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Space Heating with Water Heater

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  • Space Heating with Water Heater

    I have a question for those that seem to always propose a boiler & stating that a water heater will not keep up, provide adequate heat, and wear out very quickly in regards to space heating.

    I am not HVAC qualified at all so this is why I ask.

    I HAVE however worked on, installed, & replaced water heaters with the system designed by others to be used for space heating. They weren't any type of special super high efficiency or btu models either. Nor is there a separate tank. Just to give an estimated replacement size I've seen is 50 gallon/50,000 btu/around 1300-1500 square feet. And the water heater is the only source of heat.

    Most of the houses are also not super insulated either. Only one I can think of that I have worked on had open cell icynene.

    Our climate (SouthEast) is obviously milder compared to others. But regardless of what you might be able to Google around and find as "average winter temps", we can have temps for a few days in the single digits or weeks with mornings in the 20's. (Depending on cold fronts.)

    So here's my conundrum & question.

    Some in this forum have years of experience, installations, licensing, & sizing as proof that they know what they are doing with HVAC. And they seem to always recommend a boiler for space heating with water saying that a water heater just won't keep up & wear out quicker.

    Meanwhile, every person that has a water heater system (slab heat or forced air heat exchanger) that I've worked on always says their heat is great, bills are low or reasonable, and the water heaters have been lasting around 12 to 15 years. Well, as we all know, that's a "reasonable" amount of time to last in just "normal" water heating conditions these days.

    So my question is: How is this possible? Are the licensed HVAC experts here incorrect? If so, why is the math wrong?

    I'm not doubting anyone here and confess I might be the least educated about the questions I ask.

    I'm just trying to understand the conflict in expert recommendation vs. what I've seen in actual operation.

    I am seriously asking so as to learn and/or see if others may re-think some things.

    Thanks for any answers.
    Last edited by BobsPlumbing; 09-13-2013, 07:17 PM.

  • #2
    Re: Space Heating with Water Heater

    Just a correction or addition.

    The house with the icynene insulation was a much more elaborate system.

    It had solar water heat, a Rinnai tankless, & storage tank/s providing radiant through the slab & second floor with radiant plates.

    I was wrong to mention it as a similar system as just a water heater with a circulating pump.


    • #3
      Re: Space Heating with Water Heater

      If the math is correct, then the setup will work. However:

      - tanks have to be rated for space heating
      - most tanks are not calculated with an I=B=R btu capacity. that has to be figured using an estimated efficiency or combustion tester (50 kbtu in is X (?) kbtu out after efficiency losses)
      - a manual j heat loss has to be performed
      - the tank will not last as long as any boiler, due to excessive condensation from many more heating cycles
      - the tank will not be nearly as efficient as a boiler
      - some municipalities do not allow the same tank to be used for both potable and space heating
      - the tank usually has to be reconfigured for low pressure usage

      It can be done but properly is another thing. I wouldn't do it in my house, my hunting camp or my dog house. IMHO it's something a handyman would do because he can't install the proper equipment.

      ... it was plumbed by Ray Charles and his helper Stevie Wonder


      • #4
        Re: Space Heating with Water Heater

        Everything Punky said is spot on. This configuration is pretty common here in the DC area but only in condos. and occasionally English basements where the heat load is small, typically less than 1000 square feet. One or two person occupancy and one and a half bath setup. The tanks are rated for space heating and have side taps for the space heating loop which serves a fan coil. No low pressure loop just for space heating needed in this setup. Because the fan coil needs a higher water temp. the water heater has to be equipped with a tempering valve. My experience with these heaters is they rarely make it past 10 years. The higher temps coupled with much higher and longer cycling always leads to an early death. Regular preventive maintenance has little or no affect on extending the lifespan.

        I've serviced a few that use a standard gas water heater and low pressure heat loop that serve a few cast iron radiators in an addition. No potable water connection. The homeowners point of view is, he could buy 3 water heaters over the lifespan of a typical boiler and still come out ahead plus a much smaller foot print than a typical cast iron boiler. The building inspectors here have yet to catch on that a regular water heater is not rated for space heating.


        • #5
          Re: Space Heating with Water Heater

          Thanks for the input.

          We pretty much never see any radiators of any type. We do see some slab radiant and forced air with the water heater heating is much more common.

          It makes me think what's a good recommendation for one area maybe isn't a good recommendation for another.

          In some areas it may be better to install a simple & much less expensive water heater design that will handle both your hot water & heat for around 12 years. Then install what will likely be a more efficient new one.

          In other areas it may be better to install a much more expensive and elaborate boiler system that should be more efficient and last longer.

          More questions:

          Why would a "regular" water heater not be ok to use for space heating? Provided the components (piping, etc.) are potable approved?


          • #6
            Re: Space Heating with Water Heater

            Originally posted by BobsPlumbing View Post

            Why would a "regular" water heater not be ok to use for space heating? Provided the components (piping, etc.) are potable approved?
            I think it's more regulations or certifications than anything else. The tank may be of a thicker steel, but I doubt it. I think it's similar to mobile home vs normal-home units. BUT, have a problem and have the wrong tank installed... no insurance for the customer and you get sued.

            ... it was plumbed by Ray Charles and his helper Stevie Wonder


            • #7
              Re: Space Heating with Water Heater

              We have designed and installed combi domestic water and space heating systems all over N. America with great success. We use condensing water heater many of which have been operating for more than 20 years, but a common tank water heater will do if installed properly protecting tank from sustained flue gas condensate. This is especially true in low temperature applications such as radiant floors, walls and ceilings. These are not open systems rather, HX separated.

              Fan coils can be hooked up "direct" and "open", e.g. First Co. fan coils, but as suggested should be protected with a mixing valve.


              • #8
                Re: Space Heating with Water Heater

                Just to add
                IBR -institute of boiler and radiator manufacturers is a defunct and been absorbed by the hydronics institute, that is now also defunct and no more.

                IBR is not a boiler output, like AGA (American Gas Association) or DOE (Department of Energy) ratings which are essentially the input less efficiency, which is not to be confused with AFUE (Annual Fuel Utilization Efficiency)

                IBR is merely a math calculation to account for piping loss delivering the net BTU to the conditioned space. Which is not needed if the heat source and piping our in the conditioned space.

                IBR was developed to help the heating contractor match the boiler to the emitter. If they had so much in IBR radiation, the boiler needed the same IBR.

                This is old school stuff and barely applied anymore except by those that don't really understand its purpose. Since AHRI has absorbed all these organizations, in less than a month, your not allowed to publish and use the IBR symbol, so don't look for it much longer.

                Definition of IBR
                I=B=R Net Rating – the output of the boiler less allowances for piping and pickup in accordance with Hydronic Institute test procedures. This is the rating of to use if sizing based on the amount of radiation in a building. The steam ratings are passed on a piping and pickup allowance of 1.333 and water ratings are based on an allowance of 1.15.


                • #9
                  Re: Space Heating with Water Heater

                  What condensing water heaters have been available for 20 years?