I have a 4 zone water based heating system. One S-25 BF Cast Iron In-Line Pump, 1/12 hp heat pump for all 4 zones. The spring circulator breaks every year. It just broken again eventhough the assembly does not require any lubrication on S-25 BF. Before this model I had S-25 which required lubrication and I could see that on those old pumps lubricant oil was causing issue so I bought this one. Now it broke again.

Why is spring ciculator breaking every year?

Is there heatpump w/o circualtor that I can use in place of S-25 BF. Do I need a more powerful pump?

  • FYI; A circulator is not a "heatpump" even though it is a pump used on a heating system. A heatpump is a completely different heating system.
    – Ecnerwal
    Commented Nov 23, 2016 at 16:09
  • You said that you replaced the pump. What exactly did you replace? Did you replace the complete pump, including motor or just the pump assembly or just the bearing assembly?
    – d.george
    Commented Mar 11, 2017 at 16:17
  • By the way, an Armstrong S-25 is not a maintenance free pump. It still needs lubricated.
    – d.george
    Commented Mar 11, 2017 at 16:19
  • What is the water pressure on the boiler?
    – d.george
    Commented Mar 11, 2017 at 16:34
  • Thanks for responding. 1. I replaced it with new pump. Most recently was sent directly from Armstrong as the last one broke. Commented Mar 13, 2017 at 16:11

2 Answers 2


One possibility (and it's just that - a possibility, a guess, not definitive) is that your system leaks, and makeup water is being constantly introduced by an automatic fill valve.

The problem with that is that new water comes with oxygen dissolved in it, and oxygen rusts cast iron. So if your pump plugs up with rust, and fails, that might be your cause - as you have not provided any information on the symptoms of failure or state of the failed pumps, it remains a guess. If the failures are normally at the start of heating season, that's another clue in this direction - lots of time for the pump to rust while not spinning all summer.

If that is the case, a stainless steel pump might be the solution to your problem, though another solution would be to stop the leaks.

Systems with cast iron pumps and no leaks just rust out the oxygen that's in the initial fill water, and then rust no more, as there's no more oxygen.

  • Thank you so much for your answer. There is shut-off valve that used to leak and even the zone-valves used to leak but not anymore. Luckily Armstrong sent me replacement for the pump. However, for the time being I had the older spare one which is now installed. But even this pump is making a grinding noise. Actually this was the original pump that was replaced by new maintenance free one (S-25 MF/BF) which went bad. I have taken some pictures and made the audio recording of the noise. I don't know if this can be helpful in diagnosing the root cause of this issue. Commented Dec 6, 2016 at 4:55
  • Here is the audio. SE did not let me post it. filedropper.com/201612050031 Commented Dec 6, 2016 at 5:21
  • Here are links to photos in case SE decided not to display those. filedropper.com/1_218 filedropper.com/2_197 filedropper.com/3_108 filedropper.com/4_64 filedropper.com/5_40 filedropper.com/6_32 Commented Dec 6, 2016 at 5:24

The problem with your pump has nothing to do with new water being introduced into your heating system. Any new water never touches the coupler or that part of the pump and NO you do not need a stainless or bronze pump, as mentioned elsewhere. However, Your Armstrong pump does need lubricated yearly. You need to remove the cap on the oil tube and add a few drops of special pump oil sold by a pump dealer. And make sure you only buy a genuine armstrong pump, not an after market piece of crap. Some motors have the oil tubes and some do not; look closely. But that is not why the coupler needs replaced so often. The problem with coupler breakage is usually that the rubber motor mounts become worn or soft and allow the motor to sag down in the motor mount. This sagging allows the motor and pump shaft to run out of line, and will cause excess wear. You could take the motor to a repair shop and have them repair the motor or just replace the motor and/or bearing assembly. This pump and the Bell and Gossett series 100 pumps are what we call "BULLIT-PROOF" since they last so long and are almost trouble free. (both of these pumps look almost identical). If you want to replace the pump with a "wet rotor close coupled" pump, make sure you buy one that has the same characteristics as this pump. (head and flow). A larger pump can introduce noise into your heating system.

  • Have a good knowledge of the subject you are writing about. The pump mentioned is not a wet rotor pump so the new water introduced into the system will not come in contact with the coupler.
    – d.george
    Commented Feb 8, 2017 at 14:19
  • Water need not touch the coupler at all for the coupler to break. If the wet parts are frozen in place by rust, and the motor starts, the coupler breaks. And the wet parts can definitely be frozen in place by rust if there is a leak, seen it many times.
    – Ecnerwal
    Commented Feb 8, 2017 at 15:14
  • So I ask; why was my answer downgraded. Does someone think my answer is not accurate. If so , please comment and tell me why, since I probably have the most accurate answer.
    – d.george
    Commented Feb 9, 2017 at 14:21
  • On a i/12hp. pump, the motor isn't strong enough to break the coupler since the motor is split phase and not a capacitor start. Larger pump motors may break the coupler.depending on the coupler used.
    – d.george
    Commented Feb 11, 2017 at 11:46
  • Thanks everyone for you response. I really appreciate it as I am learning a lot through this ordeal about HVAC systems. So as I said last year I had replaced my circulator with lubrication (maintenance) free model for Armstrong S-25 MF, 174031MF-013 to be exact and after running for over 3/4 months it broke. Since this was under warrant Armstrong sent me a new one and now it's in operation. It's been running only for about 1 month or so. It hasn't gotten that cold yet.It's been running only for about 1 month or so. It hasn't gotten that cold yet. Cont. in next comment... Commented Mar 10, 2017 at 2:46

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