We have a 10" thick basement slab with rebar too and bottom. We'll be installing 1/2" pex @ 12" centers, and have the option of placing the tubing at 2" deep or 8" deep with the rebar.

For this application with high mass, will the depth make any differance in response time or heat striping? Has anyone tried using wider than 12" spacings with a thick floor, and what were the results?

good lord. top will be better, but that's a hugely thick slab and I would think twice about using it for heating. if your basement is fully buried and you are ok with fully heated space all the time... and you're using variable water temps... fine. but if you have any walkout, fixed water temps, or anything else that could result in swings I would consider that probably "uncontrollable".

I would not go wider, and if anything I would go tighter to make the slab more responsive at least in one direction (heating up).

I assume the 10 slab is about engineering requirement? other than that it is about 60% to thick. consider placing 1-2 inch foam on top of 10 inch slab and pouring a 2 1/2 inch top fro the heat section,
or consider a sandwich system like the RHT floor panel placed on top of the slab with a simple insulation in between.
!0 inch is thick and unruly.
If you must go 10, try to locate pex in the upper 2 inch area.
Dan

Yes, it's a little thicker than I'd originally planned, but's that's whats needed for the load. It's a matt/raft foundation in a walkout basement (only 26ft is exposed), supporting two upper stories of ICF with concrete (Quaddeck/Insuldeck) floors. Total of just over 1 million lbs of structure, for 3,000 sqft.

Most of the basement is fully buried other than the small walkout section. The buried walls are R22 and the slab will be R20. We've have a small electric baseboard heater in each room to help with spikes in heat load, so the slab just need to keep up with the normal heat losses. Constant water temp, could wire in variable flow rates if needed with a three speed pump.

The main and upstairs floor slabs are a bit more reasonable, only 3" thick with 7" deep beam pockets on the Insuldeck. At 73lb/sqft, should be an easier amount of mass to handle.

why not go with footings to support the structure?

Good point, unless there is some reason the slab is working like a bridge. recently we provided a system where the 6 foot city sewer pipe ran under the home, the slab was sso fortified a mac truck could drive on it. 8-12 inches and
loaded with 3 course of 5/8 rebar.
Perimeter footings certainly will save cost on concrete and are more manageable, also might have the engineer look at a thickened edge mono pour as alternative.
Dan

As someone in the process of building my own home in CA, with my own thick basement slab - but only 24" wide foundations and relatively small keyways, I'd pay no attention to the radiant design folks as they attempt to redeisng your foundation for you. After all, they are neither liable nor insured for that sort of work!

With an uninsulated slab such as yours, all of the heat that you put into the slab is going to leak out of the slab. The question is where is going to leak out of? If you are determined to heat out of it, without topping it with insulation and then more concrete, or insulation and then thermalboard, or insulation and then something else, you'll have more of the heat leak out of the top of the slab - where it is supposed to leak out of - if you place the tubes near the top of the slab. You are essentially going to be heating the earth - at some rate that depends on the soil and water content around and underneath your basement, and by placing the earth-heater near the surface, you'll have more heat escaping into the air than into the ground.

You might also find that it is impossible - as in physics impossible - to heat concrete and the basement without exceeding the maximum water supply temperature though the PEX. I don't know what the maximum water temperature through concrete such as yours would be, but if the leakage through the ground is great enough, you may not be able to pump heat into the building as fast as it leaks out. You might tbe able to get up to, say, 57 degrees in your basement, at which point the 145 degree water would be pumping heat in as fast as it leaks out through the ground. Or, you might get up to only 54. It probably depends on what the average below-ground soil temperature is for your area, as well as the composition of the soil, and the water temperature that your engineer will allow.

Jeff

Correction, the entire slab is insulated underneath, with 4" of Highload 60 encasing it. We could get away with Highload 40, but I'm concerned with long term creep.

So the slab is R20, our ground temp is around the 55° range, so should be good for heat loss that direction.

I live in a home with radiant in concrete. No expert here but I walk on my floors in bare feet everyday.

