Ducted and ductless mini split system installation cost - reasonable?

[Pickngrin]

Hello-

I am looking at having a Fujitsu mini split system (A/C and heat pump) system installed in my old (1921) home (~ 1500 sq ft). Due to aesthetics and lack of open wall space on the first floor, a ducted (not ductless) mini split with 2 ducted units was recommended for the first floor, and then two ductless units in bedrooms upstairs. I have found only one local contractor that installs and services the ducted versions, which are relatively newer than their ductless counterparts. I realize that there are so many factors that go into cost, but I am trying to get a general sense of whether the proposal I received (~ $18K, quite a sum of $) is reasonable. I'm in the metro Boston area.

Thanks for any input.

OccamsShaver

 

[Dana]

Just curious, but who is the Beantown area contractor who installs ducted Fujistus?

Not all models are the same, and can affect cost, as can the complexity/difficulty of the duct design & implementation. What, exactly is in the quote?

Last year I was working on a project on ~3200 square foot house in Martha's Vineyard where one of the proposals was for a 2-ton/3 zone AOU24RLXFZ multi-split compressor with two ASU7RLF heads and one ASU12RLF head , plus a 1.5 ton 18RLFCD, quoted at roughly $15K.

The homeowner instead went with a pair of 2-ton 3- zone systems instead with three 7RLFs on the second 24RLXFZ, in part because the contractor insisted on installing the ducts & slim-duct cassette in the attic, above the insulation rather than in the full basement where it would operate far more efficiently. The 6-zone/2 compressor all ductless solution was about the same money, but after MassSave rebates it came in at about $11-12K out of pocket, which was also financed at 0% interest via MassSave.

Any ducted mini-split should be hard-piped, no flex duct, and if you have a full basement or a crawlspace, that's where it needs to be. Punching holes in the attic and running ducts there results in duct losses and air-handler driven air infiltration.

 

[Sponsor]

 

[Pickngrin]

Just curious, but who is the Beantown area contractor who installs ducted Fujistus?

Not all models are the same, and can affect cost, as can the complexity/difficulty of the duct design & implementation. What, exactly is in the quote?

Last year I was working on a project on ~3200 square foot house in Martha's Vineyard where one of the proposals was for a 2-ton/3 zone AOU24RLXFZ multi-split compressor with two ASU7RLF heads and one ASU12RLF head , plus a 1.5 ton 18RLFCD, quoted at roughly $15K.

The homeowner instead went with a pair of 2-ton 3- zone systems instead with three 7RLFs on the second 24RLXFZ, in part because the contractor insisted on installing the ducts & slim-duct cassette in the attic, above the insulation rather than in the full basement where it would operate far more efficiently. The 6-zone/2 compressor all ductless solution was about the same money, but after MassSave rebates it came in at about $11-12K out of pocket, which was also financed at 0% interest via MassSave.

Any ducted mini-split should be hard-piped, no flex duct, and if you have a full basement or a crawlspace, that's where it needs to be. Punching holes in the attic and running ducts there results in duct losses and air-handler driven air infiltration.

Thanks for your response. The contractor is Papalia Plumbing & HVAC out of Acton. I do have a full basement and that is where they planned to install the ducted system. Unfortunately, the rebates no longer cover the ducted systems. I can do the 0% MassSave HEAT loan, but I am questioning whether this proposal makes sense, given the price tag.

 

[Dana]

Again, what exact equipment is being proposed? Did they specify hard-piped duct? Did they (or anyone else) do a credible Manual-J heat load calculation?

It seems like quite a lot for a 1500' house. A co-worker of mine has a 1700' house of similar vintage in Arlington that's heated with a Mitsubishi FH12 in the living room, another FH12 in the upstairs hall (oversized for the heat load, but chosen for the higher cfm of the head), and an FH09 in the doored-off kitchen (sized for the cooling load. Even THAT is a lot of equipment for his house. (I had proposed a mini-duct in the basement to manage the entire first floor, and another mini-duct unit in the back of one of the bedroom closets, building soffit chases to serve the other upstairs rooms, but he couldn't find a contractor willing/capable of doing the duct designs.) The three mini-split solution came in at about $12K, less than $10K after rebate subsidies. One of the upstairs bedrooms gets a bit cool if it drops below 0F outside (his outside design temp is +9F), so they kept the oil-fired steam system as the "hail Mary" backup, not that they really need it. It would have cost them money just to rip it out anyway.

