Sanity check for minisplit selection
As a mechanical engineer I am somewhat embarrassed to admit that I don’t know the NEEP tables and tools inside and out. HVAC is not my core expertise. So I am putting this out to the brain trust. The house in question is my own that I am building in VT, zone 6. It’s a Pretty Good House, with a main level covering 1600 sf. I’ve had a local engineer and the folks down at Energy Vanguard do heat load analysis and we’ve reached agreement that the heating demand is 15000 Btu/h at the design temp of -10F.
My local engineer specified an 18kbtu Mitsubishi unit (SLZ-KF18NA ceiling cassette, SUZ-KA18NAHZ compressor). When I plug this into the NEEP tables I first find that this combination of equipment is not listed. I do find a similar compressor, the SUZ-KA18NAH2 with an entirely different ceiling cassette. I also find that there is no manufacturer’s data on this unit at our design temp even though the manufacturer indicates that it will work down to -22F. Using this equipment combination I find that it provides 90% of my total heat load, but it is only modulating for 50% of the time, with 40% cycling on/off. I also see that these units’ turndown ratio is only about 2.5 – part of the reason it has such a narrow “Goldilocks” zone.
My HVAC technician suggests using equipment from Custom Comfort, a Northeast brand, probably manufactured by Midea. When I look at the CCOXHA4C12 cassette paired with CCOXHASZ12HP, things are entirely different. This also provides over 90% of my total heating load, but it provides 78% of its work when modulating and only 11% low-load cycling. This seems much better, from a 12000 btu unit rather than the 18k specified. The thing is, it reaches its “balance point” at +5F and supplemental heat is required below that. As it turns out, we have close to 200 sf of slate flooring on this level that will have electric radiant installed. With Schluter rating its Ditra Heat at 12.7 W/sf I can get 8600 Btu out of the floor, making up the entire difference at -10F. FWIW, we also have a sealed combustion woodburning fireplace that puts out 48000 Btu when we want it.
So it looks like the Custom Comfort matches out requirements better, not to mention being significantly cheaper. My only concern is the heat pump not being able to keep up with all of our load. Considering our backups, is this really a serious concern or am I just being over-careful?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Yes, there are some downsides to the ceiling cassettes—efficiency, turndown, condensate, cost.
The high head wall units are more common, more efficient, have higher turndowns, higher outputs, and cost less. They are also easier to service, especially if they are located in an exterior wall. Look at MSZ-FS18NA/MUZ-FS18NAH. Unless there are over-riding aesthetic concerns, I would recommend a wall unit.
I think a ducted unit is a better place to start. You don't have to do anything fancy for ducting, can be mounted inside a thick wall with return on the bottom and supply at the top. This lets you use a decent air filter and if you ever want to, it can be ducted to all the rooms.
Something like this can cover all your load and have much better modulation:
https://ashp.neep.org/#!/product/32101/7/25000/95/7500/0///0
The 1.5 ton Custom comfort is a good option as well, don't know if that can be mounted vertically though:
https://ashp.neep.org/#!/product/168984/7/25000/95/7500/0///0
If you really must go for ceiling cassette, try to find the ones that can support jumper ducts (I know LG units but some others as well). This would let you distribute the air to other rooms.
Thanks for the redesign recommendations but that ship has sailed. We have a large open plan space and a centrally mounted cassette will do just fine. Better, in fact than a wall unit and there's no room for ductwork. We've already provided space for its installation and a route for the linesets. So back to the OP, Is it crazy to use a 12k unit that needs 6k-8k of supplemental heat on the very coldest days but that modulates well and provides 100% of the load for the vast majority of heating days? Considering that we have up to 8k worth of radiant floors in the far reaches of the space. And a 40k-50k sealed fireplace.
Your strategy sounds fine as far as loads go—you’ve got two backup heat sources and your heat pump is probably sized for at least 95% of your heating hours. If you are worried put in a length or two of electric baseboard or panels for peace of mind.
From a practical standpoint, a ceiling cassette comes with the downsides already mentioned: pumped condensate, buried linesets, higher install cost, higher service cost, lower efficiency.
