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Need help choosing ductless HVAC proposal.

DanVT | Posted in Mechanicals on

I am giving a lot of detail here. I hope the right amount. I’d sure appreciate any help.

I just did a major solar install, so I am planning to put in mini-split ductless HVAC units in my house. These will be used mainly for heating in the winter (it gets really cold here). I live near Woodstock, VT. We had weeks of subzero temps last winter. I am planning on putting in the hyper heat units now that they are available (I put a single one in my studio last year and am very happy with it). I have gotten three proposals for installing the ductless units, and really don’t know which makes the most sense.

My house is an 1820’s Cape. The footprint of the house (minus attached shop, garage, and porch) is 60 x 25’. This is what is shown in the rudimentary plan I’ve attached. If the plan is not legible, I can attach a larger file. Exterior walls are mostly timber framed (but with 2 x 4 studs in some places). They have fiberglass insulation, except for the end ground floor bedroom, which has blown in cellulose. Windows are 5’ x 2 ½’, triple track with un-insulated glass, and some very old windows with fixed storms. I plastic some of them in the winter. Basement is a rubble stone foundation that has been completely foamed. Second floor is typical of a Cape with reduced footprint. It stays warm enough just with heat coming up from the first floor. Attic has ~30” of cellulose insulation. We did an energy audit a few years back, with a blower door test, and reinsulated some areas (the cellulose) and foaming, and sealed leaks, etc.. Off the mudroom is a shop space not well insulated in the walls, though the cathedral ceiling is foamed. The picture window in the kitchen/ living area is double paned, but not insulated glass and it is old.

I heat mostly with wood (~ 5 to 6 cords) using a fireplace insert in the parlor and a large woodstove in the living/ mudroom area. House stays mostly toasty, though the end bedroom is cool (and the bathroom of it is quite cool/ cold). I have a forced hot air oil furnace system that rarely kicks on when I am burning wood, but is used more in the shoulder seasons (and when I am not home to keep the wood fires burning).

I am looking forward to reducing the amount of wood I have to burn, and the time I spend cutting wood.

I have gotten three proposals. The first two are essentially the same, calling for three indoor mini-split units (at #1, 2, and 3) connected to a single condenser (at #1). They will use Mitsubishi units. The indoor units are sized as such: #1, bedroom: 12,000 BTU; #2, office in line with door to the parlor: 9,000 BTU; #3, living area: 18,000 BTU. Outside condenser at #1 is 42,000 BTU. Total cost ~$12,000.

The third proposal is for three indoor mini-split units (at #’s 1, 2, and 4) and two outdoor condensers (at #’s 1 and 2). It will use Fujitsu units. I have read negative comments online about Fujitsu’s reliability, though they seem less expensive and more efficient than Mitsubishi. This plan calls for a second condenser because the contractor said the length of the outside Freon run from the Condenser #1 site to the indoor mini-split #3 site is long enough (~50 feet) that it will negatively impact the efficiency. I also find the placement of indoor unit #4 less obtrusive than #3, which is in the main space where we hang out. The indoor units are sized as such: #1, bedroom: 7,000 BTU; #2, office in line with door to the parlor: 7,000 BTU; #4, mudroom: 15,000 BTU. Outside condenser at #1 serves indoor units #1 and 2. Outside condenser roughly at #2 serves indoor unit at #4. Total cost ~$7,700.

Clearly the size of the units in the different plans is quite different, as it the cost. Any help would very much appreciate.

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  1. Expert Member
    Dana Dorsett | | #1

    First, the multi-split units have dramatically lower efficiency than dedicated single-head units, one head per compressor. This is partly because the minimum modulated output of a multi-split is in the mid-to-high range of any individual head, so whenever only one head is calling for heat it can't modulate down to it's highest efficiency lower speed.

