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Heat pump question

Christoph Stevenson| Posted inGeneral Questionson

Hey there. I stumbled onto this forum after looking for heat pump information for hours.

Anyway, my house is roughly 700 (more like 650) square feet, split entry. All the bedrooms are on the lower level and the upper level is pretty much one big room. Downstairs I am looking into getting a 7000 btu head for out main bedroom/hallway. That’s the easy one. Upstairs I’m not sure between a 12000 btu or a 15000 btu. Multiple installers have told me multiple things. I figured the residual heat from downstairs would rise up and heat a tiny but so the 12000 would do the rest, but I’m not entirely sure.

The climate I live in is on the coast of nova scotia. So it’s rarely too hot (max 25 C in the summer except for the odd day here or there) and in the winter it usually dips to minus 10 C on average but there’s exceptions (random -20 days here or there or worse). Weather is typically all over the place but no real extremes. Thanks for the help.

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Replies

  1. GBA Editor
    Martin Holladay||#1

    Christoph,
    You can't size your heating equipment until you perform a heat load calculation. If you study up, you can do the calculation yourself. Otherwise, you'll have to find a qualified energy consultant, HVAC contractor (a smart one), or mechanical engineer to do it.

    For more information, see:

    How to Perform a Heat-Loss Calculation — Part 1

    How to Perform a Heat-Loss Calculation — Part 2

  2. D Dorsett||#2

    Unless it's the leakiest uninsulated house in NS you probably don't need more than a 9000 BTU (3/4 ton) mini-split for the upstairs. The good ones can still put out almost 11,000 BTU/hr @ -15C/+5F, which is about the 99% outside design temp for most of NS. At -20C it may be as low as 10,000 BTU/hr, but a 3/4 ton Mitsubishi -FH09 or Fujitu -9RLS2-H would still be delivering ~8500-9000 BTU/hr even at -25C.

    At 11.000 BTU/hr for 650 square feet it's a ratio of ~17 BTU//hr per square foot, which would typically mean 2x4 /R11 walls, U0.60 windows and an R19 attic, &/or an unusually high window to floor area ratio. A lot of houses with that description come in under 15 BTU/hr per square foot of conditioned space if they're pretty tight, with modest window/floor ratios.

    http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf

    http://www.mitsubishipro.com/media/946493/fh_product_guide.pdf

    http://smartgreenbuild.com/pdf/Fujitsu-RLS2H.pdf

    It wouldn't be a disaster to bump up to a 1-ton head, but a 1.25 tonner would potentially lead to lower comfort and a lot of on/off cycling rather than modulating during the shoulder seasons. On/off cycling results in much bigger room temperature swings, and lower efficiency.

    If that's actually more like 350' up + 350' down it's an even crazier BTU/ft ratio, and there's no way you'd want anything bigger than the 3/4 ton unit upstairs.

    But run the heat load numbers, see where they end up. As long as you're not more than 1.5x oversized for your load at -15C you won't run into too much on/off cycling, and oversizing by 1.25-1.5x gives you a modest efficiency boost too, since they get better efficiency when modulating at part load than at full speed, and it'll (almost) never have to run full speed.

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