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Mitsubishi multi-zoned system for passive house too much?

mjezzi | Posted in Mechanicals on

I’m building a 4 bedroom passive house with about 3,200 sqft of conditioned space including a home office and a 2 car garage and 1 car garage (workshop) totaling around 800sqft. I would like to use an 8 zone mitsubishi system with horizontal ducted indoor units. Zones will be assigned to each bedroom (zones 1-4), the home office (zone 5), the main living area (zone 6), the 2 car garage (zone 7), and the worshop (zone 8).

I have a few questions:

1. Can each bedroom and the home office be their own zone? It’s a passive house with really low requirements. The smallest heating and cooling capacity of the 8-zone unit is 54k and 48k btu/h respectively. This would address the sleeping comfort customization for each room. Some people like to sleep in cooler temperatures and others like to sleep in warmer temperatures. Is this 8 zone unit too large for the home where it won’t work efficiently?

2. Since I can go up to 8 zones, I thought it would be nice to be able to condition the garage and workshop on an as-needed basis. Anyone see any issues with this?

As always, the feedback I received here has been amazing! Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    A 3,200-square-foot Passive House is unlikely to have a 54,000 BTU/h design heating load.

    In fact, one of the two design paths for the original German Passivhaus standard requires that the house to be designed with a peak heating load of 10 W/m² (3.1718 BTU/h per square foot). That means that a 3,200 square foot Passive House would have a peak heating load of 10,150 BTU/h. So your proposed heating system is 5 times too big.

    But if you're building a Passive House, you don't have to guess. Your Passive House consultant has completed the PHPP software, right? So you have a detailed room-by-room heating and cooling load analysis.

    Therefore, you know exactly what your loads are. Have you talked to your Passive House consultant?

  2. Jon_Lawrence | | #2


    I have been spending a lot of time recently trying to finalize a multi-zone system for my project which has similar loads to yours. I was also at the NY Passive House Conference this past Friday and spoke with the reps from Mitsubishi and Fujitsu. Some of my takeaways from those conversations are:

    1) The minimum operating capacity of the compressor listed in the marketing brochures is not what you should expect in the real-world. I was told to expect something closer to 1/3 which means that if you have a 4-ton unit, it will be putting out no less than 16k btu's even if the indoor unit is only calling for 4k btu's.
    2) The system needs some way to deal with the excess refrigerant (assuming the minimum operating capacity of the outdoor unit is greater than the capacity of the indoor unit calling for heating or cooling) and it does this by opening the expansion valves in the branch box thus allowing the refrigeration to flow to the other indoor units. The fans will remain off but it will have some impact on the temperature in those other zones.
    3) In this situation, the system will cycle and there will be a large efficiency hit.
    4) This is a problem for high-performance homes. They don't see the same issue with drafty, poorly insulated homes set-up as multi-zones because the minimum loads are typically greater than the systems minimum capacity.

    One of the reps I spoke with who has spent time in the field working with high-performance builders to diagnose the problem said a good work-around is to cut back on the zones, use a non-ducted indoor unit for the main floor which tends to have larger spaces and less walled off rooms, and use a single ducted ceiling or multi-position air handler for the bedrooms. Most of the manufacturers now have ducted units that can handle higher static pressures.

    This has impacted my plans as follows:

    1) Size the system based on an aggressive Manual J. For example, I had my engineer and my installer both perform separate Manual J's and they both set the infiltration at simplified and tight. I asked my installer to change it to blower door and the CFM equivalent of .6 ACH. Doing so reduced the loads by 15%.

    2) Reduce the number of zones so that there is a better match between minimum zone demand and minimum outdoor unit operating capacity. It would be nice if every bedroom was its own zone, and the marketing folks would like you to believe that it works well, but the reality is it does not.

  3. ohioandy | | #3

    Jonathan's right--minisplits are an amazing technology, but they're not capable of overcoming bad HVAC design and installation. Take it from people who have installed way too much capacity--this is most noticeable in a bedroom, where lying quietly in the shifting breezes for eight hours a day allows plenty of time for regret. The indoor head cycles around attempting to make you comfortable, but it just can't do it. Reduce, reduce, reduce. Take your Manual J seriously. A well-sealed and insulated house doesn't need more than a couple zones anyway.

