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  1. #1
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    Default Water Heater Pulling Double Duty

    I looked at a house yesterday that had a radiant floor heat in a 1st floor addition. It was a hot water loop and the loop got it's hot water from the only gas water heater in the house which also supplied all the potable hot water in the house. I've never seen the potable hot water heater pull double duty like this and also be the heat source for a radiant heating loop.

    The tank was about 8 years old but showed more corrosion and deterioration in the combustion chamber than I typically see for a unit of the same age.

    Aside from extra run time for the water heater, are there any problem with this arrangement I should be considering? I'm not sure how I feel about the potable water being piped through the heating loop for the room addition.

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  2. #2
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    Default Re: Water Heater Pulling Double Duty

    They do make units specifically designed for this dual function from my understanding. Not something common around here.
    It's also common for guys to cheap out and use a regular hot water tank for the heating purpose. It works but the unit tends to die sooner. Fairly common around here during the boom and checkbox idiot reports.
    I can't see using a standard unit for dual duty being safe, efficient or economical. if done properly it could be feasible but lifespan would be an issue.
    My biggest immediate concern would be what type of piping did they use for the radiant heat. if it's standard copper pipe, it may be ok if it was soldered with 95/5. if the heating guy decided to save a few bucks and use 50/50 thats a problem.
    if the piping is one of the standard orange poly type tubing i would write it as dangerous and hazardous due to cross contamination. i would request the Seller or Installer provide written documentation showing the type of tubing used is approved for potable water. if they cannot provide documentation immediately, cease and desist use of all water in home until cross contamination hazard is eliminated.
    Beyond the piping, i would advise my client to consider budgeting for and installing a separated and/or improved system. It sounds like some sort of system upgrade is needed in the near future anyway. The client should be fully aware of the potential costs and take those into consideration.

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  3. #3
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    Default Re: Water Heater Pulling Double Duty

    Thanks Markus. I couldn't see what type of piping was used under the floor but it did have copper supply and return lines running toward that room in the basement.

    "It takes a big man to cry. It takes an even bigger man to laugh at that man". - Jack Handey

  4. #4
    Darrel Hood's Avatar
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    Default Re: Water Heater Pulling Double Duty

    Marcus,
    Where would the closed heating loop experience cross contamination? Cross contamination with what?


  5. #5
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    Default Re: Water Heater Pulling Double Duty

    There should also be a tempering valve installed since the radiant supply is usually cooler than the water for supply.


  6. #6
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    Default Re: Water Heater Pulling Double Duty

    Assuming at this point that the installation is a standard hot water heater, which it seems to be; both the potable water and for the radiant heating water would be running through the same tank system. In that case, if the piping for the radiant floor is some sort of Poly this or that instead of copper, the cross contamination would be the piping itself, if that piping is not rated for potable water.
    Obviously if its a separated tank recirc system and has all the check valves, tempering valves then it probably wouldn't be an issue.
    Assuming the radiant heat is run in a loop, where is the return line run to? Tee'd into the drain valve, Tee'd into the cold water inlet, Tee'd into the TPR valve. All scenarios I have seen.
    What if the return is tee'd into the cold water inlet or even the drain, what happens when there is a pressure drop on the cold side (owner running all the sprinklers in this heat). Take height of floor pipes above water tank, pipe configuration and water quantity into account and one has to ask whether there will be sufficient back pressure from the floor heat water to push its way into the cold supply?
    Haven't you ever been on an inspection where the first few feet of cold water pipe of the tank is actually hot? Granted its usually a bad tube but not always.

    Last edited by Markus Keller; 07-05-2012 at 08:13 AM. Reason: addition
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  7. #7
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    Default Re: Water Heater Pulling Double Duty

    Even if there is no chance for cross contamination, it seems to make better sense to a have a separate water heater or boiler dedicated just for the radiant heating loop to reduce wear on a tank that is serving both purposes.

    "It takes a big man to cry. It takes an even bigger man to laugh at that man". - Jack Handey

  8. #8
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    Default Re: Water Heater Pulling Double Duty

    A couple of things:

    Why on earth would you spend $3,000 for a hot water boiler for an in-floor radiant heat system when you can get by with a $500 water heater? So you have to replace it every eight years, the numbers look better for the water heater.

