1/4" pex for in-floor heating?

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Old 05-17-16, 09:16 AM
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1/4" pex for in-floor heating?

I'm renovating a small basement bathroom that was build on an uninsulated slab. I'd like to level the floor and put in heated tile. As with many basement projects I have headroom issues and I also can't raise the floor too much as it will make the threshold weird.

My plan is to be down synthetic cork underlayment (found a product called cerazorb) as a thermal break, then run my pex, then pour self leveling compound to both level the floor and bury the pex.

The cerazorb product is completely new to me and I don't even have it in-hand yet (already ordered it). I've done lots of pex work but given that I'm trying to keep the floor assembly as thin as possible, I'm considering using 1/4" pex. I can't, however, find any online references to using such a small pipe for heating. I'm only heating 20sqft and have ~140F water. I believe it should work fine but I'd appreciate some input.

The other question I have is what is the best way to really keep the pex down against the floor while pouring over it? Just one bit of pipe floating up will force me to pour again and raise the floor further. My best idea right now is many small bits of Dow Weathermate construction tape.

BTW, yes, I've considered electric heat, but if I can do this with hot water then I can heat with natural gas which is 1/4 the price. I also considered 1/4" flexible copper for even smaller overall diameter but read about issues with copper and cement.

Thanks!
 
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Old 05-17-16, 03:50 PM
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As far as I know.... 3/8" is the smallest size available for radiant floor heating.
 
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Old 05-17-16, 10:48 PM
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1/4" is available at many stores online. 5/16" too. By "for radiant heating" do you mean to say, with oxygen barrier? I actually don't need oxygen barrier for my application. I'll be heating the floor with an open loop system from the domestic hot water heater using a stainless steel pump. I've done this before and it works great, just never used any pex smaller than 1/2" so 1/4" seems weird to me. I feel like it should work, just looking for some more opinions.

5/16" is the smallest tube I've seen referenced for floor heating. Uponor Fast Trak goes down to 5/16" pex.
 
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Old 05-18-16, 12:55 AM
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The rule is that radiant floor heat is easy to do and very easy to do wrong. It is a matter of getting enough mass in the hot water to heat the mass of the slab faster than it is losing the heat. Using 1/4 inch tubing is going to make that necessary mass of hot water difficult to obtain and still have the temperature differential from the water in the tubing to the slab needed to effect the heat transfer. Using hotter water is NOT the answer as under certain conditions it could allow the slab to obtain higher than safe temperatures. Running a slab much more than about 95[SUP]o[/SUP] at the highest is asking for trouble in my opinion.

But go ahead and try it. It might work out okay for you even though it is against the principles of radiant heat. On the other hand, if it doesn't work don't blame anyone but yourself.
 
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Old 06-13-16, 10:10 AM
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Just to follow up on this, I did eventually end up using 3/8 pex simply because of the poor availability of 1/4 and 5/16 - not just the tubing but also fittings. It just seemed like too much of a hassle to get me that extra 1/8". However it did end up adding a bag ($30) of self leveling compound to the cost.

The method I used was to thinset the cerazorb mat to the concrete as per manufacturers instructions, then lay the pex. I first laid on the pex simply by laying bricks on top of it to hold it in place, then I had to figure out how to hold it down while I poured the slc. Tape and glue wouldn't stick to the cerazorb mat. I ended up using short loops of steel wire, skewering a sharp end of the wire through the mat then tying it around the pipe. It worked great! My pipes are installed and embeded with slc but I won't be able to test the system for a while.

To follow up on the other comment, I'm pretty damn sure 1/4" tube would work just fine for a 25sqft space. You can pump hot water through 40ft of 1/4" pex way faster than the heat can radiate out of the walls of the tube, so the limitation has nothing to do with the mass or amount of water. You would only run into a problem if your loop were much longer. When the limitation is from the surface area of the tube walls, you either pick bigger tube or space it closer together. I'd guess 1/4" would work fine at 6" or less spacing. Uponor seems to think 5/16" pex space 4-12" is just fine for floor heating applications up to 250ft, and they have a product line designed around 5/16" pex.
 
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Old 06-15-16, 05:32 AM
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My concerns will be,
Stripping... what did you place your tubing at on center, the thinner the pour the easier it is to get an uneven surface temp.
Cracking of the mass, with thin pours over something as flexable as cork, you run the risk of having the mass crack due to weight and thermal stress, which can telegraph thru the tiles and you will end up with cracked tiles.

I have a small BR that I did uponor 5/16 at 3" o/c and I have about 1 1/8" of concrete. I don't have any cracks as I used the ****er membrane under my stone tile. I do have some stripping, its not aweful, but I know it's there.

Good luck with your BR, I hope none of my concerns develope for you.
 
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