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Community and Q&A

HVAC for Detached Garage

alcoprop | Posted in General Questions on

I am planning a nice energy efficient detached 48x60x14 building/workshop and I am the builder (what I do for a living) I asked a question about sheathing a couple weeks ago. My plan will be 2×6 walls with R23 Rockwool, 7/16 osb with taped seams, Tyvek, 1.5″ of EPS or Polyiso and most likely Intello on the interior walls and ceiling since I have some left over from a big project. It will have raised heel roof trusses with R60. I will have 2″ of sub slab insulation also. The building will have 3- 12×12 garage doors, 1 man door and 6 windows. I will do some extra gasketing around the garage doors to help with air sealing. The building will have a dividing wall that will separate the 2,880 sqft into approx 1,000 and 1,880.  I am in climate zone 5a. I am in the country and propane is my gas source. What would you do for HVAC? I was planning on going ahead and running pex for in floor radiant heat. And then I figured a couple (or more?) mini split heads. I would have to run a gas line over to the building for the boiler on the radiant which wouldn’t be a huge deal. Open to suggestions/opinions on what you would change and what system you would go with. Thank you!

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  1. paul_wiedefeld | | #1

    Hi Shaun,

    You plan to use it as a garage? How often?

    I think the radiant floor may have a place if it’s being used as a garage. However, a floor that size and thickness will take many hours to warm up, so hopefully you’re getting a lot of use out of the space. I know propane is expensive, but how much does your electricity run? You could get more responsiveness out of forced air and if your electricity is around the national average, a lot more economical system. It would also be easy to plan for solar on a new build and you’ll be able to fit a huge array with those dimensions.

    1. alcoprop | | #2

      Thanks for the reply! It will be used quite a bit. Would like to keep the min temp at around 55 most likely. My woodworking shop will be in it on one side and then I will have a lift and other car stuff on the other side! Our electric is $0.0504 per kWh and our LP is $2.79/gal.

      1. paul_wiedefeld | | #3

        Good to see! Got an electric bill handy? Utilities break down their costs in such different and complex ways, I suspect your electric price is probably higher (but hopefully not). Is it a commercial rate schedule?

        At $2.79/gallon (inflation caveats of course), you’re at $31 dollars per million Btus if your boiler is 98% efficient.

        At $.054/kwh, a heat pump would run you about $5.25/MMBtu with a COP of 3 (easily exceeded if you’re keeping lower indoor temps). Even if your total electric rate is $.15/kWh, still half propane.

        I think some attention should be paid to the ductless splits - their filters aren’t designed for dusty environments. A ducted system could fit some large and deep air filters to keep the equipment clean.

        1. alcoprop | | #4

          I have an older bill That is where I got that $.050. Something else I should look for? I could do a ducted unit with exposed ductwork. Should I have a professional Man J done so I can pick the correct equipment? Thanks for the help

          1. paul_wiedefeld | | #5

            If that rate includes generation, transmission and distribution, that should be pretty comprehensive. Basically, every cost that’s per kWh should be summed. The manual J is a good plan, the garage doors are a wildcard.

  2. tundracycle | | #6

    I faced a similar issue. Mine is garage+foyer+bathroom on ground level and woodshop/photo studio + office on the second level. DC is in a closet on the 1st level. This is Minnesnowta.

    Heat Part I: In-floor radiant was only realistic heat choice as running any kind of forced air when dust is being made just doesn't work unless you like to stop every 10 minutes to clean/change filters. I have 4 zones; Garage (which is fully insulated separate from the rest of the zones), Foyer+bathroom+DC (t-stat is in bathroom and this zone is mostly to keep the bathroom warm, the foyer is just along for the ride), Office (which is fully insulated separate from everything else) and shop/studio space. The plan is to keep the garage @ 42°f unless working in there and the shop/studio @ 50°f unless working in there. The bathroom (+ foyer) will be kept @ 60° and turned up when others are there for a photo shoot. It also has a Toto heated seat. Office likely 68-70°f.

    Cool part I: I installed a large variable speed exhaust fan (7400 CFM IIRC) in the mezzanine above the office. On many days I should be able to open some windows, turn this fan on at a low speed and have sufficient cooling (and exhausting of some airborne wood dust). This will also be used when I switch from woodworking to photo shoots by cleaning as best I can and then turning the fan on high and going through the space with a leaf blower to get the final tiny bits.

    Heat/Cool Part II: 3 head heat-pump ductless mini-split. Two ductless heads in gables of shop/studio space and a ceiling cassette in the office. These work quite well for cooling the spaces in summer and even get enough cool air sinking down to the bathroom. They are not a primary heat source but can help to warm things up faster than waiting on the slabs to warm up. I hope to avoid running these very much when making dust.

    Circulation: I also have a duct + 650 CFM blower for air circulation. One end in the gable of the east end and the other at floor level in the west end foyer.

    Ventilation: The DC closet can vary the amount of return air to the shop vs outside so in some cases I'll crack a window and let the DC and/or big exhaust fan do some exhaust side ventilation. I do not have an ERV or HRV ($$$'s). I'll monitor CO2 levels to see how things go and if necessary install one but I'm already over budget on this project. My hope is that there's enough space that with windows/doors opening/closing and maybe running the circ fan a bit that I'll get enough ventilation.

    1. alcoprop | | #7

      Awesome!! Thanks for the info!

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