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How do you adjust the bypass valve in a hydronic system?

My 20 year old house has a functioning 5 zone system. Water returns from each zone controlled by a thermostat and is collected in a manifold. That water is then pumped by a single speed grundfos circulator pump through my 163 MBH burnham gas boiler to a hot water manifold that supplies the finned baseboard radiators. Between the two manifolds there is a bypass with a gate valve in the middle. Both manifolds and the bypass are 1.25” copper.
How should the bypass valve be set? Both mechanical contractors in my small town have had a look but neither has provided a good answer. Closed and open seem like wrong answers. There is no easy way to measure flow or pressure in the system other than the temp/pressure gauge coming out of the boiler. I need a simple answer to what I am sure is a difficult question. Slightly open? 1/3 open? Half?


  • coloradojeff
    coloradojeff Member Posts: 3

    The valve in question is on the left, just above the WH
    HVACNUT Member Posts: 5,744
    If the bypass is piped near the inlet side of the pump (circ is on the return from your description) then the objective is a 20° difference between the supply leaving the boiler and the return south of the circulator on the discharge side.
    Assuming the system has a large water content, it's the poor mans answer to regulating return temperature at or near 140° to help prevent shocking the cast iron block with cold return temps and possibly short cycling of the circulator.

    There are better options for thermostatic regulation.
  • coloradojeff
    coloradojeff Member Posts: 3
    Like a differential bypass valve instead of a old gate valve?
  • Zman
    Zman Member Posts: 7,542
    I am not convinced you need it at all.
    Cast iron boilers are not very finicky about low flows and it would take over 175' of baseboard to make the boiler condense (assuming about 7,000 elevation)
    Is the circ a taco 007?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • hot_rod
    hot_rod Member Posts: 21,853
    WAG, a guesstimate as flow will change as zone valves open and close. With low mass heat emitters really not a lot of value
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Gilmorrie
    Gilmorrie Member Posts: 185

    Like a differential bypass valve instead of a old gate valve?

    In any case, a gate valve is not well suited for that purpose, which is throttling. A globe valve would be preferred.