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Looking for a heat source for my new Radiant Slab

Basically I have a 480sq ft 24x20 foot slab I just poured with (2) 300ft 1/2" PEX lines running though it, insulated with 2" rigid pink foam board on the bottom and sides. No windows, 1 7x8ft insulated garage door and a 36x80 door. 12' ceilings. Will be well insulated - R19 on walls and R38 in Ceilings. This is a small garage so I am trying to find a unit that will give me the most space possible. Will be run on propane.

I like the idea of tankless units because I can mount it on the wall out of the way, I have a Tankless DPW Quiteside in my house for heat and hot water, So I was thinking maybe a ODW in the garage and using it for the radiant heat but I called Quiteside and they pretty much said it would not work, something with flow switches and stuff - I like the idea of the condensing unit with PVC discharge and the ODW unit is in my price range - DPW unit is now out of my price range. I see a lot of people use Takagi units but they don't seem as efficient as the Quiteside but look like they are designed on the same principles.

Another option would be a tank heater but this is starting to take up more room, but if it is my best option then it is what it is, I can't afford the $2000+ plus ones everyone talks about, this is my detached garage I want to keep at like 60-65* year round. I will probably only be living at this house for another 5 years or so, so a very expensive boiler would not pay itself off for me plus it would take up a lot of space.

What do you guys suggest for me?



  • hot_rod
    hot_rod Member Posts: 21,898
    do you know the load?

    assume 480 sq. ft at 20 BTU/ sq. ft. would be a load of 9600 BTU/ hr at design conditions. Do a load calc if you can with any of the radiant design software available online, to get your exact load..

    With that small load could electric be an option? A small electric tank water heater with a 3000W element = 10,230 BTU/ hr.

    Or a small gas/ lp fired water heater, there are some stubbies available that could be shelf mounted to save floor space.

    Either electric or convential water heaters may be more expensive to operate than a 90% boiler or tankless but depending on how cold your climate is and how often you heat the space???

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • oceanstatetuning
    oceanstatetuning Member Posts: 2

    True, I would like to stay with a more efficient unit BUT my load is so small that it's probably not even worth it. Electric is not a bad idea, I like the idea of not needing a hole in the side of my garage for the vent. Heat would be going on roughly end of November to probably May or so. I went on here:


    Coldest month, January is 19* average lowest with a average temp of 36*, It gets cold here but nothing extreme... I think I only say it hit the single digits a hand full of times last winter. Although it would suck to get a $1000 electric bill.
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