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thermal shock in a slab

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ecletrical
ecletrical Member Posts: 22
in Solar
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.

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  • atomic
    atomic Member Posts: 9
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    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
    ecletrical Member Posts: 22
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    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
    NRT_Rob Member Posts: 1,013
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    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 ;)
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
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    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.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Karl_Northwind
    Karl_Northwind Member Posts: 139
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    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
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