air-source mini-split heat pump capacity ratings explained

From the comments here:

“66. I’m going back to post #38 about minisplit capacity and dewpoint. The information in that post contradicted what I understood to be correct, so I checked in with my engineering contact at Mitsubishi. Here’s what he wrote:
“The only relevant factor in determining the available heating capacity is the outdoor wet bulb temperature. This graph below represents guaranteed heating capacities at various wet bulb temperatures.”
This graph shows that the Hyperheat models FE09 and FE18 produce 100% of their nominal capacity down to about 3F wet bulb, and the FE12 down to about 12F wet bulb (I’m eyeballing this graph). In the case of the FE18, I take this to be the capacity they publish at 47F dry bulb, 21,600 BTU/hour, which they show as the maximum capacity in their submittal sheet at 5F dry bulb as well. This graph states that it includes the correction for defrost.
He goes on to say:
“Rated capacity is the capacity as the equipment was tested in a lab setting to obtain the published efficiency values, maximum capacity is the actual capacity that the unit is capable of producing as it operates in the field, you should always use maximum capacity when selecting/sizing equipment for an application.”
So I don’t believe people need to be concerned about dewpoint of the outdoor air when sizing the Mitsubishi Hyperheats. Of course, at temperatures approaching 0F, the wet bulb and the dewpoint get closer together than they do at higher temperatures.”

I would also add my 2 cents to the comments at the article about heating a house solely with minisplits:

Issues which are being compounded:
1 – point source heating
2 – point source cooling
3 – ventilation/fresh air — intake vs exhaust in bedrooms
4 – ventilation — cost of options
5 – heating bedrooms
6 – cooling bedrooms
7 – window sizes effect
8 – low low temp performance
9 – reliability concerns
10 – supplementing w/ wood vs small gas heaters vs electric-resistance heat
11 – the effects of attachment parenting (young kids sleeping in the same room as parents) and/or dogs sleeping in the room

Commments (as someone who lived in such a house in MA for 3 years):

Point source heating works is fine. You can really compare minisplits to a wood stove, because there is almsot 0 temperature differential, which is frustrating because one can’t cozy up to a minisplit, or dry soaking wet clothes after sledding, or keep one room warmer or cooler than others.

Point source cooling is a bigger problem especially if you are co-sleeping and have 4 people in one room because that’s a lot of body heat and if you don’t have a minisplit right in the room it’s less than ideal in the summer even with the door open. 1 or 2 people was fine, even better if one switched the HRV direction I bet! Good idea.

… I like the idea of doing exhaust for ERV/HRV from bedrooms. That would probably be a good idea. Luckily it’s also easy to reroute the supply/exhaust ducts so an easy retro-project.

Even R-5 windows are cool in the winter if you have a small room and you a sitting or sleeping right next to one.

we never had problems with 0F heating even though our minisplits were only rated for 5F.

On the topic of back-up heat, I would probably go for a wood stove and off-grid/hybrid/grid tie solar PVs if I were still in the house. An all-electric house is kinda frustrating when the power goes out and it’s bright and sunny after the storm and your grid-tie PVs can’t do anything. 🙂 Those little honda generators seem nice but I hate changing oil/maintaining stuff. But honestly, a $50 inverter attached to a car battery ran the fridge just enough to keep things over a 4 day outage. So how much money does one really want to throw at a minimal problem? A cooler in the car would have worked fine too since it was winter, and it’s not bear country.

Parts can take a little while with a minisplit (control boards), but luckily things rarely happen. Our residential mitsu model was difficult to debug I guess (commercial models might have better troubleshooting tools?) so it was one easier to just replace a whole compressor unit.

Was minorly annoyed I couldn’t set back our mitu system below 59F I think it was for when we were occasionally away. Not that lower would have added up to much in $.

OK, other than that… minisplits in a superinsulated house were fine. Silent (as I sit here and listen to my current noisy forced-hot air system). Comfortable. Cheap/almost non-existent bills.

OK that’s it. Just wanted to chime in a little with little pros and cons.


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