If it were my house I’d probably do a wall mount for those reasons. Mitsubishi says its high wall is good for 30+ feet of throw, and in a low load reasonably tight house, the ERV ventilation system (I’m assuming that’s part of the picture) will even out temp differences so your indoor head does not have to be centrally located.
Hope that’s helpful.
That certainly seems reasonable. The one thing I will say having just installed a ceiling cassette: make sure the unit has a follow me setting, and in conjunction with that a wired remote, though if the Cielo smart remote works with it this may work, as it’s ir is much better and it has the follow me command. I was getting a huge amount of short cycling out of mine due to it reading the temp inputs at the unit, and because it was a ceiling unit the issue was much worse than my experience with the wall units. This was compounded with the original remote that had to essentially be pointed directly at the unit. Anyways, just a heads up to make sure your unit can run off remote temp sensors.
Thanks for that. I can certainly see that being an issue. Our current living space is conditioned with a wall mounted minisplit that lets the temperature in the space fall 5-10 degrees before it realizes that it's losing ground because it senses the air temp at the top of the room, not at the dumb wall mounted remote. I was considering a remote thermostat, but now I'll insist.
The NEEP sizing tool will show you how much extra energy you need to cover by aux heat in a year. If the cost of that is reasonable, there is no big issue under sizing a bit.
Sometimes oversizing can improve your COP as the unit will be running at lower load most of the time. This only works if you don't get into cycling issues though.
DC_Contrarian has posted before a really nice spreadsheet before getting a seasonal COP based on Neep data. If you can dig it up, might be worth while to play with it.
Try to avoid using a condensate pump. Not sure if the ceiling units can be plumbed without one, but it is another point of failure that is best designed out.
Here's a link:
https://docs.google.com/spreadsheets/d/14v7VA7I9W6ctAcoPxDZDnEAI4Xvi7JO0g4Rgbe-BHHU/edit?usp=sharing
To use it, make your own copy. You need to know your heating load. You need to go to the NEEP.org website and pick a candidate heat pump (they don't give you much help with that, unfortunately). From the product page, click on "Advanced Data -- System Sizing" and put in your zip code and calculated heating load and click on "Run System Sizing."
In the upper right hand corner of the resulting graph there are three horizontal bars. Click on the bars and select "Download CSV" from the menu. Take the data you get and plug it into the tab labeled "Data from Neep.org" in the spreadsheet.
On the NEEP.org product page there is a section labeled "Performance Specs." Copy the portion of that table that has heating data into the tab on the spreadsheet labeled "Heat Pump Performance Curve," under where it says "paste from NEEP.org website."
On the "Summary" tab put in your calculated heating load and design temperature under "Assumptions."
If all the pastes are done properly the "Summary" tab should now have a detailed list of temperatures, with each temperature having electricity usage at that temperature, how many hours per year at that temperature, and total expected usage. If the heating load at that temperature exceeds the heat pump output, it will show how much backup heat is noted. At the top it will show total projected electricity use.
This is useful for comparing different models and configurations. As Akos notes, sometimes a unit that appears to be oversized actually ends up with lower annual electricity cost because it can run at a higher COP and with less supplemental heat.
This also shows that sizing heat pumps for cold weather is far more complicated than traditional sizing.
Can you say more about your sealed combustion wood burner?
I have a wood stove in a tight, super insulated house and am curious as to how others have gone about integrating wood burners into these settings (air sealing, fresh air intake etc) as it is rare to see an mostly discouraged.
Hi Peter:
I am not part of the "brain trust" but a GBA member with a 24,000 btu high wall mount in an fairly open space of about 500 sq ft. (heat pump sized for expansion to sharing in other areas, and currently in a not greatly insulated house). My comment offering is related to how you will use the heat pump relative to the other identified heat sources. I have lived with my Pioneer (brand) unit for a little over 2 years in zone 6a in New Hampshire. It has worked well heat output wise. But I was surprised that I found that the blowing air movement was somewhat disturbing to me. It has to blow pretty hard to keep heat equalized around the space. Sitting where the unit is blowing on you (hot or cold) is not the most enjoyable. So I think you may find that your floor radiant heat is going to be very desirable and enjoyable and so you will use it more often than you may have planned on, so I would very much think you should consider that in deciding on the size you need.
All the best with your great project.