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Q&A Spotlight

One Minisplit or Two?

An architect wonders whether he will need one or two minisplit units to heat and cool his two-story home

Is one minisplit head enough? In a tight, well-insulated house with low heating and cooling loads, will one ductless minisplit head be enough for summer comfort? That's the question an architect in Connecticut is asking.

Christopher Vernott is an architect at work on his own home — a tight, well-insulated house in southeastern Connecticut — and the time has come to rough-in the heating and cooling system.

Because of the double-stud wall construction, triple-glazed windows, and careful air-sealing, his heating and cooling loads are low, he writes in a Q&A post at Green Building Advisor.

Manual J calculations, the standard means of sizing heating and cooling equipment, show he will need 11,000 Btu per hour of heat on the first floor and 5,300 Btu per hour on the second floor. Cooling loads are a little higher: 12,500 Btu/h on the first floor and 7,000 Btu/h on the second floor.

“My safe design strategy includes a [Mitsubishi] Hyper Heat head sized for the first floor load and a minisplit ducted cassette on the second floor with two to three short runs supplying the bedrooms,” he writes. “My second design, somewhat more risky, includes one Hyper Heat head sized for the whole house and located on the first floor.”

Both plans include an energy-recovery ventilator (ERV).

Vernott isn’t as concerned with heating as he is with cooling, in particular the south-facing second-floor bedrooms.

“If I exhaust the first floor with ERV pickups close to the floor level to grab the cool air, and then supply the bedrooms on the second floor with the tempered incoming fresh air, will it be enough to cool down the bedrooms a little?” he asks.

That’s the topic for this Q&A Spotlight.

It depends

GBA senior editor Martin Holladay doesn’t offer much encouragement for Vernott’s one-head plan. Pumping tempered fresh air into second-floor bedrooms won’t be enough to keep them comfortable, he says, referring Vernott to articles on the particular challenges of designing heating and cooling systems for…

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6 Comments

  1. George S | | #1

    short cycling
    "If I put a single head in the second-floor landing area, 5 ft. 6 in. by 7 ft., will the unit potentially short cycle in such a small area, even with all the bedroom doors open?"

    You have a load calculation 5Kbtu/h for heat and 7 kbtu/h for cooling.. When I put in a heat pump the method the vendors used was to pick from the next size up, after calculating the required loads. I think there are 9 kbtu/h units. Oversized means they can handle the design day.. The variable speed compressor allows them to "wind down " to minimize short cycling on low load days

    Again, the new high efficiency split duct heat pumps have a variable speed compressor that minimize short cycling. "High end mini-split heat pump technologies utilize variable speed compressors and fans allowing for a reduction in cycling losses, improved part load control, and enhanced humidity control."*

    That said there is a design alternative.. the ducted minisplit ** , if you want to consider yet another alternative, but I thnk these are mostly for people who don't like the look of the head (go figure). The other alternative is to just put the unit on the bottom floor, and if you are uncomfortable in the summer add an overhead fan, or even a window unit (used only on the most miserable muggy hot days)

    The post says you are in southern CT. From my experience in Massachusetts, there are usually only 3 or 4 days in the dead of August when an overhead fan is not enough

    *http://www.nrel.gov/docs/fy11osti/52175.pdf
    **http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/55010

  2. Veronique Leblanc | | #2

    Green building designer woman wants comfort first!
    First of all, you machos should update and realise there are many women who design and build green houses nowadays...and therefore they build what women want!
    I did retrofit and expand a house in Westchester to 5200sqft and we had that discussion many times with my team. My priority is comfort all the way so I did put 3 mini splits not 2 in a 2.5 storey house: one on ground floor, one on 2nd floor landing and one in 2.5 th floor master suite. And I don't regret it while I paid for them!
    Main facade is South but master suite on top floor also has western windows. And yes, roof overhang does not shade lowering afternoon summer sun. If like me, you have views, outdoor shading will alter them and might suffer outdoor weathering quickly. I love my mini splits because they are so silent and do not blast cold air on your head. They also have dehumidifying function with drain. No window box would match this comfortable silent system. And no, I won't let doors open inside: I want my privacy... Doors exist for that, right???
    You can check out our website for more details and pics at http://www.mamaroneckpassivehouse.com
    And contact the team to visit.
    Veronique

  3. George S | | #3

    So....what do women want.?
    Re...

    "First of all, you machos should update and realise there are many women who design and build green houses nowadays...and therefore they build what women want!"

    My best guess is they want different things (in regards to HVAC, and , probably, life in general)

  4. Christopher Vernott | | #4

    The mini split approach we are using
    I can report from the field that we have decided to go with option 3 - 12k BTU mini split head on the 1st floor and a 9K BTU head at the 2nd floor landing. The $99 window shaker was not an option because we have tilt & turn European style windows which means they are not compatible with window shakers. I still wonder if we can condition the house enough in the summer with the 1st floor head only. I hope to report back on this in July or Aug. next summer.

    GBA responses gave me comfort that short cycling would not be an issue with the 2nd floor mini-split head. The variable speed compressor and the power of convective forces will handle this issue. We have decided to exhaust the bedrooms with the ERV, and the grilles will be located at the far corner of the rooms away from the landing area. Even though the ERV is low volume, it should help pull the conditioned air into the bedrooms from the landing area.

    Even if the 1st floor mini-split head is not enough to condition the house in winter and summer, it will probably work out that we cool with 2nd floor head only, and heat with 1st floor head only. Here is a novel idea: a single wall head unit that can change it's mounting location based on heating season or cooling season. Thanks all!

  5. Kevin Dickson, MSME | | #5

    Dual Hose Portable AC/Heat Pump
    Here's a good looking solution that is a couple notches above the window shaker option, but still under $500: http://www.compactappliance.com/EdgeStar-Extreme-Cool-14000-BTU-Dual-Hose-Portable-Air-Conditioner-Black/AP14001HS.html?mtcpromotion=Criteo%3EAir_Quality%3EPortable_Air_Conditioners%3EResidential_Units%3EAP14001HS&utm_source=Criteo&utm_medium=retargeting&utm_campaign=Air_Quality-Portable_Air_Conditioners-Residential_Units

    In a new home, the rough-in couldn't be simpler - just a pair of 5" PVC pipes through the wall.

  6. Bernard Yee | | #6

    We are going to use one...
    One unit for 2 stories. New home, not quite PGH standards but, pretty tight and insulated. Big difference is we're in PacNW, Seattle, where there are few really hot days. Our thoughts - we have stairs to the roof and can prop hatch to let out heat, and we can try window coverings and ceiling fans. We can try the upstairs minisplit in a couple years if it doesnt work. Our heating load is less risky, with radiant wall panels in the upstairs BR on those rare really cold nights.

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