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Burnham boiler making loud knocking sounds

discman017 Member Posts: 2
edited February 2017 in Gas Heating
My home’s boiler has recently started making a lot of noise. It sounds like pebbles are crashing around inside when the boiler is running. When the boiler turns off (but the circulator pump is still running), the crashing subsides within a few seconds.

It’s a four-zone closed-loop system:


It’s a Burnham boiler that was installed in 1988. Info about the boiler model is written on the wall inside the boiler closet:


I realize 1988 is pretty old and I’ll have to replace the boiler at some point. On the other hand, I’ve heard that these old Burnhams can run virtually forever if properly maintained, and I’d like to keep it running as long as possible.

I found these articles outlining possible causes of kettling:

One possibility is that the water is getting too hot, due to a variety of causes: faulty aquastat, failing circulator pump (leaving water in the boiler too long), or pressure too low.

But when the boiler shuts off, the gauge on the boiler reads 25 psi and 190 degrees, which seems reasonable:


This site has a nice table showing the boiling point of water vs. pressure:

At 25 psi, the boiling point of water is 240, so it seems like I have a significant buffer between 190 and 240.

Given that the temperature seems OK, buildup of mineral deposits on the heat exchanger (causing localized boiling) seems the most likely cause. We have hard water, resulting in significant mineral deposits on faucets, for example, and on valves and pipes in the boiler system:


From my reading, it seems like I should try adding an inhibitor to dissipate buildup and reduce future buildup. Fernox F1 seems to be the most commonly recommended:

Does this seem like the right thing to do? If so, my problem is where to add the inhibitor. I've watched three or four videos showing how to add inhibitor to a pressurized system, and they all make use of large bleed valves on radiators. For instance:

I don't have large radiators like this. I have small baseboards:


In my entire four-zone system, I’ve only ever found two bleed valves (making purging the system extremely challenging). And these bleed valves are tiny – I’m not going to be able to add inhibitor through these:


So where do I add it? The only place I can think of is through the air purger:


So I’m wondering, could I do the following:

1. Close off all zones.
2. Drain enough water from the boiler to depressurize and make space for the inhibitor.
3. Remove the air vent.
4. Pour inhibitor into the air purger, using a funnel.
5. Install new air vent. (These are cheap and seem to fail frequently, so I might as well install a new one.)
6. Add water to repressurize.
7. Purge. (Hopefully not too difficult, since I’ve closed off the zones.)

Does that seem like a reasonable way to get inhibitor into the system? And am I on the right track in the first place, with the idea to add inhibitor, or is there something else I should investigate or try first?

Thanks in advance for any help diagnosing and solving this problem!


    EBEBRATT-Ed Member Posts: 10,323
    Yes. It sounds like you are on the right track. Your temperature and pressure are normal. Removing and adding the inhibitor through the vent tapping is as good a place as any. I have not used Fernox.

    The only other suggestion would be to have the burners and combustion checked (with an analyzer) to make sure that the combustion is in order.
  • discman017
    discman017 Member Posts: 2

    I have not used Fernox.

    Thanks! Is there a different inhibitor that you would recommend? Fernox seems to get good reviews, but looks like it's available mostly in the UK.