hydronic PEX...where to put them?

[lowrider2]

I'm building a 50' x 80' shop/hanger with hydronics in the shop half (2000 SF). Plan is 8 loops of 1/2" PEX but where to put it? Some opinions say in the 4"of sand under the 4" slab and on top of the 2" foam. Others insist in must be inside the slab.

It seems the tube on top of the foam with 4" of sand and 4" of concrete on top gives the largest thermal mass...slow to heat and slow to give up heat. It also puts the tube under the sand and less vulnerable to damage while pouring the concrete, lifting the reinforcement wire and away from the worker's boots and rakes.

Any experience or thoughts??

 

[Randy in Maine]

In the insulated slab please.

 

[LJFULLER]

When I did mine I set the tubing in the slab. After pouring my footings we back filled and compacted the sand leaving an 7" void. Rolled out the vapor barrier and then 2" of rigid insulation throughout the entire floor. The rebar grid was placed on 2" concrete dobies and then the pex tubing was suspended under the rebar. The actual pour was 5" thick with all of the pex 3" below the finished surface. Once i figure out how to add a picture I'll try and add it, makes it much easier to understand what we did.

When planning this all out I found a couple of good resources that helped me out. Radiantec has a bunch of online do it yourself guides, http://www.radiantec.com/pdf/Radiant_Heat_Design_and_Construction.pdf

 

[HoosierBuddy]

Like this.


UP

Concrete

PEX

Woven Wire

2"-HD Polystyrene

Compacted Base

DOWN

Phil

p.s. Then find someone to draw up a diagram of where it all went down, just in case.

 

[LJFULLER]

Trying to add pictures but can't get multiple pictures up so I'm going to have to do it one at a time

 

[LJFULLER]

Next

 

[lowrider2]

How 'bout this: http://www.electromn.com/pdf/BC011.pdf

It seems to have a lot of advantages...larger thermal mass, less chance of breaking tube due to crack in saw cut, can drill holes for bolting things to the floor with no danger of hitting a tube and so on.

 

[LJFULLER]

Having the pex that low does have advantages as you mentioned. I think the sand still needs to be compacted so their reference to "loose Pack" concerns me. I put my pex under the rebar grid so the saw cuts could be lower, a lot more work but it made a 2" cut very safe. I had to plan all of my equipment locations in advance and record those so I have areas with no pex for my lifts, compressor, and a few other pieces I want bolted down.

 

[lowrider2]

Good point LJ! I agree that graded sand in not the best for this purpose. We're over top of the Kootenai aquafer and there are wonderful soil types available for compaction. I'm putting in compacted gravel under the slab and I'm wondering if this approach makes sense. It seems a larger thermal mass would be a good thing...slower to heat but it would give up heat slower too...maybe using less thermal energy to keep warm. I think the key to that is the insulation to limit the heat sink the earth provides.

I saw somewhere that earth berm homes use this technique to heat with a large rocket stove and it seems the same logic would work with hydronics.

I was hoping someone has tried this arrangement so they could comment on it's effectiveness...they are out there for sure.

 

[Randy in Maine]

Why do you even need the 4" of sand under the concrete? All of that concrete (~ 50 cubic yards) will give you plenty of thermal mass (~100 tons). Don't over think this.

 

[lowrider2]

Good point Randy but for every sq in it's still only 4" thick. I don't mean to get hung up on the mass issue but it seems the more you have the longer it takes to cool or heat and the longer cycle time in between heat applied to the mass. Maybe I'm missing something 'cause there are a lot of folks who do it with only concrete over the foam and it seems to work just fine.

Anyone else have any thoughts?

 

[Radix2]

What advantage are you expecting from more thermal mass?

There is no steady state efficiency improvement for more mass.
Only with a very low mass system could you possibly get to the short cycle problem with your boiler.
The heat lost due to door openings and the like would not be less with more mass ( although the temp swing would be less)

If a large part of the mass is made of dry sand, you will also have basically an insulator around your pipes this should decrease efficiency by raising steady state water temperatures and losses through the foam and into the earth. It would be better if the added mass was more concrete or a thermally conductive material.

 

[lowrider2]

OK, OK...in the concrete it goes!

Thanks for the thoughtful responses.