Bathroom Reno - Adding new tile and heating

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Jechow

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I'm looking to remodel my ensuite bathroom. It is currently under heated by forced air and the flooring is sheet vinyl. It is a small area that would need minimal heating. I am thinking of laying down tile and adding an in-floor heating system. My concern is that the electrical servicing the room is "full" and the only other circuit is the GFI plug. Bring in a new line would require some creative routing. I am thinking of connecting it to the GFI circuit but I'm worried that if my wife uses her hair dryer (which i think is close to 15 A) is going to blow the fuse. What would you do? Thanks.
 

Jadnashua

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Most of the systems call for a dedicated circuit. The typical draw on an electrical floor warming system is in the order of 10-13W/sqft, so might work, but generally, installation requires the manufacturer's instructions to be followed, and if it says a dedicated circuit, that's what you'll need. A high powered hair dryer might put it over the top..

You need to understand that when it comes to tile, it is not the size of the room that is critical, it's the strength of the structure. You need to find out that by measuring the unsupported length of the joists, their spacing, their height, and ideally, their species. There are load tables and a few calculators around that can help you there (check www.johnbridge.com and use their 'Deflecto' calculator if you have a subfloor). Without knowing those specifications, there's no guarantee tile will survive. Many places in the SW are built on a slab, so deflection isn't an issue, although cracks can be (you won't know until you remove the vinyl)! Without under slab insulation, the response time and efficiency of infloor heating/warming won't be as good as you may like. They call the electrical systems floor WARMING, not heat. But, in a mild climate, it may provide the heat you want.

Others have reported, depending on the green rules that may apply, it may require a timer on the thermostat limited to 30-minutes at a time. This won't work well for heating. One installer in CA had that issue with the inspector (and the state codes). Have not looked at AZ's rules.
 

Dana

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I'm looking to remodel my ensuite bathroom. It is currently under heated by forced air and the flooring is sheet vinyl. It is a small area that would need minimal heating. I am thinking of laying down tile and adding an in-floor heating system. My concern is that the electrical servicing the room is "full" and the only other circuit is the GFI plug. Bring in a new line would require some creative routing. I am thinking of connecting it to the GFI circuit but I'm worried that if my wife uses her hair dryer (which i think is close to 15 A) is going to blow the fuse. What would you do? Thanks.

I would run an I=B=R type heat load calc on the room or use a a reasonable online load calculator (using very low air leakage assumptions) to come up with the total design heat load at the 99% outside design temp before choosing how to serve that load. Simply throwing 1000 watts of resistance heating under the at would certainly heat the room (probably even with the windows wide open) but it's a bit like swatting flies with sledgehammers. If the heat load number is calculated in BTU/hr divde by 3.412 to convert to watts.

Bathrooms typically have very low heat loads due to less window area, and yours is at least partly heated (probably mostly-heated) by the hot air furnace. For example, the main bathroom in my house is pretty small and has only one window, and only one side is an exterior wall, and has a design heat load of 713 BTU/hr @ -15C outdoors, with an interior design temp of 20C. That's about 200 watts to fully heat the room. Even 100 watts of auxilliary heat would be overkill at my house, since that more than covers the total heat load at the average mid-winter outdoor temperature.

Limiting the size of the heated floor area to the minimum needed to cover the load lowers the peak draw. If the bathroom can be heated with 300 watts (~10o0 BTU/hr) at the 99th percentile temperature bin even without the hot air furnace, it'll be just fine with 100 watts of heated floor, maybe even 75 watts, drawing less than one ampere- and it wouldn't need it's own separate circuit. When only heating part of the floor, think about where in the room it might offer the highest comfort premium (not under the toilet, and not under the bath mat that would impede the heat transfer- probably in front of a wash basin where you might be standing barefoot for awhile, but it's your call.) Not that I'm endorsing this particular product (which came up in a random web search), there are under tile products out there smaller than 10 square feet that would likely fill the bill, drawing well under 2A peak.
 
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