Gloworm Fuel Saver

[Lesbutler]

Heating system has pumped hot water and uses a 20 year old wall mounted gloworm fuelsaver boiler (new heat exchanger fitted 3 years ago)..

System works fine when motorised valve is open ie pump silent, radiators hot etc.

When motorised valve shuts, pump is noisy, and loud bangs and boiling noises from boiler and pipes. Overflow into tank in loft when this happens.

Tank in loft contains plenty of water and is feeding OK.

I have drained the system and put flushing fluid in and refilled but to no effect (probably worse if anything).

The boiler frequently shuts off on the overheat stat.

No changes have been made to the system for 10 years. The problem developed a couple of moths ago and got progressively worse.

The pump is on the cold side of the boiler and the overflow and feed to the hot water tank are on the hot side.

Any ideas?

 

[ChrisR]

Revised after rereading question.

If the problem has got gradually worse over time then it's probably not a switch, say. May be there's a bypass which is blocking up with sludge?

 

[namsag]

When the clock or stat stops calling for heat and the motorised valve closes have a look and see if the boiler actually turns off straight away,if not the microswitch in the valve is stuck causing boiler to stay on with no where for the heat to go causing banging and overheat to trip.

 

[croydoncorgi]

There should also be a pipe to bypass the zone valve(s) to allow the boiler to cool down after the valve has closed.... On older systems, this could be just a bit of 15mm pipe with a nearly-closed gatevalve. A proper spring-bypass valve works better. Any boiler requiring 'pump overrun' or having 'low water content' usually requires a bypass.