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Community and Q&A

Mitsubishi mini-splits

Andrew_BostonArea | Posted in Mechanicals on


I’ve read a ton of stuff on this site and learn something all the time, so thanks in advance to all. This is my first post, so please be gentle. 🙂

I live in the Boston, MA area. I am finishing my basement. I want to have the option of both heating/cooling, but I have headroom concerns, so I naturally gravitated to the idea of mini-splits for the basement to avoid any more duct work. In addition, the duct work for my first floor takes up a lot of headroom in the basement. When I pointed that out to several contractors, they suggested incorporating the first floor HVAC into the mini-split system and split the first floor into two separate ducted air handlers, each with a backup hydro coil for heat coming from my existing oil boiler. This will eliminate 90% of the overhead ductwork causing issues with headroom. I have really only discussed this with Mitsubishi dealers. The equipment suggested has been:
– MXZ8C48NAHZ2 as the out door unit
– A SVZKP24NA and a SVZKP12NA to cover the two sides of the first floor
– Two MSZFH09NA’s and one MSZFH06NA for the basement’s three rooms

I got wildly varying quotes in terms of cost months ago. Some contractors only had three 6,000 BTU systems in the basement. All agreed I needed three heads in the basement.

Now that I am close to doing the install, a couple of the contractors have said that their Mitsubishi rep has advised against this plan, even though it is done according to the spec. I’m getting very little detail as to why. One dealer said he was concerned with the small rooms in the basement getting over-heated or over-cooled with the first floor units are calling, even if the basement units are off. I saw that mentioned in an article here as well. The reason for the three units in the basement is that two of the rooms will often have their doors closed, with double-wall and under-door soundproofing, so there won’t be a lot of air flow into them and, in addition, there could be a lot of people in the media room so it was recommended to make that one larger than its size may dictate. See attached to try to help explain.

So, I am at a bit of a loss of what to do. I like the idea of getting back my headroom, being more efficient and having more control over the two sides of the first floor. But, I don’t want to do this if it won’t work correctly or won’t be efficient. I read the article on this site that says 1:1 mapping of indoor to outdoor unit is the best, but I really can’t fit 5 outdoor units outside! At max I could fit two outside units. So I’m wondering if having one outdoor unit for the two indoor 1st floor units and another outdoor unit for the three basement units is better, but I want to optimize my spend and prefer one if it will work just due to outdoor space issues.

Sorry for the long-ness of this, but I’d appreciate any help you can provide. Why do you think the Mitsubishi rep is advising against this? I’ve yet to find a contractor that I feel knows more about this than I do, so it is troubling… if you know of anyone in the Boston area, that alone would be very helpful to know! If you think different equipment will work better, please let me know!

Snips of the first floor and basement layouts are attached with my crude drawing of the plan.


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  1. jwasilko | | #1

    We're also in Boston, and just finished a Mitsubishi project. We worked with Boucher Energy Systems, who we can highly recommend.

    We had fan coils (natural gas boiler) and AC units, and one of the AC units was leaking in the indoor coil. We took this opportunity to switch to heat pumps, as we are served by a town utility and have solar panels, which makes our all-in electric rate under $0.10/KWh.

    Our system has a MXZ8C48NAHZ2 outdoor unit and the indoor air handlers are SVZKP30 (first floor) and SVZKP24NA (second floor). We had a manual J calc done and downsized from 4 tons on the first floor and 2.5 tons on the 2nd floor. We're still a bit big from a cooling perspective but nicely sized for heating.

    Our economic balance point is 15F, and the system has had no problems heating to there.

    The hydrocoils for the Mitsu air handlers are the Aquecoil Cased Coils. They're nicely built:

    We're using Honeywell Prestige IAQ thermostats and the Mitsu thermostat interface.

    We do have a 6K head in the basement for some summer AC and dehumidification, but I can't really help with your room-specific questions.


  2. kelchm | | #2

    "I’ve yet to find a contractor that I feel knows more about this than I do, so it is troubling… "

    I'm all too familiar with this feeling. Unfortunately I think it's very true within the HVAC industry.