    UNLESS: The boiler needs to heat water above 140 deg, a hot water heater won't (shouldn't) do that. You need a real boiler.

    BUT if you need a separate closed loop forget about the $500 water heater. I've yet to see one in that price range that will have a separate section for domestic water and heating water.

    AND if you try to use a single heater for both purposes you'll have a problem with the stagnate water in the heating loop and the water starts to smell and feel slimy.

    Ask me how I know . . .


  9. #9
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    Default Re: Water Heater Pulling Double Duty

    I see very few water based radiant heating systems, And this is the first one I've seen where the potable water heater doubles as the radiant heat source so I appreciate all the feedback and input

    You're right Rod. A boiler for the system which only heats one room in the entire house would be overkill and an excessive expense. A $600-$800 gas water heater tank would likely most the best economical sense.

    Actually, the entire radiant heating installation in this house was excessive. The room with the radiant system had HVAC supply registers for the gas fired furnace so it already has a primary heat source. The radiant loop just seemed to be more than what was needed.

    "It takes a big man to cry. It takes an even bigger man to laugh at that man". - Jack Handey

  10. #10
    Leigh Goodman's Avatar
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    Question Re: Water Heater Pulling Double Duty

    [QUOTE=Markus Keller;202128]They do make units specifically designed for this dual function from my understanding. Not something common around here.
    It's also common for guys to cheap out and use a regular hot water tank for the heating purpose. It works but the unit tends to die sooner. Fairly common around here during the boom and checkbox idiot reports.

    How does the style of inspection report influence the installation technique of any system?


  11. #11
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    Default Re: Water Heater Pulling Double Duty

    Wondering how they controlled the room temperature?
    I would assume they still wanted hot water in summer requiring radiant flow as well. (possibly while trying to air condition simutaneously)
    As someone said, not very efficient (or comfortable)


  12. #12
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    Default Re: Water Heater Pulling Double Duty

    There would/should be thermostat and zone valves. Summer months the valves would be closed as per the thermostat setting.


  13. #13
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    Default Re: Water Heater Pulling Double Duty

    Quote Originally Posted by Sal Castro View Post
    Wondering how they controlled the room temperature?
    I would assume they still wanted hot water in summer requiring radiant flow as well. (possibly while trying to air condition simutaneously)
    As someone said, not very efficient (or comfortable)
    If domestic hot water is needed the city water pressure would push the water thru to the outlets. The water would not flow thru the radiant loop unless it was designed in series which would be very silly.

    A system like this would require a bronze fitted pump that would pull water from the tank and then push it thru the in floor radiant. Unless the pump is running there would be no flow thru the loop because the water will take the path of least resistance. Because there is no flow in the warmer months the water in the loop gets stagnant. Therein lies the problem.

    Another problem is that the water serving the radiant will be the same temp as the domestic water which is too HOT for in floor use.


  14. #14
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    Default Re: Water Heater Pulling Double Duty

    More than half the house here in Santa Fe have hydronic heat.

    The domestic hot water must be isolated from the radiant in-floor distribution lines. Mixing radiant water with domestic could be a health hazard (bacteria).

    If a hot water system supplies both there should be an isolation device such as (Bell & Gossett) B & G type Hydronic BPX brazed plate heat exchanger.

    BP400-20MT - Bell & Gossett BP400-20MT - 125,000 BTU/Hr Hydronic BPX Brazed Plate Heat Exchanger With Mounting Tabs

    The radiant water supply is isolated and would require a separate circulating pump and zone control valves and possibly mixing valves on the domestic hot water side and the hydronic heat side as well. The boiler / water heater can then be operated at higher temperatures.

    Check out this site for radiant heat supplies: PEX - Radiant Heat - Radiant Heating - Plumbing Supplies - PexSupply.com

    Charles @ PreVue Property Inspections, Santa Fe, NM
    http://www.prevuepropertyinspections.com/
    "How can someone with glasses so thick be so stupid?"

  15. #15
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    Default Re: Water Heater Pulling Double Duty

    Of course I should have known there would normally be separation and controls, but I read too quickly ,

    " It's also common for guys to cheap out and use a regular hot water tank for the heating purpose........."

    and without seeing it wrongly assumed it must have been hastily done by a DIYer. oops


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