Sizing a Multisplit Air Handler
What are the practical effects of undersizing a multi-split air handler for its’ zone load? We are looking at a Mitsubishi multi-split system for our HVAC replacement–Outdoor unit MVZ-4C36NAHZ-U1 with an SVZ air handler in zone 1, and either one (12k) or two (9k) units in other zone(s) depending on whether we go with 2 or three zones total. If we go with a two-ton air handler (SVZKP24NA) it has a heating capacity of 27000 BTUs, but the heating load of the zone is 28817 BTUs, leaving a 1817 BTU deficit. Moving up to the SVZKP30NA leaves us about that much more oversized. The house (in Zone 4) is 110 years old with some opportunities for improved air sealing and insulation, so I find myself wondering whether it would be better to deliberately undersize and look for ways to reduce heating load down the line, but I don’t want my family shivering through winter before I can make those improvements.
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Hi Jameson,
You may find this article from GBA contributor Allison Bailes helpful, as he explains his thinking behind and calculations for undersizing his Mitsubishi minisplit air handler.
Thanks, Kiley. That was a helpful read: I found this heartening:
"Besides, I know that even when I do a Manual J load calculation correctly, it still comes up with a load that's about 10 to 20 percent higher than the actual load."
What are the two 9k heads for? If these are bedrooms, unless they are huge, the typical load is closer to 3k. You can sometimes get a 6k wall mount to work, but 9k is way over sized (speaking from experience as I have one in my bedroom).
Akos, I share your concern around the size of the units, at least the second one in the study. This is the upstairs of a Craftsman bungalow (a "1/2 story") with low ceilings and limited wall space, so floor units work better. Unfortunately, Mitsubishi does not make a 6k floor mount. We're considering the second unit even though single 12k is sufficient for the load because circulation is a problem with the study door shut (a necessity with both of us in phone meetings working from home). I'm actually talking to our contractor now about shoehorning a slim-ducted unit under the eaves behind the north knee wall, which would address sizing and circulation concerns.
Before going too far down the road, if your house footprint is similar to the 2nd story+knee walls, I would question 29k heat load for the main floor in Zone 4 with a house with double pane windows and basic insulated walls. If you have previous fuel use bills, I would check based on that if it matches.
The idea solution would be to get a ducted mini split that can be mounted vertically and mount it on the wall between the bedroom and the office. The ducts can be simple runs as they just poke through the wall on either side. You might need one longer run in the bedroom becasue of the width. The whole thing can than be walled in to look like a jog in the wall.
This is probably more intrusive than you care, the less intrusive would be to go with a 2 zone multi for the second floor with a 9k floor console for the bedroom and 6k wallmount for the office. You should be able to put the wallmount on the wall beside the door to the office (see attached for similar install in a low 1/2 story, cutoff in the picture, beside the wallmount is the door). You can connect these to a MXZ-2C20NA2, no need for hyper heat as these will be used mostly for cooling.
For the main floor unit, go with a dedicated outdoor unit for the air handler. These have much better modulation range than multi splits, so even if you oversize them a bit, all you loose is humidity removal in the shoulder season. This should be a hyper heat as it will provide most of the heat for the house. If the loads are less, you can probably go with one of these:
https://ashp.neep.org/#!/product/31992
If the loads are really higher, I would still go with the above and just add in enough backup electric resistance heat to the air handler to make up the difference. The amount of time these will run is not worth the cost of the larger unit.
Akos, I appreciate your input. That ~29K load includes the first floor (~940 sq. ft, no wall insulation, mix of single- and double-paned windows) + an additional basement tv room (~180 sq ft). Sadly the placement of the door in the study wall in combination with the slope of the roof means a wall unit on that wall can only be place about 40" off the floor, blowing directly in someone's face. So I'm still vacillating on equipment selection for the upstairs, but I'm at least comfortable with the smaller air handler for the downstairs based on feedback here and Allison's article Kiley linked above.
Did you ever find an adequate solution for your 2nd floor? I'm trying to find solutions for my house (zone 5a) and we have a 1/2 storey with a bedroom and office that doesn't cool/heat properly with our current furnace/ac set up. I don't want wall units as the space is small (250 sq/f) so we were recommended a 12,000 btu air handler in the knee wall with ducts to both rooms. But perhaps you found a solution that you could share?
Thanks!