I would place it in the top 2 inches. The rsponse time may be slightly better and your feet in the winter will be much happier. Nothing like warm feet on a below zero day ( like our last several days)

now here is a idea,
you have this huge thick slab,
go with the 2 inch on top pex
and as a kicker what about doing a 8 inch set of pipes and attach to a group of solar collectors, say 4 4x8 panels rigged as a drain down system, sunny days dump heat, summer time domestic h2o. %30 tax credit labor and material on solar hot water side (tank, panels control)
Does that blow the budget? Waste of good cash?
you have a tremendous fly wheel there with that 10 inch slab.
you may consider installing the lower course of pipe for future use, connect later.
Dan

Dan I think that is a stellar idea. That extra tubing could even be connected to wood stove in the future. It is a small up front cost that could reap big benifits in the future.

What exactly is this slab doing to make solar more attractive?

You need a tank for DHW anyway. The solar will do summer DHW and a fraction of winter DHW in most cases, with little to nothing left over for heating. If you upsize the panels to do winter heating, he can nearly as easily just use the top level in the slab as a charge media to receive the solar energy and it would do more good for his heating there anyway. Unless you want to pump actual solar panel water into the slab (not exchanged, heating system water) to save a coil in the tank... which would almost pay for the PEX.

If you're going to heat with solar, put a coil in the top of your tank to extract and let the entire heating system draw from it whenever it can. If you're not going to heat with solar anyway, having a 10 inch thick slab doesn't change the dynamics of that one bit... not unless you aren't doing DHW and can save the entire tank but for most people that would be silly since DHW is the easy money for solar... or unless you have a truly huge array (far more than 4 panels, I think) you need to store heat for in the winter.

I like PEX as much as the next guy but wood stove loops and bottom layer slab layers are reaching a bit.

That's right, a wood stove, solar loop, geo loop
So I suppose the general thought I had was to turn the whole slab in to a battery, And yes the cheapest way is likely to run from the collectors to the slab in a direct drain down system, bypassing the domestic H20, Bypassing any available tax credit to offset, the credit would probably make the domestic side a wash.
Is it right to place additional pipe on the 8 inch deep rebar, not sure the question is could the upper portion of the slab overheat if it was placed on the top 2 inches? I am avcotating a dumb system, on or off based on tempature available on the panels, no slab sensor fine controll logic, just dump heat.  4 panels is small but if the intent was to temper the 10 inch slab, that is better than not at all. is there a value added for the life and comfort of the building with this, not sure. Something is better than nothing, keeping the concrete rock a bit warmer than stone earth cold...
The pipe is the least expensive part of the infrastructure and can never be placed once concrete is cast,
Enjoy your Sunday,
Dan

it's also a complete waste of money if it doesn't address a design issue. and it doesn't. 4500 feet of pipe like you just advocated for (8" o.c. 3000 sq ft) costs a bit more than a modest copper coil in a tank for solar heat extraction...

nothing here even remotely applies to geo (that's an interesting one, are you going to pump heat from the bottom of the slab to the top?), and hydronic loops on wood stoves is a pretty poor idea in most cases IM so very HO. though with 3000 sq ft of tubing already in concrete, I don't think mass storage shortages are really going to be a problem there either, without more than doubling the pex bill, should someone decide they want to play around with potential explosion hazards in their living room. Me, I'd put the money into a blast shield around the wood stove.

the only function a lower layer of tubing would serve would be IF you had a hugely powerful heat source it could increase the responsiveness of the slab in the "heat up" direction. so once a year it would shorten the startup time.

Is the basement 3000 sq feet? or the whole house?

Heating the water for radiant by way of a wood stove does not require a blast shield. There are many who already do this.

Sorry: might only be 1k sqft in the basement. even then, 1500 ft of pipe is closer to the cost of a coil, still more, but not as versatile on the solar end. at 1000 ft of pipe, you're probably close to a wash and you're just wasting labor. the rest of it is odd or poorly conceived no matter what the cost/size is.

I do apologize for overstating the case though by missing the 3 story comment kicker made. that certainly should lower the shrill tone of my perceived "voice" quite a lot. Now it goes from criminal to merely silly ;)

and I'm aware people do stupid things, like putting hot water coils in wood stoves. that does not make it a good idea.