Tankless water heater as a heating source for radiant floor heating in bathroom

Discussion in 'Tankless Water Heater Forum' started by antonvk, Feb 2, 2020.

  1. antonvk

    antonvk New Member

    Feb 2, 2020
    The bathroom in my house doesn't get enough heat from the forced air primary heating system (it is too far from the furnace, and there is only a small duct going there due to the crawl space restrictions), so the temperature in this room is typically 3-4 degress F lower than the rest of the house (other rooms in the house are fine). I want the temperature in this room to be a few degrees above the rest, so I am thinking about installing a hydronic radiant floor heating underneath the subfloor from the crawl space.

    3 out of 4 walls in this bathroom are the external walls, attic above the room and insulated crawl space below, tile floor, wooden subfloor. The heat loss for this room is likely around 5.500 BTU/h (calculated as a ratio to the entire house), but different calculators give different values in the range 3,200-9,700.

    I want a closed loop for the radiant floor (not mixing the space heating water with the domestic water), so either a separate heater, or an external metal plate heat exchanger to the domestic water heater and an additional pump, or a new dual purpose heater (I need to replace the existing water heater tank anyway).

    The amount of water in the space heating system is about 3.4 gal.

    What heating source options would you recommend for this space and perhaps domestic water heating system, and what are their pros and cons?
    • Completely separate small heater
      • Is it allowed to use a small heater solely for space heating, if this is a secondary heating source for this room?
      • Tankless or tank
      • Gas or electric
        • I hear that electric is much cheaper to install than gas, and maybe even not more expensive to operate for my scale (the gas is more expensive, but not too much for one room, and the heater doesn't require annual maintenance)
        • However, people say that electric has low recovery rate, which is insufficient for floor heating purposes
    • An external heat exchanger for the domestic heating tank
      • It seems that the external metal plate heat exchanger (with the piping) is more expensive than another small heater
      • Plus another pipe
      • Plus heat loss in the exchanger
    • Dual purpose water heater (maybe even with two internal heat exchanger)
      • Brianwhite CombiCor is not available anymore, and I am not aware of any other
      • Plus people say they were extremely unreliable
      • Plus people say that it is impossible to balance the space and domestic heating power in them, so, in order to satisfy the domestic water heating needs, their space heating part will be grossly oversized for my scale, which means short cycling
    • Combi boiler
      • Expensive
      • Space heating side will be grossly oversized for my needs
  2. fitter30

    fitter30 Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Tankless water heaters won't turn down low enough and need more volume of water in the system. Radiate floor systems water only run 5*-10*f over thermostat setting. Remember 90*f floor is warm on the feet. A 2000 watt 20 gallon electric water heater is 6824 btu's.1/2" pex tubing flowing 1 gpm is only 30 btu's per foot of tubing. Towel bars either electric or water don't put out enough btu's.
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  4. Everett DeHart

    Everett DeHart New Member

    Jul 8, 2019
    Why not put a small electric baseboard heater and thermostat in there? Certainly cheaper than what you're considering and if you're only looking at a 3-4 degree temperature differential should be cheap enough to operate.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    Unless you have a huge amount of window area &/or the largest bathroom in Canada it's unlikely the design heat load is even 3000 BTU/hr, let alone 5-9K. It's definitely NOT proportional to it's relative square footage compared to the house size and whole-house load. It's usually MUCH lower than the proportional average, since most people put their big view windows and exterior doors somewhere other than the bathroom.

    That's not enough thermal mass to work with to keep even the smallest gas-burners from short cycling when the zone radiation can't emit a major fraction of a low mass boiler or tankless water heater's load.

    Electric mesh type in-floor radiant operating off a floor thermostat is probably what makes the most sense here from a total comfort point of view. I suspect you don't need more than 200 watts / 682 BTU/hr to make up the shortfall of the ducted air system, but going as much as 2x that isn't going to be crazy, as long as it's operating control is running off the floor temp, not the room temp.

    BTW: Rooms at the end of the duct run that don't keep up is more often a symptom of furnace oversizing than deficiencies with the ducts. The duty cycle is way too low to fully heat the rooms at the end when the system is 3x oversized. Nate Adams (a consultant & contractor in Cleveland OH) does a reasonable job of explaining that in his short videos and downloadable chapters.
    Last edited: Feb 22, 2020
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
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