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Differential pressure and flow rate

chakil
chakil Member Posts: 28
From Modern Hydronic Heating: For Residential and Light Commercial Buildings, Siegenthaler, John : "Systems that use several zone valves in combination with a fixed-speed circulator can experience undesirable operating conditions when only one or two of the zones are on. The problem that arises is an increase in the differential pressure supplied by the circulator when only one or two zones are operating. The increased pressure can cause excessively high flow rates and flow noise in the active zone circuits."

I don't understand how closing one or two zone valves would result in an increase of the differential pressure and in an high flow rate?
isn't an increase in differential pressure should result in a decrease of flow rate?



Comments

  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,317
    Lets say you have 3 zones and each zone needs 2gpm for a total of 6gpm. So you buy a pump that will pump 6gpm against the resistance of the 1 zone that has the most resistance (usually the one with the longest run of pipe)

    So now you shut 2 zones down, the pump is still trying to move 6gpm through the 1 remaining zone. It can't because the pipe is too small so the discharge pressure of the pump rises to try and push the water through. So the flow rate on the 1 zone that is calling goes up

    This causes a higher differential pressure and more flow than the one remaining zone is designed for causing velocity noise.

    But in reality your confusing two different things. By closing zones you have altered the piping system and everything changes

    Your right about an increase in differential pressure reducing the flow. With one zone calling the pump will try and move 6 gpm through 1 zone it can't so it will move say 4 gpm still to much for that zone. Because the pump head increased it pumps less water and more differential pressure. But still too much for the remaining zone.

    Lets say you only have 1 zone that needs 6 gpm so you put in the same pump as above. So the pump runs and you get your 6 gpm. Now someone starts to close a valve in the discharge line from the pump. The pump head or resistance the pump pumps against goes up, diff pressure goes up and flow goes down
    chakilZman
  • chakil
    chakil Member Posts: 28
    Thank you for your detailed and clear explanation. it was very helpful and understandable
  • hot_rod
    hot_rod Member Posts: 21,853
    A visual of what Ed described.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    chakil
  • chakil
    chakil Member Posts: 28
    Is closing a zone valve always result in steeper head loss curve or it depend on the zone circuit resistance, if it is for example with a finned tube baseboard emitters or radiant floor ... ?
  • hot_rod
    hot_rod Member Posts: 21,853
    Take a look at this journal, it may help visualize hydronic circuits and the interface with the circulator.

    On the graphic I attached above the blue lines are the various circuit "system curves" when plotted over the pump curve you establish the various OP operating points.
    Idronics takes you step by step through the calculations.

    https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
    chakil
  • chakil
    chakil Member Posts: 28
    hot_rod said:

    Take a look at this journal, it may help visualize hydronic circuits and the interface with the circulator.

    On the graphic I attached above the blue lines are the various circuit "system curves" when plotted over the pump curve you establish the various OP operating points.
    Idronics takes you step by step through the calculations.

    https://www.caleffi.com/sites/default/files/coll_attach_file/idronics_16_na_0.pdf

    Thank you very much for the document
    I liked this passage : " Think of this as similar to what happens
    with the “resistance” to traffic flow on a 3-lane highway
    when one of the lanes is closed."