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Will I crack my floor if...

Here's a bit of an odd question.
I'm building a house, slab on grade. It's closed to weather and well insulated. It has 7 in-floor heating zones.
I haven't yet hooked up the heating system - some components are on back-order. It's getting cold working in here (not freezing - about 50F).
Our heat supply is an outdoor wood furnace. It's already running, heating other buildings.
The thought came up to connect one zone direct to the main feed from the furnace, to get some heat in here until our full system is ready to hook up. It would be easy to do, and I'd just control it by turning on/off the main circulating pump, but I'm wondering if I pipe 160-180F water through one of seven zones, will I risk damaging the floor slab due to the uneven heat?

Thanks for the help!!


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    160 to 180? You might. Won't say you will, but there's a good chance of it (I presume you are referring to radiant tubes imbedded in the slab).

    You really don't want the water temperature fed to the slab to go over about 120 to 130 -- and you don't want the slab surface temperature much over the mid 80s.

    So... what do you do? You create a loop with a mixing valve which recirculates part of the return water from the slab back to the slab, and draws only enough water from the boiler to keep the feed, mixed, water up to about 120 to 130.

    You do have to have two pumps -- one for the boiler loop, and one for the slab.

    Standard practice...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • TheCave
    TheCave Member Posts: 2
    Thanks Jamie!
    I think the mixing valve is one of the back-ordered parts, but that does make sense to limit the water temperature going into the floor.
    I've got a rad, more in-the-way but maybe we'll hook that up until we can hook up the complete system as it should be.
    Thanks again for your help!
  • BDR529
    BDR529 Member Posts: 292
    Run outside reset, 3-way motorized mixing valve and a room sensor not a thermostat.