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Advice on minisplit inspector approval

Matt Watson | Posted in Mechanicals on

I’m building a new home with Mitsubishi mini-splits. There are three units total, two 9K units and one 12K. Manual J was done and came it at 24K for heating in zone 5. The house is a simple stacked square 2 story of 847 sq. ft. each floor and a 550 sq. ft finished work shop with one 9K unit. The 12K unit is on the bottom floor and the last 9K unit is at the top of the stairs. The building inspector is not wanting to sign off on the HVAC system because he does not think that the one upstairs 9K unit will get heat into each of the bedrooms, he thinks that I need a unit in each bedroom. Short of replacing the wall mounted unit with a ducted unit, is there any way to convince the inspector that I will meet the building code requirements? He is assuming the doors will be closed at night time on the bedrooms and I don’t know if that is a requirement in the code book or not.

Thanks in advance for your replies, Matt.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Matt,
    It can be hard to educate a building inspector, especially if he or she is stubborn. The cheapest way out of your dilemma, by far, is simply to install a small wall-mounted electric-resistance heater (or a length of electric-resistance baseboard) in each bedroom.

    Once you get your occupancy permit, you can sell the unnecessary heaters on Craig's List. Or leave them in place, for use during rare spells of very cold weather.

  2. Expert Member
    Dana Dorsett | | #2

    If installing resistance heaters, look at the room-by-room load calculation and install the minimum amount necessary to cover the load in each room. That way even if you leave it in place your peak power draw to the house doesn't go up (much.)

    Code typically requires that the room is capable of being heated to 68F at the 99% outside design temperature. If the Manual-J used a higher indoor temp and a colder outdoor temp (many will use the 99.6% outside design temp or colder), re-run the calculation.

    With the room load numbers you may be able to use the U-factor of the partition walls & doors to prove that heating the common area to 75F or maybe a bit higher will heat the doored-off rooms to the code required 68F, but you'll probably need to have a certified P.E. run those numbers to satisfy the inspector. In zone 5 that doesn't usually work in a code-min house, but often does at "Pretty Good House" levels, assuming the bedroom windows are small enough.

  3. Charlie Sullivan | | #3

    My guess is that an inspector who is a stickler on this would not be one to get into the detailed analysis, and would be satisfied with anything that looks like a heater, regardless of the output rating.

    Another thing to consider for situations like this is a Chiltrix air-to-water heat pump which then allows the use of very small and inexpensive heat sources in individual rooms with similar efficiency as a minisplit. The downside is just that it's an unusual system and finding an installer who knows or is ready to learn how to install it right may not be easy.

  4. Jon R | | #4

    IMO, "people will want to keep bedrooms doors closed" and "people won't like 7F+ temperature deltas" are reasonable code assumptions.

  5. Matt Watson | | #5

    Thanks all for the replies. I will see if he will buy off on the electric wall mounted heaters.

  6. Anon3 | | #6

    Buy a $10 heater and put one into each room.

  7. Expert Member
    Dana Dorsett | | #7

    The plug-in space heater approach can cover the load, but it doesn't meet code. A code-compliante heating system has to be both hard-wired, and thermostatically controlled.

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