Myson Economist boiler banging

[divoch]

Old Myson Economist WM 60 /80 BF boiler. When I turn heating OFF either at the programmer or by the room termostat while the boiler flames are ON (boiler thermostat ON) there is banging in the boiler for a while. This does not happen when the heating is switched OFF while the boiler flames are OFF (boiler thermostat OFF). It is an open vent system with two motorised valves (heating and hot water) , Grundfos pump and bypass.

When the programmer timer is ON and the room thermostat is calling for heat ( which right now is most of the time during the daytime) boiler and pump work normally and quietly with just the boiler thermostat swithing ON and OFF .

I suspected that pump overrun was not working correctly but I am not sure now. It turns pump ON after heating was turned OFF but not immediately. Looking at the wiring diagram ( see enclosure) it seems to me it cannot be immediate.

Any suggestions as to what could be the reason for the banging would be appreciated

 

[divoch]

Attachment that should have been enclosed

 

[AC Power]

"Banging" is usually water hammer.

There are suppressors for that, and various piping methods can reduce/avoid it. Look it up in Google. You might find that securing the water pipes in areas where they can vibrate/move might reduce it.

Water hammer occurs when the flow of water is stopped abruptly.

It's like slamming the brakes on in your car.....your dog, coffee, road atlas, etc. all want to project to the front of the car upon heavy braking!!!

 

[divoch]

Thanks for a reply. That could be the cause althoough I hear the banging noise standing next to the boiler and it seems to me that it is coming directly from the boiler. It happens only when heating is turned off while boiler flames are visible, i.e. boiler thermostat calls for heat. That is why I suspected temporary overheating.

 

[AC Power]

Ok, maybe I am misunderstanding:

Do you hear the sound only when the flow of hot water STOPS? This cannot necessary be determined by assuming that the applied voltage to the zone valves means the flow of water is permitted (or flowing, thus heating). You need to recognize AT WHAT TIME does the water valves CLOSE, thus preventing the re-circ of hot water, thus the system is no longer heating and the thermostat is satisfied.

If you hear a "thump" or "cludd" upon the STOP of hot water circulating, that is water hammer.

 

[divoch]

Thanks for your reply
As to the precise timing of what happens when I turn OFF heating at the programmer I will need to investigate further but, as i am going away tomorrow it will be after my return on 5 February.
I think that when I flick the switch to turns off power to the boiler while thermostat is calling for heat and flames are present it seems the solenoid does not turn off flames instantly and that is why I can see them still for a little while after switch. I do not know whether pump stops before or after zone valves close and how soon after I flick the switch. I suspect that pump stops first . If it did not there is a bypass so water can , somewhat restricted, still circulate even when both zone valves are closed.