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Maximum safe hot water supply temperature to a concrete floor with tile surface

I am in the process of building a new house. I have hydronic heating in 5 different zones. One zone is dedicated to a towel bar. I have a 110kBTU Crown Phantom boiler. The subfloor is 1 inch plywood with the 5/8 PEX stapled to the plywood with 1.5 inch of dry pack and the red mat with red sealer (forget name) for anti crack and then a 1/4 inch motar with 3/8 inch porcelain tile. The boiler is currently set at a maximum temperature of 130 with a declining temperature depending on the outdoor temperature. The system typically provides water at 123 to the floor and subsequently to the towel bar. The towels are not all that warm I would like to increase the maximum operating temperature to at least 135 and tighten up the temperature curve so it does not go below 125. Does anyone have experience operating a hydronic system with a 135 to maybe a 140 setpoint. An additional 10 degrees to the towel bar will make a big difference because I ran it at 135 and it felt great but concerned about the main floor system.


  • hot_rodhot_rod Member Posts: 14,374
    What you are experience is the challenge of mixing two different type of heat emitters on a single temperature system.

    The radiant doesn't need or want to run any warmer that what is required to cover the load. Was there a design done on the system before it was installed? It should indicate the SWT required to cover the design condition. Sounds like the 130 SWT is the design day condition?

    Warming the surface above maybe 80- 82 starts to get uncomfortable, and the floor will tend to overshoot after the stat turns off, if it runs too warm.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • delta Tdelta T Member Posts: 815
    135 is hotter than I would ever put to that floor. 120 is pretty high too....I would replace the towel warmer with an electric one.

    just my .02
  • kschefflkscheffl Member Posts: 5
    It was designed by an HVAC company. They set the maximum temperature at 130F for the minimum air temperature.
  • kschefflkscheffl Member Posts: 5
    I don't want to waste the energy on an electric towel bar. I might just have them recirculate water from the hot water heater through the towel bar and take the towel bar off the floor heating system.
  • GordyGordy Member Posts: 9,530
    edited October 2016
  • SWEISWEI Member Posts: 7,356
    kscheffl said:

    I don't want to waste the energy on an electric towel bar.

    Hydronic towel heaters cost 3-4X what electric towel heaters do. Combine that with the availability of warm towels during shoulder seasons and ask the owners...
  • hot_rodhot_rod Member Posts: 14,374
    Currently your towel bar only warms when the boiler system runs, correct. While an electric may cost a bit more when you compare fuel costs, it may not amount to all that much over a heating season. How large is the bathroom? Is the towel bar the only heat source. Door left open most of the time?

    I found with my electric towel bar, closing the door, allows the bar to warm towels and the room and power down. Mine is a tiny 5X6 bathroom in the shop.

    DHW is another option the bar should be made of non ferrous, low lead material.

    Depending on if the recirc line is well insulated, running that loop 24/7 may cost more $$ than the electric bar?

    I start the towel bar before I start the heating system for warm towels as I use an outdoor shower as late into the fall as I can stand it.

    In fact this may be the weekend to fire up the towels :) A timer and t-stat can help limit run time, no need to keep towels warm while I sleep, or am away at work. Programable controls let you arrange that. I suspect a wifi control is or will be available, call it on an hour before you want the warmed towels.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    The magic is in hydronics, and hydronics is in me
  • kschefflkscheffl Member Posts: 5
    The bar is installed all ready so I need to make this work. The bathroom also has hydronic heat in the floor. It is a relatively small bathroom by design. THe whole idea is to "super heat" the bathroom while taking a long shower; which I typically do. It works great I have a warm room when I exit the shower and a warm towel. It brings the towel right up to temperature while taking a shower so it is efficient. I would just like to get the towel a little warmer. THe bar has its own thermostat which is also a timer and programmable so I can have heat when I want it. I believe my best options are to increase the floor setpoint for the entire house or have a pump installed off the hotwater tank and just circulate water through the towel bar off the hotwater tank.
  • GordyGordy Member Posts: 9,530
    So long as you are prepared for room temp over shoot if you change the reset curve. With underfloor radiant the mass of the tile floor assembly above will have a flywheel effect.
    delta T
  • kschefflkscheffl Member Posts: 5
    Yes fare enough. I thought the inslab thermocouples would prevent this as soon as the slab reaches temperature it will shut off so I don't understand why it will overshoot. This is my first hydronic floor system so bear with me.
  • GordyGordy Member Posts: 9,530
    Hmmm did not know you had slab sensors. So you should be fine.

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