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Thread: tankless water heater for radiant heat

  1. #1
    DIY Senior Member pmayer's Avatar
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    Default tankless water heater for radiant heat

    All,

    I am building a workshop addition to my home and I am planning to install water radiant heat in the slab. My current plan is to use this electric tankless water heater: http://www.tanklesswaterheater.com/technical.html as my heat source. It seems like an economical, logical approach to radiant heating. I know there is a lot of negativity toward tankless heaters in general on this forum, but I believe that a lot of that stems from people's bad experience of running out of hot water in the shower when someone runs the dishwasher while they are in there.

    I am planning a closed loop system, so I believe that a tankless would not be a problem on that front. This unit is only $230 (about 75% cheaper than a comparable boiler), so it is not that risky of a venture, but I am wondering if anyone has any specific reasons why a tankless would not be adequate for radiant heat.

    Thanks for your help.

    Paul

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default heater

    You have to figure out your flow gpm. Then the btu demand for the building. You didn't specify which model you were considering, but I suspect the only ones that would work for your application would be any of the three biggest ones, and I doubt that they will sell for the price you quoted.

  3. #3
    DIY Senior Member pmayer's Avatar
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    Default heater sizing

    Hi HJ,

    Thanks for posting. I forgot to mention that the system has been sized and requires the N-85 model (one of the 3 largest as you suspected). The three largest are all the same price:$230.

    So assuming correct sizing, does anyone foresee issues?

    What is the difference between a boiler and a tankless heater. They seem to be pretty similar, but yet the boiler is so much more $.

    Thanks,

    Paul

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    The largest one lists temp rise at 4 gallons/minute as 24 degrees. My guess is that you would want at least that, and probably more. Four gallons is about 32 pounds * 24 degrees * 60 minutes = 46000 BTU/hour. Would you ever leave this off? Would it ever get below freezing? If so, the derate that by about 20% for the antifreeze you'd need. If you try to pump more than that 4 gpm, the temp rise continually gets smaller and you'd be running up against the restrictions in the device itself. You'd be also using it at a duty cycle likely much higher than designed. The manufacturer may not warranty the item, expecially if they find antifreeze in there.

    A boiler is designed for much higher duty cycles and flow rates and can handle antifreeze, if required.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Senior Member pmayer's Avatar
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    Default

    Thanks jadnashua. These are the types of things that I want to understand. The radiant heat guys have sized this and inidated that I needed about 30,000 BTU for the system. They suggested that a water heater would handle my system fine, but they don't really know anything about water heaters, much less tankless.

    To answer your question about leaving it off, I don't have any intentions of turning it off during the cold months (which in MN is October through April). I would probably set it at 55 degrees and leave it there the whole time. Then the water heater would be shut down from May to October to catch its breath.

    So if this unit is capable of generating enough heat, then it sounds like the question is can it handle the duty cycle. Great question. You are right, the manufacturer has no idea whether their units can handle radiant heat. It is such a niche market that I doubt any of the water heater companies have made any modifications to their systems to accomodate radiant, but I will check with Bosch to see what they think about it. Any idea how I could get information on duty cycle for boiler vs. tankless water heater? This definitely could be an important piece of info.

    My other follow question is this: Would a standard water heat (w/tank) be better suited to handle the duty cycle required for radiant heat?

    Thanks for your help on this.

  6. #6
    Master Plumber master plumber mark's Avatar
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    Talking tankless electric??

    You are better off putting in a wood burning boiler
    than an tankless electric water rheater....

    to heat your underenath slab



    you are looking at nothing but trouble

  7. #7
    DIY Senior Member pmayer's Avatar
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    Default can you clarify why it will be trouble?

    Thanks for your response Mark. I am not considering a wood boiler, but I would consider an electric boiler, or a conventional electric water heater if there is something inherantly innappropriate about using a tankless water heater for radiant heat. I have heard of some people installing tankless heaters with radiant heat systems approximately my size and have had no problems. I have not heard of anyone who has tried a tankless heater and had problems. That said, I am fully aware that we are not dealing with many data points on this, and I have not talked to or heard of anyone that has been doing so for 5+ years. That would certainly make my decision easier, but I don't believe that adequate evidence exists one way or the other. I will certainly give carful consideration to the opinion of any master plumber or other qualified people, but I just want to understand in more scientific terms why this will or will not work, rather than a general bias against tankless.

    jadnashua hit on something that I am concerned about, but I want to clarify it the advantage is simply boiler versus tankless wh, or if a conventional water heater would have a similar duty cycle advantage over tankless. If the duty cycle issue is a wash between tankless and conventional water heaters, then my perspective is a bit different as there are considerable data points to suggest that a conventional water heater can handle the needs of the system I am planning.

    Thanks!
    Last edited by pmayer; 09-10-2006 at 12:20 PM.

  8. #8
    Master Plumber master plumber mark's Avatar
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    Talking tankless heaters

    Here is the information I have colected over the past few

    years on tankless water heaters.....


    I usually just send my "I am hell bent on getting a tankless heater"

    customers to this page and it scares the living daylights out of them



    http://www.weilhammerplumbing.com/houseofhorrors/

  9. #9
    In the Trades Bob NH's Avatar
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    Default

    I would never install any kind of electric heat in a slab on the ground. No matter how much you insulate, you are going to lose heat to the earth. And electric heat requires 293 kWHr per million BTUs, or about $36 per million BTUs at the rates we pay in New England.

    A standard water heater is usually 4500 Watts, which is about 15,350 BTU/hour.

    You could use a simple electric baseboard or electric radiant heater system and save on both power and capital cost. You would not lose by having hot water pipes in the earth and you would not need any circulation system.

    If you have natural gas or propane you could use one of the small wall-vented gas heaters.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    Most of the system don't want you to run a recirculation pump because it causes the thing to run too much...this would be worse, I think.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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