Installation of radiant heat in slab on grade - new construction

Jim Habermehl

I am designing a hedronic radiant heat system for our new home. I am planning on using a polaris hot water heater for heat source with a heat exchanger. The construction will be slab on grade and my question revoves around the proper approach for the installation of the tubing in the slab. I have received several opinions and have seen several written suggestions and am looking for the "recommended approach". My potential concrete contractor is susggesting: 4"radon rock on subgrade(required by code), 2" styrofoam insulation, pex tubing, 2" layer of sand over tubing, wire mesh and 5 inches of concrete. I had planned on using the radon rock , foam insulation, and then one layer of rebar, tubing another layer of rebar (2' on center) on top of tubing, and then concrete poured into rebar and tubing. I have seen other variatioins of approach as well. I would appreciate the recommended approach.

A 2nd question - Do I need to use oxygen diffusion barrier pex tubing or just plain pex tubing? Thanks for your thoughts.

Weezbo

*~/:)

oki...radon rock and pipe,sealed radon barrier(looks like poly sheething except opaque white,special tape,then the insulation ....now comes choises..staple the pex to the foam,could do ,(better) lay sheets of 6X6X9 on the foam tie foam to it,throw rebar on that (better yet)put cinder knock outs from the soldiers or bond beams to prop up the wire mesh tie the mesh keeping it within the form 3 " away from the exterior,with rebar laid out over the pex tied with X 20d overlaps,pick it up as you pour...then the next step up, gets into welding and pre tensioning,this is something you would need licenses and engineering stamps and stuff to" get"

Mad Dog

Why the 2\" layer of sand over tubing???

Why not encase it in the concrete? Seems like the heat tranfer of the monlithic mass would be more beneficial. Mad Dog

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Constantin

Some answers...

... don't use sand to hold the PEX pipes. You might as well use styrofoam - sand doesn't transfer heat well, concrete does. Therefore, you should encase the PEX in nothing but concrete. The usual spacing is 12" OC for 1/2" PEX, with 6" OC along exterior walls. The 1/2" PEX loops should be no longer than 300', with 250' being the preferred length. Have a look at Siegenthalers and Dan Holohans books on radiant heating, they are enlightening!

Whether you run rebar/wire mesh above and below the PEX is up to you. Wire mesh is cheap, and in most slab installations that have been bored later, the wiremesh is usually only found at the bottom of the slab, where it does no good. By putting a layer above and below the PEX, you pretty much gurantee good slab strength, the WM will protect the PEX somewhat, and it will also resist the tendency of air-filled PEX pipes to float to the surface.

Into the radon rock I would lay some perforated pipe to collect the stuff in one convenient pipe, then run it in 4" sealed schedule 40 PVC through a roof vent to the outside. Put in at least 6 mils of vapor barrier on top of the rock, then the XPS, then the concrete/PEX/WM slab. Don't forget to seal all the penetrations air tight.

If you really care about slab cracks (and/or the prevention thereof), place rebar at 45° angles in re-entrant corners to prevent crack propagation. Ditto for window casing corners (same idea). Also, consider allowing the foundation to crack in areas where you won't be able to see it, such as below interior walls. Simply install no rebar/WM there and weaken the slab along the line where it is supposed to break by installing a joint below. Worked great for us.

Without exception, always use O₂-barrier PEX tubing in heating systems. The non-protected type only has an application in the potable water field. No exceptions. The O₂ will migrate into the tubing, even via the slab. Don't do it!

marc

sand

is for the curing of the concrete, helps the water migrate down also. instead of all the water working its way to the surface. marc

Constantin

Not in my experience...

... I may be a mere homeowner, but I just finished what you are about to start. Have a look over at Building Science Corporation and learn about proper basement wall, foundation, and slab construction. They're not going to make any money off of you, it's all for free on the website, and the specs are used by HUD and other government agencies all over the country for building construction.

The sand under concrete argument was thrown out years ago. It's simply not necessary, expensive, and will not help performance at all. In fact, it hurts it.

Concrete contractors are always going to complain and try to go with the soupiest mix of concrete to make their life easier. The higher the water content, the easier it's going to flow, but watery mixes decrease the final strength of the concrete and may cause aggregate to clump out instead of being uniformly distributed. Stiff mixes may be harder to install, and tougher to trowl (that's what they make a power-trowler for), but the end result is better.

If flow is a concern, have the concrete plant add a super-plasticizer. That will make concrete flow better than a warm milkshake. The price difference between regular concrete and super-plasticized versions is usually negligible. So-called super-super-plasticizers have been shown to self-level the concrete to a great extent, reducing labor considerably.

Anyway, we went as far as specifying the concrete mix and having a testing agency on site to ensure that the mixes conform to the slump, strength, etc. requirements. As Ronnie would say, "trust is good, verification is better".

hr

For a good concrete cure

you really want the "waters of convience" to leave the top. If you put sand down, or pour concrete on a plywood floor, the waters are pulled to the bottom and the concrete does not hydrate properly.

So the concrete pros tell me. Impatient concrete finishers will often poke holes in vapor barriers to allow extra water to drain into the subgrade.

Usually this is caused in part by the fact concrete guys like to add water to the mix at the site to make it easier to work.

hot rod

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"