 

[Pickngrin]

The rep. told me that a Manual J calculation was not required for mini splits unless the mini splits are the only source of heating for the home. Is that correct?

The proposed equipment is
1 x AOU36RLXFZH outdoor system (20 SEER)
2 x ARU12RLF indoor ducted units
2 x ASU7RLF1 indoor units for bedrooms

Thanks again.

 

[Dana]

Seems a bit overkill. The AOU36RLXFZH is good for about 40,000 BTU/hr at Beantown's +12F outside design temp, serving a load that's probably less than 25,000 BTU/hr. It's not clear why you need a pair 12RLF slim ducts and 4 separate zones? I could heat my 2400' house in Worcester at -5F with that much equipment!

I suspect the 2-ton AOU24RLXFZH and a single 12RLF ducted zone and a pair of 7RLF ductless heads would be better matched to your loads. The 7RLF heads are probably oversized for the individual bedroom loads though, and would cycle on/off rather than modulate, but a single 12RLF is good for 15,000 BTU/hr or better @ +12F.


Code may not demand a Manual-J when the mini-splits are not your sole source of heat, but getting the most comfort and efficiency out of the mini-splits demands it.

A typical 1920s bedroom in a house that has retrofitted insulation and storm windows or double-panes would have a load of less than 3000 BTU/hr @ +12F, which is about the MINIMUM out of a 7RLF head. When it's warmer than that out it'll be cycling, not modulating. The key is to have it cycle as little as possible, and at it's lowest speed, which is dependent on how many other zones are calling for heat.

The minimum modulated output of a 12RLF is 3100 BTU/hr @ +47F, the minimum modulated output of either multi-split is 6100 BTU/hr @ +47F. If (as I expect is the case) your whole house heat load at +12F is about 20,000 BTU/hr, the whole house heat load at +47F is going to come in at about 7000 BTU/hr, so the non-bedroom parts would have load of maybe 6K, or roughly at the compressor's minimum modulation at that temp. With two 12RLFs running they'll be forcing each other into a less efficient non-modulating on/off cycling mode whenever it's above freezing outside, but with just one it would be able to run pretty much continuous operation at least until the temps were in the mid-40s.

When only one of the 7RLF is calling for heat it's forced to take the full 6100 BTU/hr out of the compressor, but when more than one zone is calling it can throttle back to a more efficient and more comfortable blower speed. If you have too many zones, each with a minimum modulation of 3100 BTU/hr you can't count on the overlap- they'll all be cycling on/off. So to get any efficiency out of the 7RLFs your are better off with just ONE mini-duct cassette for the rest of the house, sized so that it's running almost continuously.

A pair of ducted systems for the first floor would be a mistake- it's less efficient, less modulating comfort, and would force the 7RLFs to run at a higher/less efficient speed much of the time. To get reasonable efficiency out of the system it MUST have a reasonable load, and if the zone cassettes are ALL oversized for their zone loads the performance will be pretty lousy.

Seriously, short of a Manual-J, at the very least make yourself a spreadsheet and run a room-by-room, zone by zone I=B=R load calculation on your house.

Keep in mind that the 12RLFCD can deliver more than 16,000 BTU/hr at the HSPF testing temperature of +17F, and even if it doesn't quite cover 100% of the load at +12F it'll cover most of it. I'd have to consult the engineering manual if it's kosher to hang an 18RLFCD and a pair of 7RLFs on the 24K compressor. If that combination of equipent works and the calculated load for the first floor is much over 15,000 BTU/hr @+12F it might be a better choice than a single 12RLFCD. But there's just no f'in' way the heat load for the first floor is anywhere near the combined 30,000 BTU/hr output of a pair of 12RLFCDs (at least not with the windows and doors closed! ) .