steep driveway hydronic heating system

ASK

My contractor wants to do the following:
1- crushed gravel foundation matching the grade (8-10 % grade)
2- cover that layer with foam backed rolled reflective foil (not 2" of foam board, saying that it would crack and compress over time)
3- wire grid for attaching the heating pipes
4- cover to pipes with 5-6 inches of concrete

I know you prefer 2" of foam to foil.
My real question now, is 5" of concrete too thick for this system to actually work?

Wellness

Pumps can easily handle pushing water up a grade, but your contractor should use rebar and chairs, not wire grid, which will get pushed to the bottom in the concrete pour along with your heating pipe. For a driveway 4 inches is a minimum pour, but with piping (and for heavy vehicles) I'd also recommend at least 5 inches of concrete.

hot_rod

This attached model is for a residential heating slab, 4".

But it gives you an idea of output difference with tube depth. Do your best to gets the tube up into the pour.

Gordy

Mid depth on the tubing. Some here use the barrier. The 250 psi XPS will be sufficient so long as sub base is properly compacted. Foil face has zero benefit.

Gordy

The more concrete above tubing the more likely btus will go to ground first, especially with poor insulation.

hot_rod

A 4" slab example, deeper tube = higher required SWT for same output

Tinman

@Wellness - In a closed system, the pump doesn't care about the height of a building, driveway, or anything else.

Zman

I have some questions for the contractor:

Is he trying to melt snow or heat the ground?
Keep in mind that heat does not rise. Thermal buoyancy only applies to fluids like air and water. Radiant heat is like light, it knows nothing about gravity.

Why does he use mesh under the slab? It has no structural benefit when left on the bottom (and it will be). Not only that but the tubes then end up on the bottom and heat the ground very effectively. You are trying to melt snow, yes?

Reflective insulation does absolutely nothing under a concrete slab. Don't waste a dime on it. There is not a single science based document to the contrary.

A properly constructed concrete slab (not mesh laying on the ground) will spread the weight of the vehicles over a wide area and you will have no issues with compressing the foam.

For best results use 2" rigid insulation, use real rebar on chairs and tie the tubing to the rebar at ~ mid slab.

Do it right and do it once.

Zman

Why 2 posts on the same subject?
Very confusing.....

Gordy

There is heavier grade road mesh. However if it is not put in right it’s still not effective at reinforcing the slab. The bottom is no place for it to effectively reinforce the slab. May as well use nothing.

Most contractors throw it in to give the customer the feeling they are going the extra mile.....in effect it’s only giving them something to attach the tubing to where it will also be less effective at melting the snow.

Jamie Hall

The gravel foundation is OK -- provided it is small, well graded gravel and compacted very well -- not just dumped and scraped into position. Then your 2 inches of XPS foam. Then the reinforcing mesh must be on chairs, so it is pretty close to midslab You can tie to the pipes to that. My preference would be to use fiber reinforced concrete -- either steel or fiberglass, but I prefer steel -- as it reduces cracking considerably. Plain concrete, even with mesh, will crack..

Stay away from the reflective foam or bubble stuff -- there are applications where it is useful. This is not one of them.

Erin Holohan Haskell

Zman said:

Why 2 posts on the same subject?
Very confusing.....

I've combined the two posts. Thanks!