  4. Expert Member
    CARL SEVILLE | | #4

    Mike - I'm with Andy. My 2600 SF house has 3 ductless heads and conditions just fine in Climate Zone 3, in fact most of the time I only have 1 or 2 heads operating. Total capacity is about 24000 btu. One head handles the first floor, and 2 handle 3 bedrooms on the 2nd floor. I use a bath fan to pull air from a hallway head into the 2 bedrooms. It's not built to PH level efficiency but it is very energy efficient. You can read about the construction and the first year of operation in my blogs on this site under Green Building Curmudgeon.

  5. walta100 | | #5

    When getting mini split bids please consider all the contractors will bid minis but most have few if any installs. They tend to bid a head everywhere they would have a register in a conventional system, and when the bids come you get mini and a conventional bid and what do you know the conventional system is much more affordable.

    Do you really need a different zone in each bed room? Probably not if they are on the same level have similar walls and windows. Please understand this is a high performance home all the rooms on the same level will end up being almost the same temp. Also the system is not going to let you heat one room while cooling the other.

    If you install a system with enough capacity to condition large spaces that will rarely if ever be used that excess capacity will cost you in operating costs every month as it will force your system cycle on and off instead of just slowing down. Consider making them separate system so the main system can have a smaller system with higher run times and lower operating costs.

  6. Expert Member
    Dana Dorsett | | #6

    An 8 zone system for a 3200' PassiveHouse will almost certainly result in LESS comfort and LOWER efficiency than 1-3 separate (read "fully modulating") right-sized mini-splits and a modest amount of resistance heaters (right sized for the room loads.)

    Massachusetts Secretary of Energy Matt Beaton's 2.5 ton 3-head Mitsubishi for his PassiveHouse in US climate zone 5 ended up being operated in air conditioning mode at times even in mid-winter, and was complete & utter overkill for heating the place. Now when they want to warm the place up they just cook something in the kitchen or play video games for an hour or something. The last thing they do is turn on the ductless in winter (unless opening the windows doesn't provide enough cooling.)

  7. mjezzi | | #7

    Excellent answers, thank you everyone! I don’t receive GBA emails for replies (not sure why), so I didn’t see the replies until now.

    Dana, your comment about separate minisplits is very interesting. Help me understand why that would be better than a 3 zone multi-zone system? Is it because you can better control the zones if they are fully independent? I know that the Mitsubishi indoor units can only operate at their rated capacity (not variable speed), is that why separate systems are better, because the “indoor” side is truly variable? I can’t imagine a scenario where one would heat and one would cool, but I suppose that would be possible with separate systems.

    Can you explain to me what the purpose of the resistance heaters is in terms of comfort? Why not just rely on the minisplits?

  8. GBA Editor
    Martin Holladay | | #8

    In homes heated with ductless minisplits, you don't need an indoor unit in each bedroom. That would be overkill. For a small house, the typical installation involves one indoor unit in the living room downstairs, and a second unit in an upstairs hallway. In some cases, homeowners decide to install an electric resistance heater in some of the bedrooms. These heaters are only used a few nights a year, on unusually cold nights.

    Separate single-zone minisplit systems are more efficient than a multi-zone system.

    For more information, see "Bruce Harley’s Minisplit Tips."

  9. mjezzi | | #9

    Thanks Martin,

    I’ve been aganozing over not having zoning per bedroom, but realized that if I had a multi-ducted single zone system for the bedrooms, occupants can have the freedom to leave their doors closed and the temp could be set low -occupants can personalize warmth with blankets. From doing a little research, sleeping in colder temps is better for our sleep anyways, so I think that will be fine.

    Unfortunately the master bedroom will be on the lower floor, so that may need to be its own zone, or share the zone with the rest of the main floor. This effectively creates a setup where each floor has its own single zone mini split that can modulate independent of each other.

    Unfortunately, there’s a independent studio that will need a mini split too since it’s detached from the rest of the home. And ideally I’d like to have mini-splits in the garage and workshop for occasional use, so I’ll probably have a separate multi-zone system for that too. That’s a total of 4 systems! That’s a lot of systems. Utility and comfort are important though.

    Thanks for pointing out how important right sizing a system is, even for variable speed minisplits.

