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Thread: How warm can hydronic floors get?

  1. #1
    DIY Junior Member bandrewfox's Avatar
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    Default How warm can hydronic floors get?

    Hi,

    I have already installed part of this system, but I am worried that the floors won't get warm enough for someone in the family who loves really warm (80 deg) tile floors in the bathroom. Here's my setup:

    Wirsbo/Aquapex 1/2" tubing
    *** Wirsbo Joist Trak - 2 parallel per joist bay ***
    3/4" old "ship lap" subfloor
    5/8" new plywood subfloor
    Tile flooring

    *** Main source of heat is forced air furnace - I am not relying on this system to warm the rooms.

    I am planning on using the floor heating as the return tubing for my hot water recirculation pump. So, I will get a non-iron recirc pump and the hot water heater will be the same one I use for consuming potable water.

    Does anyone have any experience in a similar setup? I would prefer to keep the HWH at 120deg for anti-scald purposes.

    Is it possible to warm these tile floors to 80 degrees? Are there any spec sheets/calculators for figuring this out?

    As always, thanks so much!

  2. #2
    In the Trades Tom Sawyer's Avatar
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    You have a lot of money? Heating the floor with a water heater is going to cost you a fortune. 120 degrees probably will not get you 80 at the tile. You will have to bump the water temp to around 160 and use a tempering valve at the fixtures.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Retired Defense Industry Engineer jadnashua's Avatar
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    You've got a LOT of mass there, and wood is a fair insulator...to get the floors warm, you'll need fairly significant water temps when doing it from underneath with staple up materials. It's easier to do when embedded IN the floor, not under it.
    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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    DIY Junior Member bandrewfox's Avatar
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    Thanks for your feedback. I got my plans for the "open direct" system from these guys: http://www.radiantec.com/faq/faq04.php

    They seem to be telling me that it is an efficient way to go and that 80 degree floor might be possible with 130 degree water. Then again, they are trying to sell me something, so maybe I shouldn't believe them.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    The number of tubes, the temperature of the water, the velocity of the water, and the thermal mass between the heat coil and the finished floor, ALL affect the final temperature. BUT, the one thing you do NOT have is a temperature limiting device. The return circulation water will be slightly less than the water heater's setting because of heat loss, but it could eventually approximate it, and since it is probably running 24/7, the floor should get warmer and warmer, then HOT, which could be unacceptable.
    Licensed residential and commercial plumber

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Keep in mind that if you get the floor to 80-degrees and keep it there, eventually, the room will get close to that temperature as well, especially if it is not regulated and you are relying on forced air to warm the home. The real goal should be to get the floors warm. Good insulation and sealing of leaks can help a lot.
    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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    DIY Junior Member bandrewfox's Avatar
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    Thanks for all your thoughts. I have put this project on hold.

    I was reading Chapter 8 of the Complete Design Assistance Manual (CDAM) on the Uponor website:
    http://www.uponorpro.com/en/Technica...t/Manuals.aspx

    I worked through the examples and the tables in the appendix and I think that 75-80 degree floors are very doable with 120 degree water and Joist Trak @8" on center and the floor insulation R value of 1.5 with a room set point of 70-72 degrees and between 10 and 20 BTU/hr/sqft of heat loss.

    So, I bought an infrared heat sensor and will test the area that I have already installed, and I'll use my 115 degree HWH and I'll report back here in 7-14 days with the results.

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    In the Trades Tom Sawyer's Avatar
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    The results will be that to maintain floor temperature the water heater will pretty much run non stop. How much money you got
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Retired Defense Industry Engineer jadnashua's Avatar
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    If you maintain the floor at 80-degrees, the room will eventually approximate that temp which, for me anyway, it just too hot. A more reasonable goal is to make the floors at least room temp, which for most people is somewhere between 68-72 or so.
    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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    DIY Junior Member bandrewfox's Avatar
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    Well, I have some electrically heated floors in my current house and the IR heat sensor says they are 78 degrees, but the air temp is the same as the rest of the house (heated via elect baseboards) at 70 degrees. So, I don't think the room will get too warm. If anything the bathrooms feel slightly colder than the rest of the house.

    Also, in response the "how much money you got" which has been asked a couple times, I don't understand why it would cost so much. People say that radiant floor heating is of comparable efficiency to other types of home heating. Why is my setup so much worse than others? Is it because I want to use 120 degree water from a HWH instead of 160 degree water from a boiler and that I will need to circulate the water more frequently? Why is running a HWH less efficient? If both arrangements use the same tubing setup under the floor and both give the same BTU/sqft/hr and both are 95%+ efficient, then please explain - I really want to understand so I don't waste a bunch of energy and money.

  11. #11
    In the trades Dana's Avatar
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    Quote Originally Posted by Tom Sawyer View Post
    The results will be that to maintain floor temperature the water heater will pretty much run non stop. How much money you got
    Huh? Do you really believe that a few square feet of tile can emit more heat than the burner output of a hot water heater? The HW heater will cycle on/off as the radiant floor draws off heat. Do the math! If it's not controlled by a floor thermostat or wall-thermostat the pump will run non-stop, but the burner won't. Even if the room gets SCREAMING hot running it 24/7 it's not likely to pull more than 2000-2500BTU/hr before it all balances, and most of that heat will be heating the rest of the house, reducing the load on the furnace.

    hj & jadnashua have it right- it's important to put a room-temp limit on it. In 80F floor in a 70F room emits about 20BTU per square foot of heated floor. Assuming you have something like 50-80 square feet of floor that's 1000-1600BTU/hr, which is less than 10% of the burner output of any tank type hot water heater. If the heat load of that room is less than that (which it will be, most of the time) the room temp and the floor temp will keep on rising. At 20F (colder than Seattle's 99% outside design temp) a 10 square foot U0.34 window is only pulling 170BTU/hr, and the rest of the exterior wall of the bathroom won't be drawing 800 BTU/hr, so the place (and the floor) would end up getting UNBEARABLY hot if you just let it run.

    At average Seattle outdoor temps you won't have anything LIKE 10BTU/ft of heat load, which would make the load 100-150 square foot bathroom 1000-1500 BTU/hr. but it might be close to that at the ~25F design temp. Unless you have the draftiest least-insulated house in King County you're not going to see a heat load of 20BTU/ft. at design temp with the windows closed.

    So what will happen is that the room temp will rise until the loss from the room to the outdoors & other rooms balances with the floor to room delta-T, which may not happen until that patch of floor hits 110F, assuming you insulated below the JoistTrak with batts or something. An 80F floor is pretty warm, and a 90F floor is well beyond comfortable. A 105F floor is QUITE uncomfortable, even though it won't burn you.

    For the record, I have a radiant zone with Joist Trak in my house with 1.5" of plywood subfloor (dual 3/4" ply) and another 0.75" of hardwood flooring that has no problem going over 75F with 125F water when there's a full heat load on. You have less than half the R-value in wood that I do- it should be able to hit 80F easy with 120F water- the key is to keep it from overshooting that temp.

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    DIY Junior Member bandrewfox's Avatar
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    Thank you, Dana, for your comments. It is comforting to hear that my target temps are possible with the typical Seattle temps. I will certainly keep a separate floor thermostat in the system to keep things from getting too warm.

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