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Using a water heater for radiant heat

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by McKraut, Oct 29, 2012.

  1. McKraut

    McKraut Member

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    Fellas,

    I am stuck in the house listening to the wind howl and the rain splatter on the windows. I am trying to plan my heating setup for my new boiler.
    Right now I am burning a TARM in my basement. I am putting an EKO 60 in a woodshed (75% done) and I am interested in keeping the TARM as a backup (it also burns oil). I am planning on working a DHW into the new system too. I also wanted to use radiant floor heat if possible.
    I have 2 rooms in one zone that are colder then rest and I have access to the underside of the floors for both rooms. I cannot figure out how to work the radiant floor into the boiler system (I don't see how you can temper the water to get it cool enough for the radiant system when your boiler is running at 180*). My solution is to put a tee in the closed system and run it a sidearm with our current HWH. I would set up the electronics to power a circulator whenever the HWH called for heat (instead of powering the elements). The heat from the HWH would be the source for the radiant heat. Now that I am thinking about it, I would have to use a plate exchanger because the radiant heat would be a closed system. This is getting expensive.
    Anyway, your thoughts and advice are greatly appreciated.
    Also, if anyone is interested in designing a system for me, I'd be happy to pay them. I cannot find anybody in this area that is interested in designing a system that I would install.

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  2. Bob Rohr

    Bob Rohr Minister of Fire

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    There are a number of ways to mix down the temperature for a radiant zone. Most commonly a 3 way thermostatic valve is used. It uses the cool return temperature from the radiant zone to blend with the 180F from the boiler.

    This article explains some simple ways to tie a radiant zone into a high temperature zone.www.pmmag.com/Articles/Column/69285e1f0bfc7010VgnVCM100000f932a8c0____


    Another method is to install pex tube suspended below the subfloor, not stapled against it. This is called "suspended tube" or joist bay heating. With this method you heat the air in the joist bay and it transfers the heat by convection, like a convection oven :) With that method you could run 160- 180F thru the tube, no need to mix down. Be sure to use at least 10" of fiberglass batt at the joist ends on the rim joist and at least 6" under the tube. Leave 4" or so of air space between the tube and insulation.

    For extra output in a suspended tube use the www.ultrafin product to add some extra convection. I have used this product before and it adds some extra "kick"
  3. TheMightyMoe

    TheMightyMoe Minister of Fire

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    I have read about using domestic water tanks as storage but that appears to be a long shot from what you want...

    For tempering for the radiant heat... This works by mixing cold return with a hot supply to supply the radiant with a warm supply.

    You can use a pump, and a 4 way valve.

    New Bitmap Image (2).JPG
    Or a 3 way valve...

    New Bitmap Image.JPG
    If you have boiler protection a 3 way is fine, if you do not, a 4 way is preferred.

    When you pipe this, you can plumb it like zone valves, except use a relay to turn on the pump. Depending on your controls.
  4. McKraut

    McKraut Member

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    Bob,

    Can I use the suspended tubes between the joists and NOT insulate it? I would think that the heat would only go up towards the floor, and if there was any radiant heat "lost" to the basement, it really wouldn't be lost because the basement is also heated. Also, I don't have insulation below the floor now, and I don't want the expense of putting in the insulation if I don't have to (the mice would enjoy it).

    Bob
  5. McKraut

    McKraut Member

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    TMM,

    Thanks for the info. Once the return from the radiant returns to the boiler, wouldn't it become 180*? I would think the floor would be too warm if the tube was running at that temp. Why is the HWH out of the question? If you don't mind explaining that I would appreciate it.

    Bob
  6. 711mhw

    711mhw Feeling the Heat

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    Western ME
    I use a DHW heater for backup for my radiant, It is heated thru a flat plate that is on a loop off the DHWH. THen it is tempered down with a taco mixing valve to about 110 deg.(the beauty of radiant heating) I used a radiant design Co. that gave me just what I wanted and there detailed drawings allowed me to do the complete installation my self therefore (as I look at it) paying for their design work by not hiring it out.http://www.nrtradiant.com/
    They will want a dimensioned drawing (sketch) and the other info to create the heat load/loss deal. But I'm sure that with my (zero) experiance with radiant or heating in general, I would have spent a lot more "winging it", trying to get it right, than I did with them. They work country wide and when I delt with them (they are primarly a design Co) they would credit back design fees from material invoices.
  7. Bob Rohr

    Bob Rohr Minister of Fire

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    Heat energy, like my former brother in law, is lazy. It takes then path of least resistance. If you have R-value above the tube as in subflooring and finished flooring, then the easier path for the energy to travel is downward, if there is no insulation below. You want at least R-19 below the tube so the energy goes to where you want it to go. I would not put much faith in reflective layers, a thin coating of dust renders them all but useless.

    Also the delta T between the radiant and the room temperature drives the heat transfer. If the basement is cooler than the space above, that is where the energy will want to go. It's tough to have one floor radiant system do both the upper and lower levels.

    The key to any sucessfull radiant design is first knowing what the load is, expressed in BTU/ square foot. Then the best installation method, with the lowest supply temperature requirement can be determined. Crunch some numbers or have a good design person help with this important first step.
  8. TheMightyMoe

    TheMightyMoe Minister of Fire

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    I prefer 4 way mixing valves (Hotter return to boiler), some setups do not need them.

    Your supply from boiler comes into mixing valve at 180*
    80% of it is sent back into boiler via mixing valve
    20% of it is sent into the radiant loop

    Your radiant loop returns to mixing valve at 110*
    80% of it is sent into the radiant loop
    20% of it is sent back into the boiler

    So when it mixes into the radiant loop your mixing 20% 180* water and 80% 110* water which brings you to 130.

    Now these numbers aren't exact but that is how it works, you adjust the valve to where you want it - More boiler for more heat / More return for less heat. Make sure you got thermometers on the return and supply.

    You COULD run the radiant with the HWH... I just don't see why... You already have hot water from the boiler to use in the loop.

    I suppose you COULD save by not using an extra pump/mixing valves, I just don't see it being worth it. Using your boiler pump to heat a DHW through an exchanger than using your DHW pump to heat your radiant through another exchanger...


    Honestly, if I was you, if you can run baseboards, just do that. You could also use a wireless thermostat or 2 if you don't want to wire a thermostat.
    All you would need is some zone valves or pumps... With radiant you will have to run the loops, plumb in mixing valves, insulate to make sure the heat goes into the rooms effectively, and you won't really be heating a mass like a real radiant zone.
  9. McKraut

    McKraut Member

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    Sorry it took so long to reply...I've been working too much. Thank you for explaining. I understand it better now.
  10. McKraut

    McKraut Member

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    Thank you Bob.
    I guess I, like your brother in law, was trying to take the easy way out. It has never worked in the past, and I guess it won't work this time. You're never too old to learn. I appreciate your help.

    Bob
  11. flyingcow

    flyingcow Minister of Fire

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    FWIW, i have staple up radiant, didn't insulate under the floor until a couple of yrs later. I notice an improvement after insulating. Warmer floor, and my storage was lasting a bit longer
  12. McKraut

    McKraut Member

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    FC,

    Do you think it was worth installing without the insulation?

    Bob
  13. flyingcow

    flyingcow Minister of Fire

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    The staple up did what it was supposed to do, improve the floor temp and keep the footsies warm. But there was an improvement after I insulated. It was worth it. I just used 4 inch F/G.
  14. McKraut

    McKraut Member

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    Great. That is what I wanted to know. Warm feet = happy wife.

    Thank you,

    Bob
    flyingcow likes this.

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