Infloor heat question

[plunger76]

I'm helping a friend remodel a bathroom. He would like to eliminate the section of copper baseboard currently installed. He asked me if he could put a loop of in floor heat under his new wet bed tile floor ? This prompts my question which is what is the best way to temper the boiler water going to this loop from 180 deg. to a more appropriate temp. for the in floor ?

 

[Watson524]

I'm not an expert so I hope the pros don't mind me answering but as one homeowner to another... my mom has a zone that's radiant heat in tubing under the floor of her master bathroom (and boy is that nice when you walk in there barefoot!). Anyway, it's a separate zone on her hot water boiler and that zone has a mixing valve on it to temper in the cold water and has a temp gauge right above the circulator pump to the zone so you can see what the temp of the water going to that zone is. The mixing valve she has doesn't have temp on it, just numbers 1 - 5 I think, so that's why you need the gauge to really know what's going into the zone.

 

[Sponsor]

 

[Dana]

If he's planning to run it at 180F, the best (and cheapest) approach would be to use suspended-tube (with or without finned-convectors, such as Ultra-Fin depending on what is needed) under the subfloor rather than mixing down the temp and embedding the tubing into the tile-substrate or using a plated-subfloor approach.

First order of business is to calculate the output of the section of baseboard, and make sure you have enough floor to get that much heat into the room with just a radiant floor. If the baseboard wasn't keeping up or was overheating the place, now is the time to make the adjustment. If it takes floor surface temps of 110F or something to hit the numbers you may want to add supplemental radiation such as a heated towel-rack radiator to bring the floor temp requirements down. Ideally you'd never need more than an 80F floor to heat the room, which is still pretty comfortable in bare feet. At 90F it's not going to burn but it's not super-comfortable either, at or above 105F it's downright miserable, which probably isn't the effect he's looking for.

At an average water temp of 180F 2" fin-tube puts out about 600 BTU per foot. The comfort-limit on a radiant floor is something like 30 BTU/hr per square foot, 20 BTU/hr is better) and you can't/shouldn't heat the square feet directly under the toilet. (You can under the tub, but you won't get the same amount heat transfer out of that area as under the floor.) eg: The radiant-floor equivalent of a 3' section of typical baseboard is 1800BTU/hr or 60-90 square feet of usable floor (20-30BTU/ft^2 ). The R-value of a bath-mat cuts into the heat load a bit for that ~5 square feet, but isn't a make-it-or-break-it deal.

If your peak requirements for the floor area are 15 BTU/ft^2 or so you can get there with suspended tube, but if it's 30 BTU/ft^2 the finned convector approach will still get you there. (Ultra Fin sez 41 BTU/ft^2 @ 180FAWT under a typical hardwood floor, which would be tolerable in bare feet on tile, but not exactly ideal. )

But then, most homes don't really NEED 180F AWT even on design day, and dialing back the temp at the boiler results in fuel savings, as long as it doesn't short cycle or create a boiler-destroying condensation condition. With suspended tube a boiler that is cycling on/off won't bring the surface temp much above the average of the duty-cycle- it's fairly slow reacting compared to baseboards, but it'll still keep the place warm.

 

[Tom Sawyer]

Even at 180 degrees, if it's a wood floor you won't be able to get enough tube in the bays to heat the room.

 

[Dana]

He did mention that it's a tile floor...

 

[Tom Sawyer]

Adds mass but btu is btu all day long. He's still going to need additional radiation. A wall hung radiator/towel warmer or a toe space heater in the vanity cabinet works well.

 

[Dana]

He didn't say how thick the subfloor was, or how many square feet of floor was available either- we don't even know the 99% outside design temp. There are plenty of locations in the US with outside design temps north of 30F, after all. He has shared very little of the particulars so far.

I agree completely that most bathrooms in a cold climate zone would probably need additional radiation. I did mention "...you may want to add supplemental radiation such as a heated towel-rack radiator to bring the floor temp requirements down...", and a properly sized towel-rack + suspended tube will probably yield a cheaper and more satisfactory result than hacking in a dual-temp floor-only solution.