Bringing Slab to Temperature

JH1226

I have just fired up a radiant heat system in my shop office 36 hours ago. I live in a northern climate. How long should it take to bring the slab up to temperature?

STEVEusaPA

How big.....

is the slab?  How thick?  Was it insulated on the sides and underneath?  Is this the first time firing it up?  Is all the air purged out of the system?

If it's designed/installed/operating correctly, you should already be toasty.  Unless its a huge volumous space.

Has the boiler been running non-stop? What's the supply temp heading into the slab, and the return water temps?

Did you shoot the floor with an IR gun?  What's the temp?

Tell us everything about the system, and we'll help you.

Jean-David Beyer

My experience.

I have a radiant zone that is a concrete slab with copper tubing in it. My old oil burner would quit sometimes (usually HV transformer quitting). The slab has so much thermal mass that I never called for emergency repair. Even once when it failed on a Friday night. I called first thing Monday morning.



My house is pretty well insulated. So once the boiler failed, the house would slowly cool down. I ran it around 70F, and by Monday morning it would be down to just below 60F. The numbers do not mean much, since I do not know the outside temperature. It meant I needed a sweater or two blankets. But if you are willing to put up with that (I do not recommend it as a general rule), mine took 60 hours to drop, say, 12F. Once the boiler functioned again, it started warming up the house. That boiler had no reset and was putting between 130F and 140F water into that slab. So the slab was too hot by the time the thermostat was satisfied, and the overshoot was considerable. But if I remember correctly, it was fairly comfortable in 8 to 12 hours.



Now if something like that happened with my new mod-con, I would have to do something else, since the outdoor reset is so tight that to recover from  a "setback" of 12F would take a very long time. I would just diddle the controller to boost the temperature for a while.

JH1226

Bringing Slab to Temperature

The slab is 4" thick and is a 144 sq ft office with 8' celings and 480 sq ft shop with 10' ceilings. This is the first time firing up the Munchkin T80M. The exterior of the slab is insulated with 2" blue board down 24". There is a 40' section of insulation that is not yet completed. The insulation under the slab is Barrier underslab insulation with an R6.7. The 1/2" Pex in the office was run 9" O.C. and in the shop 18" O.C. There is one zone for the office and one zone for the shop. The air was purged from the system. The outlet temp is 112 and the return is 105. I did not shoot the slab with a IR gun. There is no flooring on the slab. There is a 40% cryo-tek-100 anti-freeze solution in the system. The boiler is set to fire on at 94 and off at 117. It takes about 2 minutes to cool down and 2 minutes to heat back up.

Gordy

what is the problem?

 It seems supply, and return temps are fine. Small delta T for a cold slab though. Is the space not heating up to set point? Slab should be no warmer than 85*.

  18" O.C. is a bit wide for spacing with 1/2" pex, but then I would expect a wider delta t on the supply, and return if the slab was soaking up btus.





Some details about the system. How long are the loops? Type of boiler assuming mod/con? Near boiler piping Pics? Pump sizing? Is there manifolds with flow meters? If so what are they set at?

Gordy