    It's not just a slight hit in efficiency, it's pretty substantial. Search out the test submittal pages for the exact compressor and interior head combinations they are recommending, and compare the HSPF numbers to the 1-head cold temperature units. Take a look at the minimum output at +47F too- it's an eye-opener. For instance, the 42,000 BTU (cooling) MXZ-5C42 can only modulate down to 7200 BTU/hr @ 47F, which is well above the min-modulated output of a 9000BTU head, and the best-case HSPF is 11.0 rather than 13.5 something for the FH09 as a separate unit, about a 22% hit in efficiency:

    If going with single-head mini-splits and three compressors, the Mitsubishi -FH09NA units modulate down to a very modest 1600 BTU/hr out @ +47F. The somewhat more efficient and much bigger heating capacity Fujitsu -9RLS3H units only modulate down to 3100BTU/hr, but even that is less than half the min-mod output of ANY 2-3 head multi-split. The Fujitsu 15RLS3H can deliver 15,000 BTU/hr @ -15F, but it too modulates down to 3100BTU/hr @ 47F.

    The Mitsubishi -FH15NA delivers about 12.5K at that temp, and only modulates down to ~+5K @ 47F, but it's a decent unit. It likely puts more than the 18,000 BTU head on the 3-head multi-split proposal, and at higher efficiency to boot.

    The heads are typically named for their cooling capacity, but what you're primarily interested in is their heating capacity, so it's important to do some digging. But when used as supplementary heat when heating with wood it kinda matters what it's capacity & efficiency is at -5F or whatever your local outside design temp. Most multi-splits crap out on both efficiency and capacity by -5F, and the modulation ranges are limited.

    A lot of cape style houses have little or no roof overhangs, which is a problem in snow country. Ideally the compressor units would be wall-mounted on brackets above the highest recorded snow depth in your area, protected above the rake of a gabled roof rather than under eaves. But under eaves is still preferable to being out in the open where it is easily snow-clogged, or can be clobbered by ice-dam/icicle releases, etc. If you don't have roof overhangs, build a shed-roof to protect them, taking care that it's high enough above the unit that it doesn't impede air flow.

    In my area I've seen quotes for three 3/4 ton Mitsubishi FH units in the ~$12-14K range, but upsizing that to a FH9 + FH12 + FH15 (to meet/beat the output of your 3-fer multi-split quote at much higher efficiency) should still be under $15K. You can probably get a 9RLS3H + 12RLS3H + 15RLS3H combination of three separate units for less than that, and they would deliver substantially more heat than with the Mitsubishis. The 3/4 ton 9-RLS3H puts out more heat than the 1-ton FH12NA, and WAY more than any 1-ton head on a multi-split. With three independent units it's easier to place them such that there are no long refrigerant lines, and you don't have any single point failure modes (other than the power line coming into the house.) You could have a compressor hit by a falling tree (or any other failure) and you'd still have two others chugging away.

    And during the shoulder seasons when just one or two can keep the place comfortable, you can just turn the other(s) off, getting the maximum modulation efficiency out of the single unit rather than having heads all cycling on/off to deal with the high minimum modulation level of a multi-split compressor unit.

  2. DanVT | | #2

    Dana, Thank you for your detailed answer. As I am in way over my head on this, I understand some of it, but am unclear on some other aspects. I have put a single unit on my studio, and am aware of the snow issue here. So everything will be mounted high enough to be above our very hight snow depth.

    I still am a wee bit concerned about some of the reports I've read about reliability of the Fujitsu units. I don't know if you can say anything about this, or not.

    My take away generally from your wonderfully detailed reply is that I should consider doing with multiple compressors (at least a 2 and 1 scenario?) even for the Mitsubishi proposals, because of the greatly increased efficiency of such a set up (because of the minimum modulated output increased efficiency?). It may be even that going with such a proposal is less expensive than going with multiple heads on a singe compressor (I'll ask).

    The other key question for me is whether the units are sized correctly as there is such a big discrepancy between the Mitsubishi and the Fujitsu proposals in size and price.

