I am a newbie to the forum as well as Radiant Heat. I recently built a 32x48 shop and put pex in the floor. I am having a problem keeping the heat consistant. The thermostat is set on 62F with a 2 degree window. I check the system in the morning prior to leaving for work and then when I return home in the evening. The thermostat is reading much below the 60F set time for refire. Sometimes as low as 53F, but the Tankless water heater and pump are running as usual and will build the heat back up to the 62F mark. I have bled the system many times, but it is possible that I still have air in the system. Would this cause the system to go below the set turn on point? thanks, Tim

Are you saying that the thermostat is set for 62F but the temperature needs to fall far below 60F before it actually closes the circuit? Maybe a bad thermostat.

The thermostat is set to shut off at 62F and start back up at 60F. It does not always start back up at 60F, sometimes it goes below that set point before it starts back up.

Is the thermostat programmable for different days? One or more of the days might not be programmed at some times of the day to the conditions you want.

The system will go below the setpoint if the heat load is getting ahead of the radiation. It's likely that the thermostat is working fine, but the place cools off to 53F before sufficient heat can be pumped into the slab so that it's surface temp is high enough to keep the room at 62F.

Two solutions:

1: A PID-algorithm thermostat designed for managing the time lag of high mass radiation like slabs.

2: A slab thermostat- fuggedabout keeping the room temp constant, just keep the slab steady and let the room rise & fall a bit with the actual heat load. (This works better than it sounds, in most cases.)

If there was too much air in the system for circulation the tankless wouldn't even fire. If it has a remote attached to it, it'll probably even tell you what it thinks the flow rate is. If it's firing that heat is going somewhere (heard any loud steam explosions coming from the garage when this was happening? :-) )

Yes, that is why you need to test the thermostat to see if it is actually closing the circuit at 60F.

Thanks for all the input.

For 3 or 4 days, the thermostat set point was working fine. Temperature dropped to 60F, pump kicked on, tankless kicked on, took about 15 to 20 minutes and temperature came back up to 62F. Now it is all over the board. I have not been able to be in the shop when it starts its cycle, I always manage to show up after the system has fired up. I will be able to get some better information on Saturday, plan on being there all day replacing a clutch and CV Joints on my sons car.

I will report my findings on Monday.

You had mentioned that you "flushed" the system to get the air out. If you flushed using supply water from the tap, then per my recent experience, you will have introduced air back into the system as there is quite e bit of air in tap water. After much frustration trying to purge the air out of my 2-Zone (7-Loop) system, I purchased a decent utility pump and using a 5-gallon pail, flushed the system one loop at a time using the utility pump supplied from the pail. The return water went back to the pail as well and after several hours I purged most of the air out of my loops. Each of my zone manifolds has an air separator that bleeds off the remaining air as the water circulates. Good luck.

Who designed your system?

A new building with proper and simple controls will stay within a degree of set-even if you misapply a tankless water heater.

Purging should be done with cold-city-water at pressure. Typical purge, about 20 minutes.

Professional design before you pour, makes more sense.