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Radiant Heat / On Demand Water Heater Question

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  • Radiant Heat / On Demand Water Heater Question

    Hi Everyone,

    I am still learning about all "green" ways or other ways to install stuff, so I have a question for you'll.

    I have always done radiant heat with a boiler as the source ( Long Island Thing, I guess )

    I was just on youtube and found this

    He is using an on demand water heater, Think they make one big enough for a 12 zone radiant heat zone system running 18,000 feet of pex?

    Has anyone here ever used or installed a system like this, It's got me curious and wondering how many zones you could do

    Please let me know, I am not an radiant expert but like to learn and usually we use lots of slant fin baseboard.

  • #2
    Re: Radiant Heat / On Demand Water Heater Question

    Looks like he's using a Rinnai heater in the video. Nothing really "green" about that. It's just over 82 percent efficient. If you are going to do it right you want a condensing heater.

    You can't use this setup as a retrofit but works well as a new install. Remember, he's not using a heat exchanger between the domestic and radiant so he has full house pressure on his radiant tubing. This will lead to an early death of the Rinnai or at the very least heavy maintenance to combat the scaling of the heat exchanger.

    I've have done a few similar systems but the drawback is this will void or greatly reduce the warranty on the tankless heater when used as a heat source.

    If you are doing a job that requires 12 zones and 18,000 feet of tubing you must already know the BTU's required. You will likely need several heaters and a big @ss gas line.


    • #3
      Re: Radiant Heat / On Demand Water Heater Question

      If you run the numbers, using a tankless for radiant doesn't make sense. The tankless is putting out water at temperatures far in excess of the floor slab so you have to install a tempering valve on the water heater, which adds restriction, which effects the flow rate through the tankless and though It can be done, why anyone would do it like that escapes me. There's also the warranty issue because the tankless will be operating for much longer periods of time than it was designed for.