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Using a spa heater for radiant floor heat

I'd like to get some opinions on using a spa type heater for radiant floor heat. Here's the situation,  I have about a 600 square foot 3 season room on my house. When I poured the insulated concrete slap I Installed (3) 300' pex loops for future radiant heating. After enjoying the unheated space for several years we found that there may be 2-3 days a month during the winter that heat may come in handy. So my concerns for a heating system do not heavily rely on energy efficiency or perfect temperature control, just some occasional supplemental heat. According to my online research 6kw of heat should suffice my needs. A boiler or hot water heater seems to be overkill for my application so I am thinking about a 5.5 KW spa heater. I understand the need for a expansion tank,  pressure relief valves, pressure safety switch on the pump,  air scoop, etc.  The information I'm lacking is will the heating element last at least a couple of years with an antifreeze mix? And will such a small heater generate enough heat to be at all useful even though the overall kilowatt rating is similar to the boiler recommended? According to my research I only need a circulater pump of 2-3 gpm so I'm thinking it should keep up.


    EBEBRATT-Ed Member Posts: 15,622
    Radiant heat is not good for an on-off situation. To work it needs to be left on. It takes a long time to warm the slab. From a cold start it would take a day to get up to temp then you use it for 2-3 days and shut it down?

    I would rethink using radiant unless you leave it on
  • ratio
    ratio Member Posts: 3,652
    A quick google search shows several electric boilers advertised for use with glycol, so it seems there's nothing inherently prohibiting that, and a BTU is a BTU, regardless of how far removed from the dinosaur juice it is (electric vs nat. gas, etc.); however like EBEBRATT-Ed said, it'll take a while to heat up. As a data point, it takes several weeks to warm (or cool) the slab at my church.
  • Alan (California Radiant) Forbes
    5.5Kw x 3,400 BTU/Kw = 18,700 BTU's

    18,700 BTU's/600 square feet = 31 BTU's per square foot

    Sounds like it will work fine allowing for start-up time as discussed above.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • hot_rod
    hot_rod Member Posts: 22,258
    The small electric boilers have modulation and outdoor reset as some features, that alone make it desirable for an electric radiant.
    The spa heater may not be listed for the temperature or pressure you need to run?

    I've used 6 and 10 gallon electric water heaters. They at least are rated for pressures and temperatures in radiant. I imagine even less $$ com pared to a spa heater.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream