thermal shock in a slab

ecletrical

I'd like this solar fired concrete slab to have no temperature protection. That is, coming directly from collectors to slab without any sort of mixing.

The only protection for the slab would be diverting glycol to an indirect tank during the summer or when the glycol temperature get too high.

My question: what is a reasonable maximum temperature difference between glycol and slab that can safely do no damage to the slab.

atomic

too much heat

I have a system like this. It is very efficient, and with so much mass doesn't run too hot. BUT, of the collectors ever stagnate, the fluid will melt pex in a heart beat. How do I know? Yup, it happened to me when snow covered my PV panel for the pump, and the snow on the collectors slid off. I highly recommend a dump load that opens if the fluid reaches 130'F. You could shunt it to a water heater or other load, mine is a hot tub!

ecletrical

thanks

good warning. Yes there will be diversion to an indirect.



As long as I stay within pex specs I'm OK.



My concern is with doing damage to the slab? Any ideas on those numbers

NRT_Rob

complicated question

say the water is 200 degrees. how much energy is really in it? How fast will it cool down in your slab?



dumping a cup of 200 degree water on your slab wouldn't cause a problem. pumping 200 degree water through it all day might.



I would suspect that very few panel arrays contain enough heat to cause a real problem if you are definitely within pex specs in all cases. However, I would use a temperature limiting valve set to 140 anyway in any case where you are using storage.



Not sure what the big deal there is. it's $100 to $150 valve. pretty cheap peace of mind, unless you know a lot more about concrete physics than I do... which is entirely possible, and in which case, please share

Kevin_in_Denver_2

No Pex in a closed system period

There's no way to guarantee that temperature and pressure will stay within PEX specs. Murphy's Law governs here.

Karl_Northwind

mixing valve

put a thermostatic mixing valve with the hot from the collectors, cold teed off the piping from the floor and the pump on the mixed, with the setting at 150F.  not often will the temp top 150 and therefore mix, except in the summer, but you then have something protecting the pex.



if you have other loads for the summer ( kind of necessary) you can still use the same circulator, just use a 3 way valve or zone valve to send the fluid where you want it.

caleffi has solar rated diverting zone valve bodies, with a wide range of actuators.



best,

karl