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  1. #1
    Join Date
    Mar 2007
    Western Montana

    Default Sizing heat source for radiant floor heat

    Can a standard 40 gal gas hot water heater be a sufficient source of heat for a radiant floor heat system?

    8-10 yr old 1-story on slab, about 1200 sq ft
    Northern latitude (montana) where we get usually get at least a few weeks of the year with -20 temps (we live in a more temperate part of the state)
    Heat source is a Rheem natural gas 40 gal hot water heater rated at 38,000 BTU/HR input.
    Plus - this hot water heater (and the domestic HWH) are both in an unconditioned exterior closet (R-19 wall insulation), with an exterior vent for combustion air. So the closet and plumbing must lose at least some heat anyway. Also, seems like a waste of energy maintaining the 40 gal tank at temperature at all times.

    Of course, I have no details on the construction of the concrete slab. I can see 2" rigid foam insulation on the exterior of the slab foundation; don't know how far down it extends, or what is underneath the slab.

    The temp setting on this water heater was set to max, and it was running continuously this morning when it was about 35-40 degrees (I was in short sleeves). When I do see the occasional radiant floor heat system, it usually has a gas boiler system with at LEAST 90-100,000 BTU input.

    As a rule I don't comment on sizing of heating systems unless it is obviously very undersized. But this seems a bit 'on-the-cheap' even to me.
    Luckily, the buyer plans to buy and install a wood stove anyway.


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  2. #2
    Join Date
    Feb 2008
    Caledon, Ontario

    Default Re: Sizing heat source for radiant floor heat

    There are many variables which come into play and I don't think there is an answer other than to say without a heat loss calculation being done you will not know.

    Variables that will affect the heat output.

    1. Insulation levels
    2. location and siting
    3. number of stories
    4. number of loops
    5. depth of loops in slab
    6. water temp in and water temp out temperatures
    7. temperature setting
    9. number of windows/shading
    10. slab insulated or not

  3. #3
    Phil Brody's Avatar
    Phil Brody Guest

    Default Re: Sizing heat source for radiant floor heat

    You actually answered yourself, usually you see 90-100,000 BTU/hr and this is supplying 38,000 and running continuously. When the temp drops lower or the wind kicks up the party's over. Maybe they will just add another heater serially next to it.

  4. #4
    Join Date
    Mar 2007
    Near Philly, Pa.

    Cool Re: Sizing heat source for radiant floor heat

    boilers are more efficient at heating water than tanks with burners made to heat and hold water over time. The domestic potable water is usually supplied at temps. >125F while floor heating is usually only around 100-105F. A boiler with a separate circulator and smart control can fire at a reduced rate to supply the warm water for the floor as needed then ramp up to fire full blast for potable hot water, too as needed and do it very efficiently. A boiler with an indirect tank and smart control would be a much better solution esp. in a cold climate I would bet but I defer to radiant heat specialists such as Mark Eatherton in Colorado. If you really want to go for the gusto, get a modulating condensing boiler with zones and a smart control.


    Keep the fire in the fireplace.

  5. #5
    Mike Denny's Avatar
    Mike Denny Guest

    Default Re: Sizing heat source for radiant floor heat

    What a mess!! Around half the water heaters are not listed for space heating. Check your T&P valve, should be set at 30 PSI and 120 F. Too many other code violations to list.

  6. #6
    Join Date
    Jul 2008

    Default Re: Sizing heat source for radiant floor heat

    A domestic water heater when used for space heating must be listed as such. Usually this requires an ASME pressure vessel rating.

    Simple answer is the 38,000 BTUH will not cut it if used for both domestic and heating hot water.

    I agree with all of the comments above, but there is a chance you could get by on JUST heating the space with 38 MBH if the insulation and envelope are adequate.

  7. #7
    Join Date
    Mar 2007
    New Mexico

    Default Re: Sizing heat source for radiant floor heat

    Unless they've been freezing for the past 10 years, you CAN heat it with a water heater. I agree that it's not going to be a very good system, and probably will have trouble keeping up at the low temps you'll have there. Running constantly at top thermostat setting in March is not a good sign at all.

    There was a good article a few years back in Fine Homebuilding or JLC about a guy in Montana who heated his house (in floor radiant) with a tankless water heater. Said it worked just great. I'd probably look at something like that if it were my place.

    Jim Robinson
    New Mexico, USA

  8. #8
    floyd pfingsten's Avatar
    floyd pfingsten Guest

    Default Re: Sizing heat source for radiant floor heat

    It sounds like the system is undersized. You will need to do a BTU heat loss calculation. What ever figure you come up with, is the least size heating source you will need. Round what ever you come up with to the next sized unit (ie., 35000 - 3700 BTU loss replace it with a 40000 BTU unit). An undersized unit will run all the time, where as an over sized unit (too much over sized will cycle on and off too often). Either way, you will harm the unit with too many start ups and shut downs, or it will run all the time causing too much wear on the system. Good luck.

  9. #9
    David Bell's Avatar
    David Bell Guest

    Default Re: Sizing heat source for radiant floor heat

    Just the fact that the heater runs all the time at the highest setting bothers me. 1) The heater is unable to satisfy the demand. 2) If by some off chance it does satisfy, the water stored in the tank would be too hot for the system as designed. There are on demand tankless systems that would provide better results with little change in piping, and would actually be smaller than that 40 gal tank.


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