Triple aluminum plates between joists - bad idea?

SteamtoHotWater

I had the bright idea to run rows of three aluminum plates between my bathroom joists. The bathroom has some pretty thick tile floors and I really think the extra output would be beneficial. But, I have never seen anyone do this. That got me thinking that perhaps this idea isn’t so bright after all.
I’d have to drill through both ends of the joists. And there are some tight, but manageable bends. But other than that, I can’t think of any downsides.
The blue area is tile. The light brown is the hallway with wood floors. The dark brown are the the joists. The grey rectangles are the aluminum plates.
While I’m asking questions: heat beneath the tub or no? It’s cast iron and level with, and built into, the tile floor. That is, it has no feet and there’s no tile beneath it.

STEVEusaPA

Did you do any design/math?

hot_rod

Unless you have two outside walls with windows? Bathrooms typically do not have a high heatloss. Knowing how many btu/ sq ft you need would answer the question.
No harm in going with that tighter spacing, but it may not be needed

SteamtoHotWater

The bathroom has two outside walls, with one window taking up a lot of the smaller wall. The energy audit guy calculated ≈ 300 BTU/hr heat loss for for the bathroom. He didn’t express any concerns with the heating bathroom. The bathroom is on the second floor, and only recently did I take down the first floor ceiling. We didn’t know what the joist and pipe situation in the bathroom would be. We also didn't know if the tub would be level with subfloor. He guesstimated that I’d probably be able to put up 400 BTUs worth of plates. The old steam radiator was supposedly capable of 1000 BTUs. With the radiator, the bathroom was warm enough. Though there are a lot of cold surfaces - porcelain, tile, some marble.

In addition to some extra warmth, I also like the idea of running three plates because the bathroom looks like it's going to be its own loop. Adding some PEX would help lengthen the run to make it closer in length to the other three loops.

Alan (California Radiant) Forbes

If it's going to be on it's own loop, put a thermostat in the bathroom to control that loop. Switching times for bathrooms don't always coincide with other zones. Also, tile floors heat differently than wood floors, i.e. it's good to keep them on separate zones.
I used to think heat under a bathtub makes sense. Not any more. It doesn't seem to do much.

hot_rod

SteamtoHotWater said:

The bathroom has two outside walls, with one window taking up a lot of the smaller wall. The energy audit guy calculated ≈ 300 BTU/hr heat loss for for the bathroom. He didn’t express any concerns with the heating bathroom. The bathroom is on the second floor, and only recently did I take down the first floor ceiling. We didn’t know what the joist and pipe situation in the bathroom would be. We also didn't know if the tub would be level with subfloor. He guesstimated that I’d probably be able to put up 400 BTUs worth of plates. The old steam radiator was supposedly capable of 1000 BTUs. With the radiator, the bathroom was warm enough. Though there are a lot of cold surfaces - porcelain, tile, some marble.

In addition to some extra warmth, I also like the idea of running three plates because the bathroom looks like it's going to be its own loop. Adding some PEX would help lengthen the run to make it closer in length to the other three loops.

How many square feet is the floor? About 25 btu/ sq ft is reasonable for a comfortable tile floor. So if you have enough floor with 25 btu/ sq ft, output, will you cover that calculated load?

Exposed floor only, under cabinets and tubs doesn’t count.

SteamtoHotWater

We guesstimated about I could get plates under 20 sq feet. In reality, I can probably get closer to 25, not including the tub.

Derheatmeister

Let's start with...Triple plates are nothing new
Personally i like warm floors when i step out of the shower or tub so yes the more tubing the better.
The closer the tubing is to the surface such as a Gypcrete on Clima panel by Viega the better it will perform.
It is not going to hurt installing tubing under the tub but will do very little on behalf of heating the bathroom.
If you do require additional heat you could also install small Towel rack if you have room for it.
I know that the drawing is probably not to scale but keep the tubing away for the Wax seal

EdTheHeaterMan

There is one thing you need to observe, Make sure you do not put the tube too close to the toilet drain pipe regardless of the number of tubes in the joist. if the water in the tube gets too hot from a malfunction some day, the wax ring will melt and then you will have some nifty odors and you will not know where they are coming from.


Now as far as adding tubing to a floor, If you could stack 28 rows of 1/2 OD tubing in a 14 ft joist space you will not get any more heat from that floor suurface. Think of the floor as EDR, one square foot of floor at any given temperature is still only one square foot of floor. if 2 tubes or 6 tubes get the floor to the designed temperature of say 90°, then that 90° surface will give you the exact same amount of heat to the room. Think about radiant wall heat on one of the outside walls, or a towel warmer at a higher temperature. Make your job easier and do the math before, you do the muscle.

SteamtoHotWater

A couple days ago, I replaced the wax seal with a rubber one. Though I'll probably still try and route tubes away from the area.

So, in summary: triple plates won't make any difference and neither will plates under the tub - is that correct?

hot_rod

I will say if all the systems I have installed the tight spacing and counter flow design of the of the TwinTran made for a very consistent floor temperature. The entire surface was evenly heated. With 8 or 9” you will feel some stripping. For the small amount of extra tube, and the additional hassle, go for it. It not something you want to do twice.

Radiant tile floors in bathrooms are best with any of the rubber or foam type closet seals. An undiscovered leaking wax ring can make for a stinky problem.