    I'd appreciate any additional thoughts and/ or clarifications. I need as much of this in truly layman's terms as possible. Thank you so much, Dana


  3. Expert Member
    Dana Dorsett | | #3

    You really need to have a peek at the "extended temperature capacity" tables for the units in the different proposals. For instance:

    A 3/4 ton Fujitsu -9RLS3H is rated at 9000 BTU/hr for cooling, but in heating mode it can put out about 14,000 BTU/hr @ -5F, and more at +5F.

    The 1-ton Mitsubishi -FH12NA is rated 12,000 BTU/hr for cooling, so it's a bigger unit right? It IS bigger for cooling, but in heating mode it puts out about 14,000 BTU/hr @ +5F, falling to about 10,000 BTU/hr @ -13F.

    The "smaller" Fujitsu packs more heating punch than the "larger" Mitsubishi, even though the 1-ton Mitsubishi clobbers the 3/4-ton Fujitsu on cooling capacity.

    Ideally you would have a "Manual-J" heating & cooling load calculation performed by a third party (not an HVAC company that has a vested interest in the numbers) and size the equipment to cover most if not all of the load at your 99th percentile outdoor temperature bin (aka "99% outside design temperature" see: ). It's a pretty rare HVAC installer that actually does a load calc, and even those that do tend to have anxious thumbs on the scale to avoid the 5AM midwinter call on a sub-zero F night from an irate & shivering customer. If the contractors can't/won't provide you heating output capacity numbers at -5F or -10F you're really shooting in the dark (and so are they). Otherwise, the only way to really compare them is to look all that stuff up yourself.

    Ideally every head would have it's own compressor which avoids the minimum compressor output problem, and it's often cheaper to do it that way. Multi-splits are useful if there isn't enough room to install multiple compressors, as long as the output is reasonably sized for the actual loads. The newer Mitsubishi units don't take quite the efficiency hit that older models do, but they still have substantial minimum-modulation on the compressors. The model numbers really matter quite a bit if going multi-split.

    If a proposed model doesn't appear on Efficiency Vermont's list of subsidy-eligible heat pumps, you might try to figure out why:

    One of the guys in my office is in the process of getting quotes for mini-splits retrofit into his previously oil-heated house in suburban Boston. NONE of the mini-split installers to date have even offered to do a load calculation, and some of the proposals have been truly insane overpriced, oversized, low-efficiency selections guaranteed to rarely modulate at his modest loads. The saner proposals have been for three ductless units, each on it's own compressor. It's looking like it'll down to two Mitsubishi installers, both recommending two MUZ/MSZ-FH09NAs and one MUZ/MSZ-FH12NA. Three FH09s could actually cover his load, but for distribution reasons from doored-off areas an -09 would be marginal on the largest zone, and the -12 won't be ridiculously oversized there.

    There have been a few quotes on three FH09s- one of which was over-the-top crazy, from a contractor who had previously pushed for a head in every room, then later three 1-ton heads on a 2.5 ton multi-split with an HSPF efficiency less than 10, and a minimum compressor output that is above his average wintertime load. (Asked to quote on three FH09s after his proposal was rejected he came back in less than an hour with a quote for $15K. Not exactly the kind of contractor to be doing business with.)

    The only reason my co-worker knows which proposals are crazy and which make sense is because ...

    A: He performed the heat load calculations himself.


    B: He looks up the manufacturers' efficiency rating submittal pages for the proposed equipment, and knows the approximate output at his +10F 99% outside design temp, as well as the minimum modulated output @ +47F (part of the HSPF testing standard.) In your case you'd have to look at the extended capacity tables, not just the submittal pages, since your outside design temp is likely to be ~10-20F lower than his.

    It's been surprising just how clueless a lot of the installers are about sizing for the load, and particularly the issues surrounding minimum compressor output and efficiency issues on the multi-splits.

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