Boiler making noise only while on gravity feed

[DavidStewart22]

My Sigma 40 make a humming noise not unlike an airplane passing overhead. It is not electrical, it is not the fan. It comes from the heat exchanger section and only while the gravity heat transfer is working to heat up the water tank. When the radiator heating pump is on it is fine. C

Could this be a seal issue?.

I would appreciate any information on this subject.

 

[ChrisR]

Kettling, due to high temp. Try a dose of Boiler Silencer.

 

[DavidStewart22]

Thanks Chris. Forgive my ignorance, but what would bring about an increase in temperature to cause this problem which only started a last week. Any detail would be appreciated as I would like to understand the problem and solution.

Thanks again. This has been doing my head in.

 

[ChrisR]

The temp in there will be higher because the flow is lower when it's relying on convection instead of a pump.
Over time scale builds up on the inside of heat exchangers. The water makes noises as it approaches boiling under the scale, and forms bubbles which collapse when they escape the scale.
The boiler probably maks less noise if you turn its thermostat down?

Boiler treatments either work to dissolve the scale, or slightly raise the boiling point of the water, so it doesn't boil.

Limescale isn't likely in Glasgow, so the "scale" would be metal oxides from the radiators or boiler itself.

The only other likely noise would be from the combustion process, but I would expect that not to occur only on HW.

 

[DavidStewart22]

Thanks Chris. Almost everyone I have discussed this subject with agrees that it is indicative of sludge or scale and it makes sense. My own boss (an expert like yourself and very experienced) agrees with your comments. However, since I cleaned out the system 1 year ago using a 3 step process I wondered if there might be another less obvious reason. One person, from B&Q coatbridge suggested the temperature sensor. I checked the resistance out and hey presto. I am awaiting a spare but since removing and refitting the sensor temporarily I have had no end of temperature related issues. Currently I am getting hot water spurting into the expansion tank. I did also drain the system and refill to get to the sensor (hopefuly cleaning it out to a degree). Hopefully will have the new sensor in place tomorrow.

Thanks again.