Armstrong S-25 PumpI have resonance in the heating system pipes. Touching the pipes near the boiler (natural gas) pump I can feel ever so slight vibration, most detectable by touching the pump motor.

Background: About a week ago I took advantage of warm weather to replace some components of my heating system (Raytherm 134 WT natural gas boiler, Armstrong S-25 pump with MF/BF bearing assembly, and motor - 1750 rpm). The components replaced:

  • Watts EXT-30 Expansion Tank (12PSI)
  • Watts 335-M2 PRV (preset to 30PSI)
  • Watts 9D Backflow Valve
  • Watts 1156f Pressure Regulator (preset to 15 PSI, turned down to 12 PSI)
  • I also changed the flange gaskets and the body gasket on the pump.
  • Bled the radiators.

Cold system pressure is 12PSI. Running pressure, after 1 week of bitterly cold sub-freezing temps, is just under 20 PSI, boiler temperature under 150F. The bearing assembly is about 2 months old (old one was leaking oil), the motor probably 30 yrs, but ran quietly before the updates). There is some very faint noise from around the pump/bearing/motor, but I can't determine which (the impeller?).

I don't recall there being any resonance / vibration prior to the above updates, I think it only appeared after weather changed to cold after the updates. Any ideas how to tacle this? Any adverse effects if I leave it be (other than the somewhat annoying resonance)?

UPDATE: The resonance is still on. I replaced the Armstrong S25 pump with a wet rotor 3-speed Bell & Gossett NRF25. The difference is that I now have absolutely no detectable vibration, and the pump is extremely quiet by comparison (with the armstrong I could hear the impeller through my rads, whereas to hear BG NRF25 I have to press my ear to the pump.)

After NRF25 installation (yesterday afternoon), and later before bedtime, I'd swear I heard no resonance. But some 12 hours later, the next morning, it's back on. The sound pitch changes depending on the pump speed selection. Low tone on speed 1, and higher pitches on speeds 2 and 3. Speed 3 also introduces water flow sounds into the system, and not surprisingly since it is faster than my furnace specs require, however I don't need it this fast, and run it at 1.

I purged the system of air. Have no vibrations. No hammering. No banging. Only the humming / resonance, and only on the ground floor and basement (where furnace and pump are located), no detectable sound on the second floor. The humming is somewhat similar to flourescent bulb running loud. I think, but only think, that it is present mostly at elbows where horizontal pipes in the basement turn to the vertical leading to ground floor rads. No hummming / resonance at furnace.

Where should I look next?

  • Found possible cause for the resonance. I wanted to try a new coupler, and as I shut down the pump and inadvertently touched the intake water pipe (.5 inch copper) it was terribly vibrating. Just following the flow (intake pipe -> backflow valve -> pressure reducer) I figured the first item to look at was the pressure reducer. I turned off the boiler and let the system cool. Once pressure dropped to 12 PSI (the resonance still obvious), I opened the pressure reducer, manually adding pressure to about 15 PSI. <p> Now the resonance is gone. <p> What can this mean? (reducer is brand new.)
    – Roz
    Dec 16, 2017 at 23:17
  • The Watts 1156F, the fill valve needs to have very clean water. If a speck of sand gets into it wrong it can cause the diaphragm jump up and down quickly. I would recommend taking that fill valve apart and thoroughly washing it. I have at times been required to install a good quality water filter ahead of the fill valve to keep it clean.
    – Paul Logan
    Dec 16, 2017 at 23:56
  • Did you figure it out?
    – rogerdpack
    Dec 12, 2022 at 20:21
  • 1
    @rogerdpack it was the circulator, replaced it with a wet rotor type matching the specs on S25 and the problem solved
    – Roz
    Dec 18, 2022 at 23:17
  • Sweet, feel free to add an answer to your own question too :)
    – rogerdpack
    Dec 19, 2022 at 18:10

1 Answer 1


I suspect that you have a warrantee issue with the Armstrong pump. Sounds like yours has a bad bearing or out-of-balance coupler, it happens. Normally these pumps are quiet and reliable. I wouldn't let this slide. I suspect that the pump will experience a significantly shortened life if left in service. FYI, there is a new class of hydronic pump on the market today. I think it is superior to this type of pump. They are call 'Wet Rotor'circulators or 'wet rotor' pumps. They are actually quieter and more reliable than this style of pump. The cost is similar. And they experience fewer service issues. Most of the major manufacturers offer this new style. Taco makes a good one and Grundfos too, however all of the major manufacturers make good products.

  • Thank you Paul Logan for replying. My comment is too long, so I will add it under the original post.
    – Roz
    Dec 16, 2017 at 23:13

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