John Klingel’s question was simple enough: what’s the best way of heating up a thick bed of sand beneath a concrete slab with PEX tubing? But the underlying issue — whether a sand bed is a good idea in the first place — quickly takes center stage in this Q&A post at GreenBuildingAdvisor.
Klingel plans to include a 2-ft. thick bed of sand between his concrete slab and a layer of rigid foam insulation. The sand is a heat sink, but Klingel isn’t sure where the PEX tubing should be located for the best result. Nor is he sure what diameter the tubing should be, or what the spacing of tubing in the sand will work best.
Some writers think a sand bed is a waste of time. Others report they’ve had good luck with them, even in extreme climates. That discussion, similar to an exchange on the Q&A forum last year, is the subject of this week’s Q&A Spotlight.
Forget the idea — it won’t work
Count GBA senior editor Martin Holladay among those who think that an insulated sand bed doesn’t add much to solar design. “Here’s my opinion — subject to revision when someone gives me good monitoring data to contradict my statement: you can put the PEX wherever you want, because these systems don’t really work,” Holladay tells Klingel.
To get a useful amount of heat from the sand during the coldest months of the year, he says, it must be hot enough to get water in a hydronic heat distribution system to at least 100°F. And that, he adds, just isn’t going to happen.
“The sand doesn’t get that hot — or if it does, it doesn’t stay…