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Q&A Spotlight

Are Seven Heads Better Than Three?

A GBA reader wonders how many ductless minisplit units he should install in his new house

Designing a ductless minisplit system. In a well insulated house, does every bedroom really need a dedicated minisplit head? Or is that just overkill?
Image Credit: Scott Gibson

John Bell, building a 3300-sq. ft. house in eastern Pennsylvania, is weighing his options for heating and cooling, and it comes down to a conventionally ducted air-source heat pump or a multi-head ductless minisplit system made by Fujitsu.

Bell sought several quotes from contractors. The first proposal, parts of which gave Bell pause for thought, was for a 5-ton conventional heat pump. A second HVAC contractor thought that a 2.7-ton system would be enough. So Bell went for a third opinion and so arrived at the ductless minisplit option.

His real dilemma is deciding how to configure a ductless minisplit system in his extremely well insulated house. One bid includes seven minisplit heads.

Each of the heads would be connected to its own compressor outside, and Bell says both he and his wife have no problems with that. Nor will they mind looking at all those fan units on interior walls. He does wonder, however, whether this plan makes any sense.

“It might be overkill with that number of units,” he writes in a post at GreenBuildingAdvisor’s Q&A forum. “I will be ventilating the house with an UltimateAir ERV. I would like to go with the minisplits for the whole house, but I don’t know anyone who has used them.”

Any advice?

That’s the subject of this month’s Q&A Spotlight.

Why so many indoor units?

Bell plans on installing one minisplit head in each of the house’s four bedrooms, one in the living room, one in the kitchen and another in the den.

He thinks that number of fan units would make for a more flexible, versatile heating and cooling system. When his children aren’t at home, for example, their rooms can essentially be shut off.

Plus, he adds, by…

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  1. wjrobinson | | #1

    Be interested to see posted
    Be interested to see posted the exact electric radiant panels used. Have not used such yet.

  2. LenMinNJ | | #2

    HVAC Sizes?
    It would be helpful if you'd post the tonnage or BTU ratings for each of the ductless minisplit systems mentioned in the article.

    We're in New Jersey and building a four-floor home to the Passive House standard (around 4000 square feet including the basement). Our HVAC system design currently consists of an Ultimate Air Recouperator HRV (200 cfm), and two 18k BTU Mitsubishi ductless minisplits, one on the main floor and one on the second (bedroom) floor. Based on the most recent PHPP run, we may be able to reduce the two minisplits to 12k BTU.

  3. Marc Rosenbaum | | #3

    Electric radiant panels
    We've used Enerjoy -

  4. Expert Member
    Dana Dorsett | | #4

    Oversizing mini-splits only benefits efficiency to a point...
    Up to about 1.5x oversizing for the design-condition load of a zone a mini-split gets a pretty good efficiency boost since it still modulates output and only experiences short cycles when the outdoor temps are high (=higher efficiency). This is oversizing is derived by the compressor & blower spend most of operating time at lower speeds, which is much more efficient than at the highest speeds across temperature. See Figure 5 , p18 (.pdf pagination) in this document:

    But when the unit is so oversized that it can't modulate output low enough to match load, and is spending a significant fraction of it's time idling, the standby time isn't free, there is still real power being burned. As understand it, keeping the compressor up to temp for safe startup and keeping the remote-control receivers active takes at few 10s of watts per unit, and can average more than 50W per unit during freezing weather just to keep the compressors happy. That power that is expended outside the house, and is not even offsetting the space heating load at a paltry COP of 1- it's a total write-off!

    Seven tiny 3/4 ton mini-splits would have a combined rated output of 63000BTU/hr, which is probably 2x or more the actual heat load for the whole house, and for the smallest & lowest loss rooms a 9000BTU unit could easily be 5x the heat load of the room, which would more than eliminate any oversizing efficiency advantage to standby idle losses.

    With seven units and a combined rated output more than 2x the peak load, at least 4 or 5 are likely to be idling at any one time, and the wasted standby power is going to average well in excess of 100W, and could be pushing 300W, the amount of power consumed by a small resistance space heater (!).

    With a multi-split approach the compressor is almost always turning, doing moving useful heat with the power rather than burning some to protect the unit during an anticipated startup.

    Most manufacturers have heads starting at 9000BTU and up, and even with multi-splits, putting a 9KBTU head in a room with a
    If this is truly an "...extremely well insulated house" the number of ductless heads can likely be brought down to one per floor, as there are many existence proofs where that works, but it does require a close eye on the room-by-heat load, which will likely be dominated by windows. Buying better windows is cheaper and more effective than buying a mini-split, and windows have no standby losses.

