPs (coefficients of performance) if you live in a cold area, even with the new fancy Mitsubishi and Daikin or Fujitsu models which operate pretty well at 0F and -5F, you are still talking a COP of only 1.5 I believe. (And has anyone verified these numbers? I don’t have reason to doubt, but how would one measure?) Anyway, the problem with a 1.5 COP is that if you assume (in the northeast US) that a high percentage of your electricity is coming from a power-plant using fossil fuels, the problem is that power plant has to burn 3 KWh of fuel to get 1KWh of electricity to your house!

So here’s the math on source energy:
3KWh burned at the plant => 1Kwh delivered to your house * 1.5 COP of heat pump => 1.5 KWh of heat delivered. (50% efficiency, source to site!)

The seasonal numbers reported (in the form of HSPF) is usually like 2.5 COP (if I recall correctly… I use the Canadian scores, which I believe adjust downward, dividing by 1.15 vs the US numbers for whatever zone North Carolina is in I believe). So 2.5 / 3 = 83.3% source to site efficiency averaged over a year maybe?

So… it would have been a LOT better to burn some natural gas or propane right at your house at 90% efficiency. You’d be 90% source to site efficiency!

Or wood! Or sun!

9. Vacation set backs. Unless I am missing something, the lowest the heat settings go on our mitsubishi system goes is 59F. What?!!!!! I would set back my house temp to 45F if I were away if I could! Maybe (I can figure out a way to fool it, like one can with a typical thermostat with those 15F setback timers you can control via X10.) What a hassle though!

10. Oversizing for design temps. I think the 90-something design temp is 6F for our area. And according to manuals, our system is only operating at roughly 30% of rated capacity at that temp. So either you have to oversize, or use some back up heaters — electric radiators seem to be what I hear people use. Or wee have a few small plug-in heaters.

11. Wet clothes after playing in the snow. How do you get them dry? In a typical northern house, I would put them on a radiator or forced-hot-air vent, in the boiler room, in front of the wood stove, etc.
Those don’t exist in this house! And we axed the clothes dryer too! I guess this is where one of those bathroom towel-dryers comes in handy. Or we use a typical dehumidifier in our big mudroom closet. Works ok. But still, just sayin! You can get floor models for air-source heat pumps I guess. Maybe that would do the trick. But that takes up more space.

12. Anything I’m missing? Let me know!

Related:
Heating with Oil or Gas: What’s to Like?
We may not like it, but heating with oil or gas is really convenient.

Is Radiant Floor Heat Really the Best Option?

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** (from our MXZ-4B36NA service manual PDF)

10-1. PRE-HEAT CONTROL
If moisture gets into the refrigerant cycle, or when refrigerant is liquefied and collected in the compressor, it may interfere
the start-up of the compressor.
To improve start-up condition, the compressor is energized even while it is not operating.
This is to generate heat at the winding.
The compressor uses about 50 W when pre-heat control is turned ON.
Pre-heat control is ON at initial setting.

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