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Rinnai ru199in for radiant floor heat

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gracecon1
gracecon1 Posts: 6
in Radiant Heating

I am a plumber by trade, not a boiler fitter. I have a brand new rinnai on demand water heater left over from a job. I have a shop with radiant floor heat that requires about 56k btu input. (I had a professional heat loss and design done for the space). Instead of buying another boiler, could I repurpose the water heater to heat the floor? It’s for my own personal use so if it’s not perfect, I don’t mind if it saves me $3k on a boiler. Thanks in advance for the advice.

Comments

  • GroundUp
    GroundUp Posts: 2,561

    Sure you can, but it's not rated for space heating and it's way too big. It'll work until it doesn't, then you'll be buying a boiler anyway.

  • gracecon1
    gracecon1 Posts: 6

    I totally get it, I understand that your trade is very technical. I just have a water heating appliance that I can’t re-sell and if it works for a few years ok, I’d be thrilled. I know it’s not ideal, but if I could use the junk I own for a winter or two, it would be awesome. I’m sure somebody on here can help me shoe horn this boat anchor into service. It does ramp so minimum output is 15k btu.

  • hot_rod
    hot_rod Posts: 28,377

    Not ideal, but plenty of them used for radiant. You will need to size the pump for the pressure drop of the unit.

    I think JLC, maybe Green Builder have run articles on how to pipe them.

    You can find small copper tube boilers for under $2,000. For a one zone system a copper tube works fine, properly piped.

    Screenshot 2026-10-05 at 10.49.17 AM.png

    Shop around, you should find a small mod con boiler for around 2 grand.

    Screenshot 2026-10-05 at 10.54.32 AM.png

    Sell the Rinnai on FB marketplace for the price of a new boiler

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream

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