  5. user-985898 | | #5

    Too many mini splits
    Thanks to everyone that has contributed!! I find that our upstairs about 1000sqft will be hard to heat and cool with just one mini because it is not as open as the downstairs. Three bedrooms, two baths,and short hallway leading to bedrooms. If I undercut the doors will that give me enough to heat and cool the rooms? That's the question I have struggled with. The downstairs is more open so I could at least eliminate one mini on that level and have three downstairs. One in kitchen that will also serve the mudroom, laundry rm, and 1/2 bath. one in family rm to serve the hallways and den. One in the master bedroom to serve the master bath and larger hallway leading back to living rm. Plus I just don't know how well the mini's perform and and for that matter, I have never lived in a house built with high R values, air tight construction, and an ERV. Its all uncharted territory for us. So we thought it would be better to have more than less. Then, if we find that the house and min's perform better than we expected I would then shut two or three off and use the others. I guess its hard to imagine two or three mini's working well in the house, whole new concept for me. If you have anymore suggestions that would be great.

  6. LenMinNJ | | #6

    2nd Floor Not Open?
    John wrote:

    " I find that our upstairs about 1000sqft will be hard to heat and cool with just one mini because it is not as open as the downstairs."

    If the ERV's ductwork, inlets and exhausts are properly sited and selected, shouldn't it handle the air distribution adequately and allow just one minisplit per floor to do the job well?

    Where are the inlets and exhausts placed on the second floor?

  7. Expert Member
    Dana Dorsett | | #7

    I'm still doubting you'll need that many upstairs.
    More is definitely NOT better, not more minis, and not more BTU/hour capacity-it'll all work against you in both up-front and operating costs.

    Without the real U-factors on your walls windows & roof and an outside design temp and calculated heating/cooling loads it's hard to say for sure, but even relatively modestly insulated houses with old-school U0.6 windows do better than you might think when the downstairs is heated and the floors are 68-70F from the heated space below. You may need to close the doors to the upstairs rooms to prevent convective heat transfer if you prefer cooler sleeping temps.

    Do the heating & cooling load calculations, then you'll know better what to expect. Leaving the doors open when the rooms/bathrooms are unoccupied reduces temperature difference by quite a bit, but if you have R40 whole wall and U0.20 windows (and not a huge amount of glazed area in any one room) odds are you wouldn't need to bother paying attention. Some temperature balancing can be achieved by sourcing the ventilation air to the doored off rooms via jump-ducts or grills from the common area heated/cooled by the mini-split head with the ventilation exhaust pulled from the rooms separately, but it's a modest effect- you get more out of leaving the door open.

    And if you're still anxious about it and want more heads, go with a multi-split approach. With mini-splits way oversized for the sub-zone loads you're guaranteed to get lousy efficiency out of it, no matter what the HSPF and SEER numbers say. (Those numbers are predicated on a reasonable sizing factor relative to the average load.)

    If extremely well insulated with great windows you could have a whole house heat load under 2 tons, and a 2-3 head 2-ton multi-split may well be your best option. Key to efficiency it to limit the number of compressors and keep the idle time on those compressors low, run-times high, and you'll never get there with a fist-full of tiny mini-splits. If you need to load-balance, resistive radiant heating can pick up the slack with less annual power use than a bunch of idling compressors.

    All good system designs start with a careful load calculation, from which the rest must flow. It doesn't read as if any of the HVAC contractors have run the real numbers, and they are probably shooting from the hip with rules of thumb or best-guesses inappropriate to a super-insulated house. Five tons would be ridiculous for even a code-min house that size, and even 3 tons may be overkill by half. Find somebody who can do the real room-by-room numbers, THEN start thinking about where to put the ductless heads, and what the appropriate sizing would be for the zones, etc.

  8. user-984364 | | #8

    On the other end of the spectrum ...
    We have a 1931 house with 1400 square feet. Replacement windows and retrofitted cellulose insulation in the walls for insulation & air tightness - measured 1400cfm leakage prior to the cellulose. Climate zone 1 in the upper midwest. We just put a single 12,000 BTU Fujitsu at the top of the stairs. We did a heat gain calc and it came out at just about 12k BTU/hr.

    The single head is not *quite* enough to keep the whole house perfectly comfortable; on our 100F+ days it got up to about 82F downstairs - not bad, but not perfect. Another 9k unit downstairs would certainly do the trick, I think. Granted the house you're talking about is twice as large, but probably also twice as insulated and twice as tight. I can't imagine 7 heads would be required just based on our experience.

    Have you considered the ducted mini-split units, if it's primarily the distribution that's worrying you?

  9. Marc Rosenbaum | | #9

    I didn't realize that GBA edited my original response when they asked my opinion. What was left out was at the end:
    We have put up to seven individual cassettes in a superinsulated house without issues, albeit on only two condensers. What we tend to do on low load homes with more closable rooms is use a ducted system or two, because then the capacity of the system is much closer to the actual load of the house.

