Turning on Heat in sub 32 degree weather

xcheese

How do I go about turning on my hydronic system once the slab falls below 32 degrees?

28x44 garage slab 4 inches thick 2 inch foam under and around perimeter. Finally got all the wall and ceiling insulated, but need to finish building controller with winter fast approaching. 4 loops 300 ft each - 1 zone. It will be heated with a water heater running glycol. Slab pour in spring of 2020, unheated last year. Looking for low temp... just keeping the air temp around 50 degrees (F)

Looking for input on how to warm the system / floor without cracking.

Would turning the circ pump on when I start the heater for the first time so it circulates the slowing water fluid keep the slab from cracking?

Jamie Hall

You're using a glycol/water mix? I won't comment on the water heater... wrong device... but let that go.

If you are using a glycol/water mix, then it's simple enough: start the circulating pump and start the water heater. That way you're warming the circulating liquid and the slab all together, and you shouldn't have a problem.

GroundUp

Either that, or simply turn the WH thermostat down to 80ish to start and see where that gets you. If it's not reaching 50, turn the water temp up to 90. So on and so forth.

Dave H_2

When the slab is that cold at startup, expect a loooong time for it come up to temp. Even the heat source is not going to see its target temp for a while. That slab is going suck out BTU's faster than it can make. The load at this point can be up to 4x larger than the heat loss.

I would do as @GroundUp suggests, start with 80ish degree water and let it run for a few days/week then start turning it up

Dave H.

Zman

If the slab is super cold, there is a risk that the water will freeze when you first fill it. I would warm the space for a few days with temp heat and be 100% sure the boiler will fire before filling with water. Unless you grossly oversize the boiler, the slab will climb in temp very slowly.

hot_rod

Is it a standard residential gas water heater? 35,000 BTU or so?
That water heater will probably be condensing for days, maybe its entire life certainly as the cold slab warms. I doubt it has enough output to shock the slab, set it wherever you need to run.