Help me understand the correlation between boiler circulator RPM / HP / Head / Flow / System Pressure - see the particulars at the bottom.

CASE: My 1986 boiler came with Armstrong S25 circ (same as B&G 100). When the circ failed I replaced it in an emergency with the Taco 007. Now each time thermostat called for heat and the boiler kicked in I would hear 3-4 gentle pops from the boiler (within first 15 seconds, and never at other times). Someone here suggested lower head on the TACO 007 maybe the reason (24 vs 32). So I replaced the 007 with TACO 0010 (closest match to Armstrong S25). Sure enough the popping sounds are gone. But now water flow sound in pipes and rads is loud, and when the boiler heats up I am hearing a wailing sound. I can live with the water flow sound, but the wailing can be annoying. Why is the TACO 0010 causing the wailing? What part of the system can I adjust to stop it?

Cold system pressure has always been, and is now set at 12 PSI. When the boiler runs hot pressure rises to about 18 PSI.


Cold system pressure: 12 / Height from gauge to the highest rad: 14 feet

Expansion tank: 12 PSI

Armstrong S25: 1750RPM; 1/12HP; 9-32 Flow-GPM (original system circ, was ideal setup)

TACO 007: 3250RPM; 1/25HP; 10-24 Flow-GPM (problem: few pops after boiler startup)

TACO 0010: 3250RPM; 1/8HP; 10-33 Flow-GPM (problem: water flow sound + wailing when boiler heats up)

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  • What is the actual question, or do you want us to do your homework?
    – RMDman
    Commented Dec 9, 2022 at 21:22
  • Why is the TACO 0010 causing the wailing? What part of the system can I adjust to stop it?
    – Roz
    Commented Dec 9, 2022 at 23:15
  • 1
    Just a WAG, but sounds like your flow rate is too high.
    – SteveSh
    Commented Dec 9, 2022 at 23:31
  • @SteveSh what is WAG ?
    – Traveler
    Commented Dec 10, 2022 at 3:06
  • WAG: Wild-Ass Guess. Speculation without enough evidence to support it but which might be worth checking.
    – keshlam
    Commented Dec 10, 2022 at 3:28

1 Answer 1


The Taco 0010 is not the closest match for the Armstrong S25. The S25 is 8 foot head at no flow and 32GPM free delivery. Those match up with a Taco 008 circulator.

The Taco 0010 is 14’ shutoff head and 56 GPM free delivery. It’s pushing much more water through the pipes, which as you noticed can be noisy.

That said, I suspect you’re hearing air gurgle around because you need to purge the air from the system.

  • Entry 10 in the legend is for the Taco 0012. It has 52 GPM free flow.
    – popham
    Commented Nov 23, 2023 at 18:18
  • @OliverStreet You're misreading the chart. Line 8 is the 0010 and has 10 foot shutoff and 33GPM open. The 009 has 35 feet shutoff and 10 GPM open, shown by line 7
    – Ecnerwal
    Commented Dec 23, 2023 at 15:02
  • @Ecnerwal - you’re quite right. The “wailing” that only shows up hot is must be system piping very sensitive to mass flow. He could find it if he could put his finger on it… literally. If it was cavitation he should have seen impressive impeller damage when the 0010 was swapped out for the 007 a month after my answer in December 2022, or maybe he didn’t look. By now the heat transfer has either been judged adequate or poor flow left some rooms cool as the burner short cycled in cold weather. Commented Dec 25, 2023 at 15:22

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