  10. Jdmillar | | #10

    Mike, I suspect you will just need one single-head system per level. The heat flow from room to room should easily match losses.

    "Occasional use" of the garage and workshop to me says baseboard heaters or similar. Will you ever get the return off a minisplit to be worth it, either dollars or carbon?

  11. mjezzi | | #11

    I had an interesting conversation with a Mitsubishi design engineer today. Apparently they are researching low load multi-zone minisplits for high performance commercial applications and they have also been talking about high performance residential applications. It’s just a matter of having enough market demand to justify the R&D. Refrigerant lines are said to be the same size. I’m considering preplumbing the house with lines in anticipation of hooking up right sized indoor units in the future once the technology catches up with high performance homes. I generally don’t like to settle, so I may deal with passive cooling until the right tech exists. We do have cool nights and a lot of wind up there anyways.

  12. Jon_Lawrence | | #12


    I had a Mitsubishi rep at my job site recently and here is what I learned that might be relevant to your situation:

    1) Single-zone modulating units (think FH09/12) beat multi-zone ducted units hands-down, end of story. The multi's never really operate at less than 1/3 of their rated capacity, whereas an 09 can operated down to 1600 btu's of heating;
    2) If you can't avoid multi-zone, keep that 1/3 effective minimum operating capacity of the outdoor unit in mind when determining how much load to cover with each indoor unit. A 2-ton unit's minimum capacity is going to be around 8k btu's so you don't want to have a ducted indoor unit covering just a 4k btu load. Mitsubishi now offers medium-static indoor units (PEAD's starting at 9k btu's) so you can cover more rooms and btu's to help meet that 1/3 capacity target. Also, opt for an outdoor unit with ports as opposed to branch boxes. The rep explained to me the reason why and the best I can relay what he said is the following:

    When the load is less than the minimum capacity, the excess refrigerant is sent to the remaining indoor units, but the fan to those units remains off. They call this thermo-off and it appears to be how most if not all manufacturers of multi-zone units deal with this situation. In a leaky house, not such a big deal, but in a low-load house, it will have a noticeable affect on the rooms served by these other units. To make matters worse, outdoor units utilizing branch boxes have a much harder time dealing with all of the excess refrigerant than the units with outdoor ports. Something like the compressor gets flooded with the refrigerant so it has to spend a lot of energy turning it back to a gas. They have witnessed this in the field and they will see the indoor unit cycle on for 2 minutes, but the outdoor unit will run for an additional 5 minutes dealing with the refrigerant. Much less of an issue with the outdoor port units, but I don't know why and I am not sure if they do yet either. Bottom line, go with port over branch box if possible.

    3) If you go single-zone, the units with an H at the end have a heater pan installed and there is an energy penalty associated with that. I was told that nobody had an issue with the units freezing up around here (NJ) despite the unusually cold temperatures we had this winter, so no need to opt for the H here, but that my be different where you live.

    Originally I was planning on a multi-zone units with ducting and air dampers, but now I will be using single zones in the common areas and a dual-port ducted multi-zone outdoor unit with medium static PEAD indoor units for the bedrooms and other enclosed rooms.

  13. pbout | | #13

    This is a great thread. I'm having similar difficulties deciding what to do on my 2600 sq ft house in CZ 03C. I have 3 bedrooms upstairs and a guest bedroom downstairs, and a big open living/dining room. Remodeling to Very Good House (aiming for 1.0 ACH, high insulation walls/roof). I've been designing using Fujistu specs. The house will have an HRV serving the common space and every bedroom as well.

    Our first look was to use a multi-zone with two zones: one head downstairs, and one head upstairs in the hallway. But my wife didn't want to have to sleep with doors open on the hottest/coldest nights, so I looked at option 2: multi-zone with small (7k) heads in each bedroom, plus a 15k head downstairs. The guest bedroom is in the NE corner of the house and before the remodel was always cold in winter, and option 2 provided backup for this room.

    Reading this, wondering if I should go back to Option 1. Or maybe even two individual systems, 1 upstairs, 1 downstairs.

    I didn't do a Manual J, but roughed our heating load by hand to approx 22k Btu/h. My 5 zone system was more like 45k, so total overkill but (maybe) more zone control.

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