Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Radiant Heat Return Lines are Cold but floor is hot

Ty76 Member Posts: 1
I have a 2-story dual climate controlled system (upstairs, downstairs) with a gas furnace and radiant heat.  Even with the thermostat off downstairs the floor is still warm all of the time.  I initially thought it was just a bad thermostat but when I go to the furnace and feel the return lines and manifold they are cold to the touch except for one of the 6 1/2" lines but the manifold is cool above it. The pump motor is not running. 

The output from the furnace goes to a Tee (one line going upstairs, one going down).  I turned on the upstairs thermostat and both sections of the tee get hot, the one going upstairs and hte one going into the slab downstairs but again, the return lines from the floor are cold. 

The system was installed last year and last winter it was butt-**** cold downstairs unless I had the thermostat on.  If I turned it down or shut it off the floors remained cool.  Since the return lines coming out of the floor in the basement are cold I can only assume that somehow the hot water is not only entering through the input line into the slab but somehow circulating but not returning to the return lines/manifold/pump. 

How can it get warm and ciruculate if the water is not coming back through the return lines and pump?  Could there be a leak and hot wate is spilling out somewhere under the slab?  If there is a leak, does the system re-fill itself? 


  • Ironman
    Ironman Member Posts: 7,119
    Water Temperature Control

    How is the water temp to the slab controlled? What are you using for heat source? It's not a furnace; furnaces heat air, not water. Is it a boiler or on-demand (tankless) water heater?

    Can you post some pics of the heater, near piping and controls?

    You should not have "hot" water going to a slab. Max should be 120* and less is usually sufficient.

    You probably don't have a leak. From your description, it sounds like there may be a piping problem or the slab water temp is not properly controlled. Maybe both.

    The water temp to the slab must be controlled by a smart mixing valve or by variable speed injection mixing. Either method uses outdoor reset to vary the water temp to match the load. The colder it gets outside, the warmer the water to the slab. This is necessary because of the high mass of the concrete. It takes hours to heat it up and, conversely, it will continue to give off its heat for hours after the thermostat is satisfied and the pump has stopped. It's called the flywheel effect.

    If the water being delivered is too hot, then the slab will be heated too hot. Even though the t'stat has been satisfied, the high mass will continue to emit heat for a long time. That's probably why your floor is warm but your return line is cold.

    With proper mixing control, the thermostat can almost be eliminated because the control will deliver just the right water temp to match the load.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Tim_Hodgson
    Tim_Hodgson Member Posts: 60
    It sounds like you have Ghost Flow

    when the 2nd floor thermostat is operating you are getting some unwanted  parallel flow through the basement loop. Turn off all the heat for a few hours until the piping cools and turn the 2nd floor thermostat up and feel the basement piping to see if it warms up.

    Best regards,

    Timothy Hodgson
This discussion has been closed.