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DIY Manual J Results in Cold Temperature

vashonz | Posted in Energy Efficiency and Durability on

I recently finished building our house (1200sqft, SIPS, lots of windows east, zone 5B, 12°F 99% temp) with a focus on airsealing.

The HVAC contractor we started with wanted to install a 36k BTU multisplit. I felt this was too big.
Did my own calcs, depending on variables came in between 18-24K BTU. Had two Mitsubishi Split systems installed (6k in Master bedroom, 12k in living room), and electric resistance in entry way, and 2nd bedroom.

It finally got cold (down to 12-15 F last night), have been leaving the split units off most of the time because its only been in the 20-30s at night. Left both on last night, set to 66 in living room, 67 in bedroom. Both modulated (one indicator light, never 2, i.e. not full load) all night, and I was actually too warm around 4am. Leaving for work set the 12k to 65, and turned off the other one.

I know this is an N of 1, and that it’s likely different from sustained low temps, but makes me feel better about “undersizing” our split units. Combined with 4000W of resistance heat back up, and a woodstove, I think we managed to have an energy efficient, comfortable, and reliable HVAC setup.

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  1. bfw577 | | #1

    You should invest in an electricity monitor for your mini splits. You can see exactly how they modulate. Seeing how they run has been a massive help in running both mine efficiently.

    Here is my first floor 12k unit that I had programmed to come on at 3:30 this morning. It will draw 1200 watts at max capacity and 180-200 watts at minimum modulation. The spike at the end to 500 watts is done to send oil through the system to lubricate the compressor at minimum modulation.

  2. bill328 | | #2

    Thanks for sharing. I am in the process of building a passive solar home and have been tortured over the right way to go with our heating system. We are in climate zone 6 at 8,000ft in the mountains of CO. We have an exposed slab floor and are planning for hydronic radiant floor heat. We were wondering if we needed any air handler units in addition to the in-floor heat. We will also have a wood stove on the west side of the house which will only service the kitchen and yoga/living space as well as migrate up to the loft area. The Manual J for our home came in at 50k BTU which seems extremely high for a passive solar home with obsessive air sealing. Any thoughts?

    1. Expert Member
      Dana Dorsett | | #3

      >"The Manual J for our home came in at 50k BTU which seems extremely high for a passive solar home with obsessive air sealing. Any thoughts?"

      Many MANY passive solar designs have excessive glazing, leading to very high peak loads. Less glass and higher performance glass is usually what it takes to fix the peak load problem. But if the house is already designed and the windows already installed, it is what it is...

      For the record, what fraction of the total load in the Manual-J is window loss?
      Who ran the load numbers? (You, an HVAC contractor, an architect, an engineer or... ???)

    2. Expert Member
      NICK KEENAN | | #4

      Do you have the Manual J? It should show room-by-room loads as well as by surface type -- e.g., walls, roofs, doors, windows. For each surface type it should say how many square feet and what the modeled heat loss is. It should also say for each surface what the assumed R-value or U-value is. From that it should be straightforward to calculate what the impact of various changes would be on the bottom line number.

  3. Expert Member
    NICK KEENAN | | #5

    A little OT, but I thought "results" in the title was a verb, not a noun. I was expecting a thread on what to do with an undersized heating system!

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