I have a customer whose electric company requires a heat sink of 1-2' of sub-slab sand above the insulation, PEX on the insulation. This is for their off-peak program. Is this a good practice, for here in MN? Which is better 1' or 2'?

Wow, a new record for tubing depth. What are the goals and the heat source?

A tank of water would do a much better job of heat storage. and it would do it without making the mass of the entire emitter ridiculous.

I agree. Significant mass in an emitter is rarely a good thing. It will turn into a heater that you can't turn off.

Not to mention, sand is a terrible heat sink and worse substrate.

Off peak thermal storage is a good idea - but not with a sand emitter. Maybe you need to pass their inspection and then disconnect it and use a better system. $.044/kwh is attractive.

We have run in to this design as well, seems that in the middle of the night when the nuclear reactor is running there is a surplus of energy (same with all energy sources) no throttles,
electricity is around .02-.03 cents KW. Not a bad price but the requirement of the large sand mass removes some control options. Because the sand mass is required it is hard to escape, We have sugested in these situations 2 courses of pex,one to fufill the sand mass dump and another in the slab at the 2-3 inch level to meet the quicker responce times needed when the building might want a bump in heat.
Smart solution, who knows you be the judge, pipe is one of the least expensive parts of the system, comfort is king so this was a solution to making a quicker responce for the mid daybump.
The sand will work like a giant fly wheel keeping the over all house warm, but no fine tuning control.
Dan

Is this a good practice, for here in MN?

Does the sand stay dry that way?

Sand along with crushed gravel is often used as engineered fill for above-grade slabs. While high mass hydronic radiant floor slabs have many advantages for energy efficient buildings, using sand for this mass and placing the tube this low in the slab makes little sense to me.

How effective can any under slab storage be from a delayed energy cost standpoint when you are heating it 24 hours a day? A couple hundred gallons of water heated on a timer would seem to make more sense,no?

heating it 24 hours a day? A couple hundred gallons of water

Their ill conceived idea is to only add heat to it part of the day. Tanks are much better, although a couple hundred gallons is too small.