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Thread: Maximum Temperature for Radiant Heat

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    DIY Junior Member greekguy7's Avatar
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    Default Maximum Temperature for Radiant Heat

    I have 6 zones in a 2.5 story building. The lower lever has radiant heat and the 2 other floors have baseboard heat. The boiler usually runs at 160-180 degrees and there is a mixing loop that mixes some of the return water back with the supply to feed the radiant zones. Is there a maximum temperature to feed into the radiant floor pipe?? I don't want to risk cracking the copper radiant pipes that are buried in the concrete slab

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Yes, there is both a practical and a safety issue with the maximum water temp in a radiant floor. The floor temp shouldn't end up much warmer than your desired air temperature, and once the temp in the slab is stabilized, the feed water normally wouldn't need to be very hot, maybe in the 100-120 degree range, depending on how deep they are and how close together, and, how well the slab is insulated underneath. How well the whole room is insulated will also dicate how much heat you need to add. Somebody that does this all the time should chime in with some max values, but they're likely much less than the straight boiler temps.

    Have you done a heat load analysis? Knowing the amount of heat needed, and the available length of the pipe and the flow rate would give some idea of the heat transfer capability, then setting the temp becomes easier.
    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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    In the Trades mtcummins's Avatar
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    I'm not a radiant heat pro, but in my system, which is under wood flooring, not in a slab, we have to run it at about 140 degrees to provide adequate heat. Concrete is much more thermally massive, and therefore shouldn't need as high of a temperature, I would guess in the 120-125 degree range. You should be targeting about 80 degrees surface temperature of the floor, generally to maintain comfortable room temperature. Is your system controlled by surface temperature, or room air temperature? Usually your pumps will have different speed settings as well, which changes the temperature requirements... higher speed requires higher heat to get same gain, but spreads the heat more evenly through the zone if that is an issue, lower speeds pull more heat out of the water per cycle, but you can end up with the beginning of the loop warmer than the end.

    You want consistent heat, so your system should not heat up and drive the temperature up quickly, then stay off for a long time. It should cycle on and off relatively frequently to maintain a constant slab temperature. You will probably need to adjust your outgoing water temperature until you reach this state.

    Hopefully someone on here with more knowledge of this will chime in with more specifics, my knowledge is quite limited.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Because the concrete is a much better conductor of the heat, as opposed to say staple up heat on a wooden floor, you need to run it much lower in temp. The goal is to keep the floor temp at a safe point where the room is warm, the flooring materials will survive, and you're comfortable.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    In the Trades Tom Sawyer's Avatar
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    With staple up a standard thermostat works about as well as anything. Staple up temps run high, about as high as baseboard temps do depending on the quality of the installation. In floor concrete can run as low as 80 and as high as 140 again depending on the insulation factor, pipe spacing, tube diameter and length of run. Here a floor sensor is a better option for temperature control because the slab has a very slow reaction time.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  6. #6
    In the trades Dana's Avatar
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    It's doubtful that you'd crack copper embedded in concrete, but are you sure it's copper? (That would be both rare and expensive!) In a well designed radiant slab you'd likely never need more than 110F water in an IL climate, but it really depends on the heat loss of the room, and the spacing of the pipes, etc. In some antique radiant design from the 1950s or earlier who knows? It might need 150F water to meet the load at the outdoor design temp, but probably would be fine with sub-100F water most of the time.

    Unless you have an embedded coil in the boiler for domestic hot water there's usually no reason to run the boiler that hot. Many older systems are so overdesigned that the radiation can deliver sufficient heat at 140F, even during the 99th percentile coldest weather. Running a cooler average temp reduced distribution losses, and increases combustion efficiency slightly, reaping fuel savings on the order of 3% for every 10F you cut the temp. If you had a condensing boiler you'd be able to save dramatically more by dropping the temps even further. With a gas fired cast iron or steel boiler the water coming back from the heat emitters to the boiler has to stay above 130F to avoid destructive condensation (with oil boilers 140F would be required.) But with a "boiler bypass" or other near-boiler plumbing strategies it's possible to run the heating loops at almost any temp, by mixing enough boiler output water in with the return water that the temp remains in a safe range.

    Most boilers are considerably oversized for the design load too, and when you cut it up into 6 zones it's often 15-20x oversized for the smallest zones leading to short-cycling inefficiencies if the heat emitters are low-mass fin-tube baseboard (but not if radiant slab.)

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