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convert to joist heating?

I'm getting ready to renovate a bathroom, and looking for advice on converting the heat in the room from baseboard to under floor radiant. The bathroom is small, only 5x8.

I'm happy with the performance of the heating system in the rest of the house, but would like to take the chill out of the bathroom tile floor in the winter months.

The house is a 900 sq foot single story ranch. All rooms are heated by baseboard convectors on a single zone. The boiler is a high efficiency modulating boiler w/ outdoor reset.

Initially, I thought about re-running the pex under the floor using the Uponor joist track system from below. However, the heating zone is plumbed with 5/8" hePEX, and the joist track seems to call for a max of 1/2" pex. Reducing the pipe diameter for the joist track, then increasing again to meet with the rest of the zone seems like a bad idea, though I'm no expert.

What about staple up systems? I imagine they transfer heat the subfloor less efficiently than with the aluminum plates, but that wouldn't reduce flow to rest of the house if I kept the same diameter pex.

A friend suggested using an electric mat under the tile floor, but I'm less inclined to go that route due to the high cost of electricity here in New England, especially where I already have the boiler.


Re: convert to joist heating?

I'm no expert on bathroom hydronic heat either; there's been a lot written on this topic by hydronic engineers like Siegenthaler and others.

Google "a little floor warming please" by Siegenthaler to get his famous article and 4 different floor design circuits covering at least 4 ways that it could be done.

I would recommend you have a technician do this rather than attempt it as a diy project; there are so many issues that have to be gotten right in order for you to have a warm bathroom floor in cold months & deliver hotter than normal water temps to that part of the system.

Due to the very limited room to put convectors in a small bathroom, it is correct for you to avoid baseboard, but another conventional option is to install one or two kickspace heaters at the base of a wash basin cabinet, etc. & combine this with installing a separate zone on the piping system.

This could be easily done with a 571-type Taco zone valve that would deliver sufficiently hot water to the bath floors, & at the same time be on its own t-stat so you could control the bathroom heat.

A very common complaint with bathroom heating is that it is OFTEN INADEQUATE---this is because the sensible thing to do is to calculate heat loss for the bath at DOUBLE the ordinary calc used for other rooms.

Thus, an 8 X 5 bath = 40 sq.ft. X 60 btu/hr (bathroom heat factor) = 2400 btu/hr X 2 = 4800 btu/hr---that's equal to close to 10' of baseboard on its own t-stat, or other convectors, or staple-up pex that will equal 4800 btu/hr.

The deal maker comes with hiring someone with enough hydronic experience to get the job done right the first time so you have a zoned bathroom circuit with enough convection in the floor that is able to get the bath floor & room warm enough for your comfort.

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