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Tankless water heaters used as boilers- Good or bad idea?

Radhead Member Posts: 2
I have a client who has an associate who is convinced that the best way to build an affordable in-floor radiant heating system is to use a tank-less heater for the boiler. I am looking for opinions from professional installers.


  • Mark Eatherton
    Mark Eatherton Member Posts: 5,839
    edited May 2011
    Affordable = Cheap???

    Not when you end up having to replace it every 5 to 7 years its not...

    I am told, but have not seen them with my own 4 eyes, that there are some condensing thankless water heaters out there that have the ability to modulate around a load. But, they are not cheap.

    AS for misapplying a conventional tankless appliance as a space heating appliance, no, I can not suggest it. I've had to replace more misapplied tankless heaters being used as space heaters than you can shake a stick at...

    You can do it right, the first time, or do it over and over and over again...

    Cheap and efficient are not ever allowed to ride on the same bus together.

    Pick wisely. Money is not easy to find, and hard to keep.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • CMadatMe
    CMadatMe Member Posts: 3,086
    Define Affordable?

    I totally agree with Mark. Ask the associate to define affordable? Does it mean I want radiant heat installed at the least cost without regard to comfort, maintenace and longevity of the heating equipment? That's not affordable, tell him to change the word to "Foolish".
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • meplumber
    meplumber Member Posts: 678
    Best way?

    Try illegal way.  In my area tankless water heaters are not allowed to be installed as central heating appliances.  That doesn't mean that they haven't been installed as such.

    As Mark stated, I have had to replace more of these than I want to count.  The big box store was recommending it to homeowners.  Boilers are designed, built and appropriately rated to be used as a central heating appliance.  Water heaters are designed to heat domestic drinking water.  Completely different operations.

    A boiler is designed to last 25 yrs.  These water heaters last 5 yrs tops in this application.  Pay now or pay every 5 yrs. 
  • Jack
    Jack Member Posts: 1,046
    look at the design

    of a tankless. It is designed for a high delta T (ground water to 120F) and variable flow (.4gpm to 4.3 at 70F rise on my biggest selling Rinnai). Now put that tankless in as a boiler and you will end up with pretty much fixed low flow and low delta T (10,20,30 F). That puts the unit in low fire and it just stays there...and beats the hell out of it.

    Early in the game Rinnai said it was ok to use their tankless as a boiler. I on the other hand, as the rep, told my customers that if they did so...don't call me. There simply were not enough hours in the day to deal with the system issues and it took what was otherwise a really nice reliable product and dragged that reputation thru the mud. Listing issues began to come up and Rinnai reassessed their position and thankfully, pulled the approval for closed loop heating.

    My recommendation is to us a product in accordance with it PRIMARY design criteria. You may drive a sqaure peg in a round hole and get away with it, but it is not a good long term investment.
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