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Thread: Tankless WH Radiant Floor Heat

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  1. #1
    DIY Junior Member cpmarks's Avatar
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    Default Tankless WH Radiant Floor Heat

    I am currently heating my attached garage with a 4KW electric workshop heater. I have tubing in the floor for radiant heat and I want to take advantage of radiant floor heat. Please note electricity is my only energy option.

    I am thinking of installing a 6KW high-flow tankless WH as my heatsource, it is activated by flow. Am I correct to assume I could use a 120V thermostat to control my pumps to cycle the system? I am thinking it should work.

    I have one zone with four loops of about 210 ft each. 1/2" pex.

    Thanks,

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Depending on where you live, you'd probably get more bang for the buck with an air/water heatpump. Rather than just getting a watt for a watt of heat, you might get as much as 4-5W for one watt input. Depending on your electric rates, that could be huge. With radiant slab, it can take days to warm it up, but once there, it's pretty stable. Much differnet feeling than warming just the air.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    In the trades Dana's Avatar
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    Your control scheme will work, but you have to keep the flow well below the max-flow of the water heater to keep from going through flow-sensors. Water heaters are designed for high delta-T and low-moderate flow, but in hydronic loops you're looking at very modest delta-Ts and higher flow. Still, it's possible to get the full 6kw in with a 20F delta-T and 2gpm of flow, which is about the flow you should try to run it. At 4gpm you could get half the delta-T, but you'd likely toast the flow sensor. Set the output temp of the tankless to something low, ~85-90F (hopefully it'll go that low) and it'll modulate the power once the slab starts coming up to temp, returning higher temp water for a lower delta-T. Otherwise it'll just bang/bang on/off at the highest power, taking life out of the heating elements, and the air temps will both under & overshoot the thermostat setpoint at times. The lower you can set the output, the more modulation you get, and more even the temperatures will be. But at some temp (probably in the low-mid 80s) it might not keep up if it's cold out. Without a real heat loss calc it's hard to say where that low-temp balance point is, but odds are you'd never need MORE than 95F water if the walls, roof & slab are insulated, and you have a minimum amount of window area.


    With the tankless set to 85F water and 2gpm flow it would still heat the slab to over 70F, and still put out the full 6KW on a cold startup, but back off to 3kw or less once the slab was up to temp and returning 75F+ water, but automatically modulates higher if the room is cold enough to be pulling the slab temp down. If you have to crank it to 95F you'd have the bang/bang control issue. There's a happy medium in there somewhere where most of the time it'll run long and slow when near the t-stat setpoint, but full power when it's many degrees below the set point. If it can't be adjusted to lower than 100F, it's not a great solution.

    But unless you're planning to keep the slab at temp it's going to be an expensive and long-delayed way to heat the garage. It could even be uncomfortably warm to be lying on working on a car unless you use a slab-sensor thermostat to maintain the FLOOR temp at 68-70F rather than trying to maintain the AIR temp in the room. Unless you have at least R10 rigid insulation under the slab (more at the slab edges) it could get pretty expensive just to leave the slab at 68F-70F all winter too, even in US climate zone 2. (And you're in a much colder climate, closer to US zone 7/8).

    A 3/4-1-ton mini-split heat pump would be pretty responsive for raising the room temp without any hot or cold spots, and have less than half the operating cost of your current shop heater, but it would cost quite a bit more up front. (But it could also air-condition.) It wouldn't likely cut it at 7AM on the coldest day/week of the year, but most of the time it would. (They're good down to -4F/-20C, some are still putting out 70% of full rating at -13F/-25C, but I suspect your outside design temps are a bit cooler than that.)
    Last edited by Dana; 03-12-2012 at 01:31 PM.

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    DIY Junior Member cpmarks's Avatar
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    So the heat pump would be to heat the air? I am looking to heat the slab but sounds like I might be better off with a real boiler and a slab sensor. Thanks for the input.

  5. #5
    In the trades Dana's Avatar
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    Yes- if you want to heat the slab, a real boiler is the right way to go as compared to hacking it with an electric tankless.

    And yes, a heat pump would only heat the air, but at half or less the operating cost of an electric boiler/slab. There are hydronic versions capable of heating slabs, but at more than 2x or more the installed cost of air-to-air versions, and MANY times the cost of an electric boiler. The output of ductless air source heat pumps it pretty limited at typical Manitoba outside design temps, but still pretty reasonable at -20C when running a low-temp slab.

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    General Engineering Contractor ballvalve's Avatar
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    No mystery here: You buy a 40 gallon $200 electric water heater and you're all set with two thermostats to play with water temperature. And a simple hook up for your sink. You can install any watt elements you want for about 10$ each. I heat a huge house in just this way.

    You want to spend thousands on a heat pump with hundreds of components to fail to warm up a garage? In a freezing climate? No payback there.....KISS!

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