Hydronic heating. When should the pumps run?

Discussion in 'Boiler Forum' started by Technophile, Feb 13, 2021.

  1. Technophile

    Technophile New Member

    May 29, 2012
    Duvall, WA
    Hello! We have a hydronic heating system that appears (to my admittedly inexperienced eyes) to have some design and installation errors (no filter, expansion tanks undersized and upside down, air eliminator in the heating return instead of between the boiler and circ pump, ...). I would appreciate help in understanding what I am doing before making any changes.

    One big question is, are the circulation pumps on at the right times or did they swap the connections?

    In the photo, the (Buderus GB142-24) boiler is just out of the frame, above. The storage tank is off to the left.
    • The black pump at right flows up, and is constantly on.
    • The red pump feeds the storage tank (outside of photo at left), with return plumbed to tee in at top right, just below the boiler intake. This pump only seems to run when the boiler is running (and perhaps while cooling down).
    • The bottom horizontal pipe is heating return, flow is due to per-branch circ pumps outside the photo.
    • The pipe above it is hot water to heating, separate circ pumps for each branch, outside the photo.
    I can see that the black pump connects the boiler pipe circuit to the heating system.

    What confuses me is:
    • Why circulate through the boiler all the time? Looks like this connects the flow circuits?
    • When the boiler is off, the red pump is off. How does heat get from the storage tank to the heating circuit? There's no check-valve on that pump. Will there be enough circulation through it driven by the black pump?
    Last edited: Feb 13, 2021
  2. fitter30

    fitter30 Well-Known Member

    Feb 2, 2020
    Retired service tech
    Peace valley missouri
    Your pic didn't come out . Take some pics of the boiler, ex tank ,pumps, piping
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    Though they're a "nice to have", not all systems require filters. They become more important when the piping & or radiators are cast iron or steel.

    Expansion tanks with internal bladders don't have a preferred orientation- they can be upside down, sideways or cock-eyed at some random angle and still function just fine. The best position for the connection of the expansion tank is a foot or two of straight pipe just prior to the intake side of the pump(s), where it will function somewhat like a shock absorber/damper, limiting the amount of cavitation and stress on the pump, but even that isn't always essential.

    Air eliminators usually DO have a preferred orientation, with the vent on the top. Most air eliminators also function better if there is at least a foot of straight pipe from any fittings or valves on the incoming side so that the flow is less turbulent as it passes through the device, which allows the bubbles to accumulate toward the top.

    With the higher pumping head of water tube boilers it's often better for the primary pump to be pumping toward the boiler, which will put the heat exchanger at the highest pressure on the loop. The somewhat higher pressure will limit the potential for the micro-boil going macro in the heat exchanger creating sizzle & bang, and lower heat transfer efficiency.

    Whenever there is a call for heat the boiler needs to have flow, even when the burner cycling on/off due to insufficient radation on the zone to emit the full min-fire output of the boiler at the operating temperature. Without the flow the boiler would not be able to determine when the loop had emitted enough heat to fall back in to the boiler's operating setpoint range.

    When the boiler is off and there is no call for heat, there should be no pumps operating.

    When you scale the picture to where it shows up it there will surely be more to comment on.

    Can I assume you've read the installation manual, and understand how hydraulic separation works, with primary & secondary pumps?
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