Hydronic Radiant Heat


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Old 02-13-09, 05:41 AM
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Hydronic Radiant Heat

I'de like to put a hydronic radiant heat system in my 200SF addition and im looking for a proven system. The addition sits on a concrete slab and has a 3/4" plywood floor ramset into it. Im eventually installing a hardwood floor in this room.

For the heating system, I'de like to lay 2x4 sleepers along the plywood. Then run a hot water loop from my boiler to the room and install 7/8" pex tubing around the sleepers. Following that, I'de like to fill over top of the tubing with sand but im not sold with this part. I'de prefer to use sand instead of concrete, or gypcrete, but I want to use the best thermal mass. Can anyone with experience comment on this process?

Here is where I got the idea:
Radiant Heat From Radiant Floor Company - The Suspended Slab
 
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Old 02-13-09, 10:53 AM
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When we go to the beach and the sand is too hot to walk on, the sand just an inch down will feel nice and cool. I don't like the sand idea, but I'm not a professional life guard so I can't comment on it.

Is the slab insulated from underneath? If it isn't, you might be farther ahead busting out the floor, putting down 2 to 4" of xps, laying out your 1/2" pex loops and then re-pour the floor. That's about as proven as it gets.
 
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Old 02-13-09, 01:28 PM
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Sand? Absolutely not. It has poor thermal conductivity, about FIVE TIMES less than concrete, and 1.7 times less than water.

The system that company is suggesting would eventually work, but the output would be poor, require relatively high water temperatures, and the overall efficiency would really stink compared to a real concrete or gypcrete pour.

Who is right -- the best way to do this is from the ground up, starting with a rigid foam insulation thermal barrier under and around the perimeter of the slab, and thermal breaks around any footings. Then rebar mesh, tubing, and a pour.

From where you are now, you would still probably want at least 1" of rigid insulation above the slab/plywood and then do sleepers and the pour on top of that. Or just go tile over the pour and eliminate the sleepers altogether. I'm not a fan of embedding wood in concrete in the first place. Moisture, rot, etc. potential is pretty high.
 
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Old 02-13-09, 03:20 PM
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Hopefully, the concrete was fully cured before the wood went down, and they installed a proper vapor barrier between?

then, R20 minimum insulation, then "Warmboard" with the desired wood on top.

Warmboard

Install I&O ... Product specific, but MUCH good install info in general

The Barrier - click the buttons on the left for some interesting articles

Is there enough headroom to afford to lose a few inches of headroom?

How about the entry and exits to the room, will raising the floor present any problems there?
 

Last edited by NJT; 02-13-09 at 05:21 PM.
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Old 02-17-09, 08:26 AM
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re

Thanks for the info. Im not crazy about the warmboard and I really want to put hardwood flooring (bamboo) in the room.
I've improved upon the first method though.

Im thinking of pulling up the plywood floor and starting from the concrete slab. I would have 2 1/4" to work with to match the existing house floor. I want to first put down reflective foil insulation (FDBF). Then put the 2x4 sleepers down. Then install the pex tubing. And pour on top of that to the top of the 2x4's. With a 3/4" floor on top of that I would achieve the 2 1/4" height. Am I going to be disappointed?

Also, do I need a vapor barrier and at what point do I put it down?
 
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Old 02-17-09, 03:12 PM
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I don't remember if anyone asked, or you answered whether or not there is insulation under the slab? And a 'thermal break' around the perimeter? If not, you'll be heating the Earth.

If you read through the warmboard install info, you will find plenty of 'how to' information.

Also, the "Barrier" link has some good reading... there is an article there that talks about the K-value of insulation... or, it's ability to stop CONDUCTIVE heat loss. While their claims seem to be valid in many respects, in ADDITION to a radiant reflector, you should also have proper insulation (R20 min). Also, a 'reflector' will do very little 'reflecting' without a dead air space in front of it. Covering the reflector with concrete will kill it's purpose.

The vapor barrier is the first thing on top of the concrete. Without a vapor barrier, any wood in contact with the concrete will ROT... and wood boring insects love wet rotten wood... did I mention MOLD?... I would use double layer of poly sheet, and overlapped 15lb felt paper on top of that. It's overkill, but that's how I would do it. Something like "The Barrier" is probably a very good vapor barrier, but probably not much in the way of insulation... but I guess it's better than nothing... and I guess if you lapped it up the sidewalls a few inches, it might form a weak thermal break for you.

By the way, all the principles behind installing Warmboard apply to any radiant job... so reading the install will NOT be a waste of your time!
 
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Old 02-17-09, 03:28 PM
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Am I going to be disappointed?
I dunno... maybe... (I have 'commit' problems ya know...)

The main thing is the insulation... I've said that already...

Next, the reflective 'bubble wrap' is just about useless, pretty much a 'myth'.

Even if you could get 1/2" of insulation in there, it would be better! In between the sleepers...

Try to get the tubing in the middle of the slab by supporting it with wire... or saddles of some sort... you can find all this by perusing those two sites I gave... click on some of the articles on the left hand side of "The Barrier" page.

I forget again... did you do a heat loss calculation on the room? If so, assuming that you can get maybe 25 BTU/sq ft out of a radiant floor, are you going to be able to match the heat loss with the radiant?

If you don't have good insulation, plan on tighter tube spacing to get the heat output you need for the room... I certainly wouldn't use 1 tube per joist space... 2 maybe... 3 or 4 along the outside walls...

For maximum heat output, don't run one single length of tubing, split it into 'loops', and feed the loops with a manifold. You get more flow through the system and the floor heats more evenly.
 
 

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