    I talked to 8 different contractors when I was trying to make a similar decision earlier this year. I did end up getting a system that does what it's supposed to do, but only after spending literally weeks of my time doing research, reading technical documentation, calling out deficiencies in the design and workmanship and logging the performance issues I was experiencing. You can see some more details on the issues I encountered in my posts:

    If I were doing everything over again I think I would front the cost to have an independent engineer design an appropriate system and then collect quotes from contractors based on that design.

    Some additional thoughts:
    1. The sizing seems very suspect to me -- I realize you are in Boston, but for the size of the home I wouldn't expect to need a 4 ton outdoor unit. Furthermore, your indoor capacity adds up to 60k BTU/h. While this is within the 130% (62k BTU/h) Mitsubishi allows on the MXZ-8C48NAHZ with a branch box and SVZ air handler, I'm not sure it's a good idea. Was a proper manual j load calc done taking into account the insulation and air change rate for the home?

    2. The 6k head in the basement office definitely seems like it will be a problem to me. Not only is it a bad idea to have such a small head on such a large outdoor unit (due to the lower limit on the turn down ratio), it just doesn't make sense for such a small room to be on it's own zone.

    3. If you have the space for two outdoor units, I would look at having one outdoor unit serve the basement and a second for the first floor. This would help keep the sizing of the units on each floor similar

    4. It is possible to disable the fan in thermal-off (either by cutting resistors on the control board or by configuring the thermostat appropriately). This can help mitigate the issue of overheating/overcooling zones which are in thermal-off.

    1. jwasilko | | #3

      I had missed the "I’ve yet to find a contractor that I feel knows more about this than I do, so it is troubling" comment. I felt the same way--first 3 contractors didn't know the Mitsubishi stuff well (especially doing them as a dual-fuel system with fan coils). All 3 had very different (and wrong) Manual-J calculations.

      Boucher finally seemed like a contractor that knew more than me, with lots of experience doing dual-fuel systems.

  3. Expert Member
    Dana Dorsett | | #4

    This Mass-CEC web page is a good resource for finding local contractors (with pricing information!)

    The whole thing sounds like way too much equipment, with most of the heads ridiculously overized for the likely loads, especially for the basement rooms. I suspect that may be what's behind Mitsubishi's reluctance to sign on. A single FH06 head (or a 3/4 mini-duct cassette) would likely more than cover the entire basements heating load, and it has an extremely small (and mostly latent) cooling load, even with a half dozen occupants (at ~350-400 BTU/hr per per person combined latent + sensible) in the media room. The basement plan looks like overkill in the extreme, but the rest of it seems like too much too.

    I suspect the SVZ air handlers may also be way oversized for the first floor zone loads. It's better to right-size them and use some heat strip (not hydro-air from the oil boiler) to make up the difference during cold snaps. Even when it's -5F outside the bulk of the heat of a right-sized SVZ on a hyper heating MXZ will be from the heat pump at a COP >1, with the heat strip making up the shortfall. (Don't oversize the auxiliary heat strip either.) At the even lower duty cycle efficiency of the already oversized boiler and fuel cost of the oil boiler there is zero advantage to a hydro-air coil. Mothball the boiler as the "Hail Mary" backup if the heat pump outright fails (rather than just failing to keep up)- don't idle it, and don't use it for auxiliary heat.

    Take a big step back and get a room by room, zone by zone Manual-J heat load calculation have a third party professional engineer run those numbers, not an HVAC contractor.

    If you have some wintertime oil fill-ups on the boiler it's possible to run a whole-house heat load based on fuel use, which may tell you just how oversized an MXZ-8C48NAHZ is for the whole house load. At +5F outside the -8C48NAHZ can still deliver 54,000 BTU/hr without strip heat, which would make it about 1.5x oversized for my sub-code 1920s antique 2400' 1.5 story house + 1600' of insulated basement in Worcester. (I sure hope you're insulating the foundation walls to code when finishing out that space.) It's probably 3x oversized for your loads. For details on how to run fuel-use based load calculations, see:

    (If you willing to share the fill-up dates and amounts & dates, your ZIP code and the boiler's nameplate BTU in/out I can run the crunch for you right here on the forum.)

  4. MattJF | | #5

    Start with the basics, have an engineer, not HVAC contractor do a manual J to find out how much heat you need. Energy Vanguard is a good place to start. Ideal would be have them design the whole system, then just put it out to bid.

