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Holiday Project - Radiant Floor Bathroom

dtrani Member Posts: 25
My house has a first-floor full bathroom off the back of the house with 3 outside walls and tile floors, so it's cold. Currently an electric space heater lives in there half the year. I don't really have room to add more baseboard (static 180* set point HW tube/fin baseboard system), but I do have easy access to the floor from below.

The floor is old, plank subfloor, then I'm guessing 1/2" cement board, then tile, so I calc this as roughly R 1.5. I think I have room for eight 48" alum. transfer plates under bathroom floor, so I was thinking of running 1/2" PEX, giving me 32' of plate.

The entire first floor of the house is one baseboard loop (one zone) with its own boiler (it's a 2-family house that we're living in as a single-familiy). I was thinking of running the radiant loop as a parallel branch and putting valves in both the radiant branch and the main loop so I can manually balance flow through the radiant branch.

My biggest concern is that 180* (avg. about 170*) water is too hot for this and I'm going to cause cracks in the grout. I have a roughly 20* dT with this system so I could plumb the radiant branch on the return end and see 160* (150* avg.) instead. My baseboard was designed for 180* set point and I need it that high to get enough heat when it's cold.

I don't want to run this as a separate zone for several reasons, mostly because I don't want to cycle the boiler.

This is not going to be a precision solution since it will only heat the floor when the rest of the first floor is being heated, but the heat runs a lot when it's cold. I can always throttle flow through the loop if I need to..

Am I way off base on any of this?


  • bmwpowere36m3
    bmwpowere36m3 Member Posts: 512
    The baseboard is insufficient to keep the bathroom warm? Or is it just the cold tiles you want to warm up for added comfort?

    Uponor's website has an excellence radiant design manual (CDAM)... that goes thru various installation methods and design parameters (tube spacing, water temp, etc...).

    You could make it a separate zone: install a thermostatic mixing valve and circulator. Wire the thermostat to only turn on the circulator. Let the "main" zone fire the boiler as needed.
  • dtrani
    dtrani Member Posts: 25

    The baseboard is insufficient to keep the bathroom warm? Or is it just the cold tiles you want to warm up for added comfort?


    Interesting. With a thermostatic mixing valve, what's the source of the "cold" water? Boiler return?

  • Dave H_2
    Dave H_2 Member Posts: 526

    Mechanically in the boiler room, in order to avoid the high water temps being delivered to the floor, you will need a mixing valve. Whenever a mixing valve is introduced into a system, a new circulator needs to be installed downstream.
    At that point, you are 99% of the way there, so adding a thermostat to the bath is not too big of a deal.
    You also should make it a separate zone. Mostly because of the response time of the two different heating systems. Radiant is a slow and steady heat whereas high temp baseboard is more of a fast and furious type of heat.
    The boiler may not cycle too much more because of the radiant. When the boiler gets to 180 and the flow for the radiant is low (probably less than a half gallon per minute), it will take a loooong time for the boiler to cool off and make it call again.

    Dave H.
    Dave H
  • hot_rod
    hot_rod Member Posts: 19,279
    For just one small radiant zone, I'd be inclined to do a suspended tube or UltraFin installation and run it at the same supply as the baseboard. No need for a mixing valve, piping, and the additional pump and expense.

    I agree with Dave, run it as a separate zone if possible, you may find you run it even when the heating in the rest of the home is off. Nice to have warm bathroom floors year 'round.

    Look into a stat like the tekmar 500 series or the Uponor with both floor and air sensing.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    rick in Alaska
  • dtrani
    dtrani Member Posts: 25
    If I were to make the bathroom into a separate radiant zone with a mixing valve and circ, am I going to have to convert to prim/sec to ensure enough water flow through the boiler? It's a cast-iron boiler.
  • pecmsg
    pecmsg Member Posts: 3,883
    One other option is Electric In Floor. If this is a small bathroom its a lot cheaper then radiant.
  • Dave H_2
    Dave H_2 Member Posts: 526
    One small zone does not require primary/secondary piping. The volume of water coming back from the radiant won't affect the boiler in any way.

    Dave H
    Dave H
  • dtrani
    dtrani Member Posts: 25
    This project went pretty well, though I decided to start with another area, a front entryway that lost its baseboards in a remodel and so was also cold. I got eight 4-ft alum plates under the space, and I have the loop in series at the end of my baseboard loop (this is temporary), so it's seeing average ~150 deg water. Even before insulation the floor is warming significantly, though it takes a while.

    My plan is to do the bathroom this weekend and then convert to 3 zones (baseboard zone plus entryway and bathroom radiant zones). The radiant zones are going to need to run much longer cycles to be effective.