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Cold water backfeeding up hot water recirc piping

I just came across this issue. Customer recently had an indirect tank replaced. A mixing valve and recirc pump are also on this system. This link gives an accurate depiction of what how it is piped.


Problem is on a high draw of water, such as the soaking tub, cold water will backfeed through the check valve and recirc pump and up the return piping to the fixture. The recirc pump is powered constantly so I would think it would rule out the issue being as simple as a bad check valve. Do I have a pressure differential problem here, where their street pressure too great for the pump to overcome. Hoping to hear someone has come across this before.



    JUGHNE Member Posts: 9,910
    Would a pressure reducing valve on the cold supply, maybe even the entire house, stop this.
  • Jamie Hall
    Jamie Hall Member Posts: 19,432
    The check valve is leaking on the higher pressure differential
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Brian S
    Brian S Member Posts: 11
    So a pressure reducing valve on the cold supply to the was actually my next thought as well. Just never heard of having to do this.

    I do know that the check valve passes. Was easy to see it pass by disconnecting power to the recirc pump and seeing it backfeed. But I do not believe this explains the problem because there is always constant power to the recirc pump which would make the check valve constantly open as well. Why would the cold water be able backfeed why the pump is circulating?
  • hot_rod
    hot_rod Member Posts: 16,881
    What type of check valve? Swing checks don't really seal tightly, a spring check or check in the pump itself would be 100% shutoff.

    Is the main HW line to the fixture undersized if it is trying to pull from two lines?

    Large flow tub fillers may need 3/4 or larger depending on length and gpm you are trying to flow.

    What is the incoming water pressure? Any softness on the hot supply? Got a thermal expansion tank downstream of checks?

    Check valves in the mixing valve tailpieces?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Jamie Hall
    Jamie Hall Member Posts: 19,432
    No. If there is enough flow demand on that recirc pipe, which I dare say there will be, the flow will be backwards through the pump, power or no power.

    One doesn't usually concern one's self with the behavior of pumps under back pressure conditions, because it's not supposed to happen (it's kind of hard on the pump) but if the reverse pressure differential is greater than the shutoff head of the pump, you'll get reverse flow.

    Some types of pumps -- piston, screw, other positive displacement types (but not diaphragm) -- this isn't true, but it is for any centrifugal, no matter how large.

    Fix or replace the check valve.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Brian S
    Brian S Member Posts: 11
    Good call Jamie, I overthought that one.
  • HomerJSmith
    HomerJSmith Member Posts: 1,693
    edited October 2018
    Could the CV of the mixing valve be inadequate? A higher CV mixing valve might be preferable.

    Look, the first thing that I learned in plumbing school, other than paydays on Fri, is that water takes the path of least resistance.

    Look for the path that the water is suppose to take and the path that the water is taking. If there is less pressure in part of the system, water will move to fill that lower pressure to establish equilibrium.

    "Look for the resistance, Luke!"

    Maybe, you need a check valve on the cold water return, too.