    The 3x basements heads are incredibly oversized. In Boston, you shouldn't even need backup hydro on the correctly chosen heat pumps.

    Where is the duct work you want to eliminate currently?

    The ideal solution is probably one ducted system per floor.

  5. Expert Member
    Dana Dorsett | | #6

    I can't clearly read the house dimensions on the plans, but it looks like may a 1500' footprint to the house(?) , with about 180' of perimeter wall, some of which is adjacent to the garage. With 10' ceilings that would be 1800 square feet of gross above-grade wall area. With a 15% window/floor ratio (yours is probably smaller than that) about 225 square feet of the wall area would be window, another 21' would be the front door so only ~1564 square feet of wall.

    Rough estimating the heat load...
    ...assuming a 99% outside design temp of +8F (Boston is +12F, Framingham +6F), for a 60F temperature difference...

    ...a 2x4/R11 wall at ~ U0.1 .. R20 "whole assembly (U0.05 ) ceiling ...

    ...U0.5 clear glass single panes + clear storms...

    ...U0.5 solid 2" wood door...

    ... the design heat load would be something like.

    Walls: 1561' x U0.1 x 60F= 9366 BTU/hr

    Ceiling: 1500' x U0.05 x 60F= 4500 BTU/hr (assuming it's an attic, not conditioned space above)

    Windows & doors: 286' x U0.5 x 60F= 8580 BTU/hr.

    Add it all up and you're at 22446 BTU/hr of conducted losses for the first floor.

    At ASHRAE 62.2 ventilation rates from Table 4.1a you'd be looking at 30 cfm (= 1800 cubic feet per hour) of ventilation/infiltration losses on a fairly leaky first floor with no bedrooms (it might be that high if the fireplace are leaky) adding another

    1800'cf/h x 0.018 BTU/cf-degree x 60F= 92 BTU/hr

    The whole first would be less than the 25,000 BTU/hr that JUST the SVZ-KP24 delivers (without heat strip) when married to a MXZ-****NAHZ compressor of sufficient capacity.

    With heated bedrooms above you're looking at no more than 20,000 BTU/hr for the first floor heat load. The first floor's cooling load will be even lower than that.

    And that's for a fairly l0w performance house- your might be a tad better than that (?).

    There's absolutely no way you'd need more than the 10,900 BTU/hr heating (unless you are NOT insulating the foundation walls, which would be a serious mistake) or 8100 BTU/hr cooling output of a SEZ-KD09 in the basement. If using joist bays to accommodate most of the duct runs and using hard-piped duct it doesn't need to be a head banger, but you'd have to do some careful measuring and thinking.

    You can probably do the whole thing more cleanly with a 2 ton 3 zone Fujitsu AOU24RLXFZH comparessor, with a 7RLFC ducted cassette for the basement and a pair of 3/4 ton ARU9RLFs for the first floor, and NO strip heat backup:

    The higher static pressure allowance on the ARUxxRLF cassettes allows you get away with skinnier ducts, than a -KDxxNA, and the ability to mount them vertically is a space saver. They don't push static pressures as high as the SVZs, but they also don't need to- you're not running a mile of skinny duct here.

    There are comparable solutions using an MXC-3C24NAHZ or MXC-3C30NAHZ compressors and a pair of 1 ton SVZs and one KD09, (or three KD09s or three 1-ton SVZs), since you're allowed to hang up to 130% of compressor capacity's worth of cassette onto the compressor.

    Bottom line, an -8C48NAHZ and three separate zones in the basement is just plain ridiculous for your likely loads. Paying for a competent and proper load calculation and putting right sized equipment in place would cost less than the proposed solution.

    1. Andrew_BostonArea | | #9

      Dana, just occurred to me I did a new post to this instead of a reply to your post so you might not have seen my reply to you... let me know your thoughts when you get a chance! Thanks!

  6. Andrew_BostonArea | | #7

    OMG. You guys are awesome. I've learned more from your posts in the past few hours than from the 5 or 6 HVAC contractors that have come to my house over the past couple months! Thank you!

    Follow-up actions from me:
    - Jeff - I called Boucher after you recommended them. Thanks! They are booking out in March at this point for installations, so I told them I'd call them back as that would blow up the schedule I have with my basement contractor. Thanks for the other info as well!

    - Matthew - Glad to hear I am not alone! After your recommendation, I did reach out to Energy Vanguard to see if they can help with a design. Let me know if you (or anyone) has a specific person there I should talk to.

    - Dana - wow, that is some awesome info! Also thanks for the idea about the third party engineer. You live in Worcester! I used to live in Sutton! You are welcome to come by any time! :) I am indeed insulating both the walls and the floor of the foundation based on the the things I have seen on this site. :) As to giving you the numbers to run the calculation for me, that would be so awesome. The existing boiler is 172,000 BTU. My zip is 02421. Here's the last year's worth of oil deliveries:
    Date Qty Price Amount
    2019-11-18 204.9 $3.070 629.04
    2019-08-06 112.9 $2.970 335.31
    2019-04-15 119.6 $3.070 367.17
    2019-03-11 149.5 $3.070 458.97
    2019-02-15 167.3 $3.070 513.61
    2019-01-21 203.3 $3.070 624.13
    2018-12-20 170.4 $3.070 523.13

    I can send the full PDF of the plans privately to you if you'd like - just figure I shouldn't blatently put my address out there publicly? Let me know if you want them! Sorry I didn't mention the 2nd and third floor originally - see below for more info. But, I think I am definitely better sealed up than you were guessing so the numbers are likely even better. On the Fujitsu, can you recommend a Boston-area dealer?

    I didn't quite understand why the heat strip is better than the oil hydro coil for the supplemental heat? My oil boiler will continue to be active, both to take care of that 2nd and 3rd floor air handler, but also my indirect hot water tank and baseboard heat I have in my garage (to keep my daughter's room above from being freezing - I realize there is probably a better solution there - but baby steps. :) ).

    - Matt F - Ditto about the third party engineer idea! To answer your question about ductwork, please see the new attached drawing with my crude lines in brown for the existing supply and returns. As you'll see, they basically go right through critical walkways of my rooms and lower the headroom. Once you insulate the floor and soundproof the ceiling, I will only have a couple inches above my head clear and I really want to avoid that. By splitting the 1st floor system in two, there are almost no soffits needed in the middle of the rooms.

    More info about my house and what I've been told before this thread:
    - The house is currently about 3,800 square feet of finished space over three floors. It was built in 2008 if that helps you understand the code for insulation back then. The basement will add about 900 square feet.
    - The current HVAC system is actually two hydronic air handlers calling an oil boiler with two single stage AC compressors (3 tons on the first floor and 4 tons for upstairs). One unit serves the first floor and a second serves the second and third floors. For the purposes of this project, I am ignoring the upstairs system, although I would plan to replace it with a heat pump as well some day if I like how the first attempt goes.
    - Several of the contractors have told me they have done a manual J to calculate the loads for both the basement and 1st floor as I originally laid out. But my mistrust of that lead me here. :)
    - I have been told the office and the media rooms in the basement were given their own unit because the doors will often be closed with a seal underneath the door for soundproofing reasons and little to no airflow to the other rooms. Do you guys not agree with that?

    I await further awesome comments and feedback!


  7. Andrew_BostonArea | | #8

    You guys got my brain going, especially with the idea of the Fujitsu that can be mounted vertically...

    Please take a look at my attached idea. The biggest change from before is that I am seeing if I can create a nook where all three rooms come together in the basement. Would a single 7RLFC as Dana mentioned be able to go there and have supplies go into each room up top with returns from each room down below? Would there be enough airflow to the farthest point of each room? Given that soundproofing is extremely important to me, will sound travel from the ducts really easily between the rooms?

    As to the first floor, will there be enough air flow from the two ARU9RLFs to reach the distances I have? In case you are wondering about the strange routes, that is due to my joists and wanting to get everything up in the ceiling as much as possible and also the desire to reuse all the existing holes in my first floor for the returns. My return for that back right unit shown is actually built into two floor joists, so is there a way to transition that into two round ducts?

    Thanks again,

    1. rondeaunotrondo | | #10

      Andrew, who did you end up going with for HVAC contractors?

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