I have a SIPS house near anchorage AK. Oil fired in polished concrete slab radiant heat. On a hilltop south facing wall mostly windows. This time of year the solar gain is incredible, gets 80F+ on sunny days and obviously the heat does not kick on. Some mornings when there IS a call for heat the boiler runs for hours reheating all that water and slab when the sun will make our house very comfortable about the time the boiler kicks off. MY QUESTION is, would we be in any danger of freezing the water in our slab if we set the thermostat back and use the solar gain to heat our house on sunny days? I figure the heat mighty stay off for a few days at a time other than to run our hot water loop. Our pex is laid on top of 2" of foam, 6" of foam within 2' of slab, 2" out to 4'. We have a 24" standoff around exterior walls ( and yes your toes do feel cold if you go stand next to a window in your bare feet)

It's hard to believe PEX in an insulated slab could freeze when you are getting more than enough insolation to make the home comfortable. Could you run the circulator with the boiler off in order to monitor the fluid temperature and see if it is ever getting down to near-freezing temps?

There's a digital readout for my water temp which I guess is actually the
water in the boiler, my real concern would be out on the perimeter near exterior walls. Slab feels FRIGID out there but tubing is 24" inboard . Maybe get a reading on the floor temp one foot inside the walls as a gauge??

If you are at any risk of freezing you PEX, you should have slab temp sensor that at least annunciates a warning (and preferably initiate action) before this occurs. Yes, freezing will first occur at the slab perimeter where the heat loss (BTU/H) is largest and where the slab temp will be coldest.

Circulate the fluid and monitor the temperature. As long as it is not getting down near freezing, the circulation will help keep the outer edges from freezing.

Some mornings when there IS a call for heat the boiler runs for hours reheating all that water and slab when the sun will make our house very comfortable about the time the boiler kicks off.

That's overshoot and is why active thermal storage (usually a tank and water) and lower mass radiators are attractive.

I get calls, especially this year in Minnesota, for frozen pipes of all kinds. Nearly every year someone calls to say an copper pipe serving fin tube has frozen. When I ask about setback (marginally effective for hydronic systems anyway) I nearly always get a positive answer and when I don't I confirm that some type of auxiliary heat has satisfied the central boiler and/or shut down a zone.

Last December I went out on a Buderus GB142 condensing boiler heating a steel building. The boiler and pump were off and the perimeter of the slab read 22°F at the surface and 42° in the center. Once the pump was restored and before the boiler was repaired we kept a steady 42°, end to end, with a modest infrared space heater supplying the only heat in sub-zero weather.

You don't want to freeze PEX in a slab, regardless of the depth, since it will burst the tube in nearly every case. If you feel you are in serious danger of burst temperatures you should add the appropriate amount of propylene glycol.

I would rather devise a control strategy to take advantage of solar gain by employing full-time or nearly full-time pumps with injection or other means of controlling slab temperature and thereby ambient.