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Grundfos 26-64 pump opening Honeywell zone valve.

I have a customer who says that his master bedroom zone overheats. The room continues to heat when the thermostat and zone valve are off and one of the other zones is on. The Honeywell zone valve operator was replaced by another contractor which didn't help and I almost replaced the valve today until I decided to check his system pump which is a Grundfos 26-64 which I believe is oversized and could be forcing the zone valve to slightly open, heating the space. This overheating happens only in the master bedroom zone which is furthest away from the pump. If I manually open and close the zone valve at the lever on the operator, it feels fine.

At this point, I don't know whether to replace the pump or the zone valve.

Honeywell spec's.: https://forwardthinking.honeywellhome.com/related_links/water/5000_series/install/95c_10932.pdf

Grundfos spec's:

Flowing 8 gpm @ 6' head.
8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

Two btu per sq ft for degree difference for a slab


  • NY_Rob
    NY_Rob Member Posts: 1,370
    I had one of those Honeywell zone valves with the rubber ball plug that closes the valve off. On mine the ball had deteriorated to the point where there wasn't much of it left and it was doing nothing to stop the flow of water past it even when closed.

  • SuperTech
    SuperTech Member Posts: 2,186
    Have you ruled out gravity flow? Are the zone valves on the supply or return? Do you have any pictures of the piping?
  • Zman
    Zman Member Posts: 7,573
    You might need both. I agree that the pump is oversized. Has it trashed the valve?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 22,256
    that is a very low shutoff pressure on the high Cv valve? Typically a 20 psi shutoff for large Cv, up to 75 psi on smaller Cv valves. No doubt even a 15-58 could push open that valve.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream