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Radiant Heating Zone Migration/"Leakage"

Moved into a new house in July - first time with hot water heat. With cold weather coming I was messing around with the system for the first time this weekend and found hot water will flow into an inactive zone when an active zone calls for heat. Seems like there's not much point in putting the effort into zoning a system like this if you can't actually prevent the flow of hot water, so I'm wondering if there is something wrong with the zone pumps or general setup?

Photos of the setup with additional details/model numbers of everything involved are below. The zones are numbered 1-6 from bottom to top. I turned on my living room thermostat (zone 6) and upped the temp to activate a call for heat. The living room radiator heated up as expected, but so did the dining room radiator (zone 1) to the point that it was also throwing off enough heat to warm the room. I had the dining room zone thermostat off. Should I have the dining room zone thermostat on, but set to a low temp (no call for heat) to get the zone 1 pump to actively close/prevent water flow? Or is it even possible to prevent water flow into inactive zones when the taco circulator is on due to a call for heat from an active zone?

I've done a bit of research and it looks like the zone pumps have flow check valves - but would those valves even be able to prevent the flow of water being moved by the taco circulator? Maybe I need to semi close the ball valves in varying degrees from bottom to top to prevent flow into the lower zones in the manifold when the taco circulator is running?

Thanks for the help and advice.

Full setup:

Boiler model details - Weil-McLain EVG-299:

Zones are controlled by Pro1 T955WH:

Taco SR506-ECP relay:

Main circulator - taco 0014-F1-3

Zone pumps - Grundfos UPS15-58FC


  • Dave H_2
    Dave H_2 Member Posts: 527
    Open the valve that is on the boiler loop. The one that is partially closed.

    The only time that valve should be used is in isolation and/or purging. Never throttled

    Dave H.
    Dave H
  • swedenumberone
    swedenumberone Member Posts: 12
    Thank you, Dave! I will give that a go.
  • clammy
    clammy Member Posts: 2,983
    Just wondering are you getting migration thru the pumps or on the return side if it the return side then your gonna need check valves on the returns . I try not to pipe in the manner that they choose when using multiply pumps always seems to lead to thermal migration and it s a issue for some other see it as nothing .also the valve your throttling should be open as others have stated . Usually I pipe them horizontal not vertical and make a thermal trap for each return it’s a little pipe cheaper then a check valve and no moving parts in your case it going to flow check valves the swing checks they installed are the wrong valve and where not needed the pumps checks should be enough to stop migration through the pumps .if it’s migrating through the pump you may have dirt solder paste stuck in the valve . remember thermal migration hi temp to low temp or non separation of pumps also does your system do domestic hot water through a indirect tank if so see if the return piping is hot into your system piping if so that may be your issue . I see that a lot hi temp indirects w no return flow check to a low temp heating system the tank migrates to the systems and system then calls for domestic heats the tank to set point then slowly migrates into the heating system until the sensor or aquastat calls again ,very common issue seems a lot of guys see no purpose to the flow check but boy do they get pissed when then have to come back and install a check they usually try to say they never have a issue lol check on pump check on return of all indirects unless you don’t mind migration but on low temp system there’s really no way around it espically once the ho notices it hope this helps peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating