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Master Plumber

Leaky Member Posts: 16
Hi all,

I'll soon be reworking a hot water loop setup (3 zones) off a Weil-Mclain steam boiler.  The customer is tired of the water boiling in the hot water zones!  Elsewhere on this great site I learned I need to pipe a bypass between supply and return.  My remaining questions are:

Do I need to pipe a thermostatic 3 way mixer between supply and return, or will a throttled ball valve suffice?  If I use the ball valve to mix, won't the HW temps be too low when there is no call for steam?

Do I need to install another aquastat?  There is already a tankless coil aquastat maintaining 170 f all the time.  Won't that be sufficient?

Should I leave a flow check on the return?  Some information suggests I should, just to create head for the circulator.  The zones are controlled with zone valves.

Thanks to all for your input!


  • Steamhead
    Steamhead Member Posts: 14,864
    edited August 2009
    Here's a hookup we did

    and it does include another aquastat from the one you already have, visible at the front.

    The purpose of the second aquastat, mounted in the supply line to the HW loop, is to make sure the water never approaches the boiling point when only the HW zone is calling. This keeps the main house (steam) from heating up when it shouldn't.

    The bypass is needed to make sure the water in the HW loop does not approach boiling when the boiler IS making steam. A throttled ball valve is fine for this, you only have to set it once.

    This job does have a flow-check on the return, not just to provide head for the circ, but also to prevent gravity flow in the HW loop. Believe it or not, this thing did circulate by gravity without the flow-check! But depending on the ZVs you use, they may provide enough head, so the flow-check may not be needed.
    All Steamed Up, Inc.
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Leaky
    Leaky Member Posts: 16
    ...and the flow check?

    Since my boiler is always 170 f, couldn't I go without a second one to limit the temp to 180?  It seems redundant....

    Also, I'll need to set the ball valve while the boiler is calling for both steam and hot water zone too, right?  What's an acceptable temp to the HW zone- around 200?

    Is the flow check necessary?
  • Leaky
    Leaky Member Posts: 16
    Thank you

    Thank you, steamhead.  I really appreciate the quick reply and the picture too.  I reposted this with a more appropriate title- will check on that thread from now on.

    Thanks again.
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