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Clunking noise when WM CG-4 is at temp.

So a few years ago (8-9) I upgraded the 1962 American Standard boiler (my house came with) with a 1983 WM CG-4 Natural Gas boiler. did this with help from here as well as from local contacts in the trades.

Two story farmhouse, located in W Michigan, built in 1890. (1850 sq ft) The system was a single zone setup with baseboard units covering every foot of exterior walls. I did a heat loss survey and was able to eliminate ~35% of the baseboard and still have more than is needed (especially as I remodel and add insulation and modern windows etc).

The system as it is now has four zone controlled by Honeywell Zone valves, the aforementioned boiler with a Grundfos UPS 15-58 FC set at its lowest speed. Water pressure is at ~15-16 lbs at temp, ~11-12 when cool. I am running the system at 175F and achieve a delta-t of ~ 15 on 15-20F days.

The shared return is ~ 50% 1" and then increases to 1 1/4" the shared supply is of the same ratio (The shared piping is ~22' and ~5' respectively).

One item that may be useful is that I have had the system open probably 3 times since I installed it, I re-did a couple of the zones and did some re-modeling of the house. Each time I had the system open I purged it and made sure the air was out of all the zones.

I purge with a garden hose at ~25# pressure and do each zone and the boiler separately.

Up to the last week or so the system has worked fine, with no real issues at all. But in that time the boiler has started making a clunking sound when it reaches temp. It is the same sound a water heater makes, albeit a deeper tone, when it is full of sediment. I have watched the unit carefully and the sound starts at about the same time either of the temp gauges on the boiler reach ~160F. 

Any help would be greatly appreciated, as I need to get this figured asap.


P.S. I will be contacting an acquaintance who is a licensed HVACE tradesman for his input tomorrow.


  • ishmael2kishmael2k Member Posts: 39

    UPDATE: I purged/flushed the entire system under pressure. Isolated the

    boiler and flushed at ~28# and then the rest of the system at 45+#. I

    then adjusted the overall pressure to ~12# cold and ran the system up to

    temp on one zone. Seemed to work fine so I brought the rest of the

    zones up to temp, still with no apparent problems. Thought I had fixed

    the issue. I also replaced the pump with a new spare that I had (still a

    Grundfos UPS 15-58FC).

    Unfortunately this morning I woke to the irritating sound of water kettling again.Am waiting to hear back from a local contractor (still) and doing research.

    Any suggestions as to how to de-scale the system would be appreciated.

    What confuses me is that the system worked fine up to the last few weeks. No obvious signs (noises) or anything.
  • SWEISWEI Member Posts: 7,356
    descaling has been recommended here a number of times.
  • ishmael2kishmael2k Member Posts: 39

    Thanks for the link. I have been looking at different descalers but had not run across this one yet. Will check it out.
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