  10. wjrobinson | | #10

    John Bell, yaa need to hire
    John Bell, yaa need to hire one of the talked about companies here that has installed many Splits over the years. At least for the design!! The yap here is good for starters but no one should design a HVAC system from some chatting at a web site. IMO. IF yaa just need a little pointer... great, but.... make a call.... write a check. Be happy.

  11. GBA Editor
    Martin Holladay | | #11

    Response to Marc Rosenbaum's PS
    I'm a GBA editor, but I'm not the one who removed the last two sentences of your response. My guess is that it was a cut-and-paste error that happened before I saw the text. In any case, the text has been restored, so your response now reads (I hope) the way you wrote it. Sorry for the slip-up.

  12. user-998246 | | #12

    Like Len said, I would think that proper air/heat circulation
    will have a big effect. If your ERV/HRV is laid out properly, it should be able to handle some of the circulation needed to get by with fewer minisplit units. Ask yourself whether a single unit would be adequate or appropriate for each of your levels with no walls, just one big open foot print. I suspect the answer is yes and then some. Then try to determine how to mitigate or resolve those walls/obstructions via ducting, placement, under cutting the doors, ERV piping, etc. But 7 units seems like an overkill and buying units, labor, maintenance to just turn them off later because thy're unnecessary is not advisable either. In any event, it sounds like an interesting project and good luck.

  13. Jesse Thompson | | #13

    ERV / HRV don't circulate heat
    I see this come up all the time, and it deserves mention. ERV & HRV systems DO NOT circulate heat or cold through a house, they bring fresh air into a house that isn't as cold or hot as it could be. In simplest terms, an ERV / HRV system is made up of two sets of tubes that don't touch each other, that run directly from the outdoors to various rooms of the house. Energy from the interior of the house is used to heat or cool incoming air, but it can't move heat around a house to keep one room from getting colder.

    It's not like a typical ducted US forced air system that swirls around all the air inside a house whenever the fan is running.

    If you put your hand up to the diffuser in a bedroom of a house with an HRV in winter, the air coming out of the diffuser will always be colder than the temperature of the air that is leaving the house. Buy a fabulous 90% efficient HRV, and it will only be 10% colder, but it will be colder none the less. A really good HRV in Maine will supply 65 degree air to a bedroom (70 degrees in - 15 degrees out = 55 degree delta T * 90% = 49.5 degrees added to outdoor temp = 64.5 degrees).

    Buy a cheap 75% efficient HRV and you'll be blowing 56 degree air into that bedroom. Do you want your bedroom at 56 F?

    They just aren't heating systems unless you add heat into the system...

    If you want a house that has rooms with even temperatures, insulate the building really really well, or supply energy to all the rooms you want to be warmer or cooler than the outdoors. Don't hope a fresh air ventilation system will circulate heat.

  14. GBA Editor
    Martin Holladay | | #14

    Response to Jesse Thompson
    I think you are introducing fresh air and pulling exhaust air from the wrong rooms. If you want to improve room-to-room temperature imbalances during the winter, you introduce fresh air to the hot room and exhaust stale air from the cold rooms.

    In the case of Carter Scott's design, that means introducing fresh air to the living room and upstairs hallway, and pulling exhaust air from the bedrooms.

    Read more about this technique here: A New Way to Duct HRVs.

  15. user-1005777 | | #15

    Safely circulating air.
    Most fires start in the kitchen. I don't like the idea of taking air out of a bedroom, as that would put a negative pressure in that room. In the event of a fire the smoke will be sucked into that room. Most fire deaths are from smoke inhalation. I would go for the HRV supplying outdoor air into the bedrooms and taking the exhaust from the kitchen and bathroom areas. Kitchen exhaust should be some distance from the stove hood. This could give you a couple minutes of time when the smoke detector starts.

  16. Expert Member
    Dana Dorsett | | #16

    But Roger...
    Kitchens & bathrooms) almost universally get exhaust-only treatment the HRV ducting, for localized humidity control and VOC aspects of cooking, not just the smoke hazard. And the blog article in the link quotes Robb Aldrich's recommendation:

    “ERVs or HRVs have to dump cool air somewhere, so it’s a good idea to dump the fresh air near where the heater is.”

    Kitchen locations are never good candidates for placing the mini-split head (at least not in heating dominated climates.) Putting the HRV supply in the directly heated area near the head tempers the cooler supply air of the HRV. Putting supply-only into the doored off bedrooms with exhausts elsewhere has a cooling effect on the bedroom in winter, warming in summer, which is exactly the opposite of what works for comfort.

    Exhaust-only at the kitchen almost always makes sense, but exhaust only at at the bedrooms for tempering the delta-T also makes sense- they are not mutually exclusive.

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