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How do I balance a hydronic heating system?

Hello the Wall,

My family owns a 1000 sq ft vacation house in Northern California built in the late 60s with a slab on grade hydronic system. No emitters, just six 3/8" loops in the slab. The boiler went a few months ago and in the course of replacing it, I discovered the system was not only poorly designed (no expansion tank, no air remover, no reducing valve, over-sized t&p valve), but also has been incorrectly serviced all these years--to correct the leaking t&p valve (no expansion tank), some bright boy plugged the discharge line with concrete; before that happened, in order to make up the water loss, the cold water inlet valve was left open to city water pressure (80 psi). The list goes on, but I think you get the picture.

Needless to say, there is also a slab leak. After fixing all these problems, I want to balance the system, as it's pretty obvious this was never done (the floor heats up quite unevenly). I've been searching online for a few hours and am unable to find a resource which would teach me how to do this. The supply manifold is buried in the slab and the six control valves on the return manifold are sweated on. The return manifold is just below the circulator (1/12 hp Grundfos). I know I have to measure flow in each loop to balance this system, but am sort of stumped as to how to go about it. Any help here would sure be appreciated.

Thanks and Happy New Year,

Scott Matthews


  • Mark Eatherton1
    Mark Eatherton1 Member Posts: 2,542
    Welcome to the Wall...

    Thanks for the excellent description of your system. If there are balance valves on the return manifold, I'd suggest you first try and balance it thermal dynamiclly. In other words, with 140 deg F water going out, attempt to acheive an average of the return temps, typically around 30 to 40 degrees F. Balance all circuits out to approximatley the same return temperature and see what happens.

    There are ways to "see" into the slab and see what the circuitry is...spelled Thermal Infrared Imaging.

    Pricey ($350/hr, 2 hr minimum) but well worth it in some cases. You may be able to borrow one from your local volunteer fire department...

    In any case, I hope you have found and repaired the leak. If not, you must. If you need help in finding the leak, there are numerous posts here at present reegarding leaks in snowmelt systems, which are really radiant floor heating systems in disguise:-)

    All I can say right now is AHHHHhhhh....That shower felt SO good!!!

  • George_10
    George_10 Member Posts: 580
    I would also suggest that you

    clean and treat the system now that you are completing the repairs. This will give you better system efficiency and reduce future problems of corrosion. You can check us out on our web site at www.rhomarwater.com under hydronic products. Wish you the best on your project.
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