Boiler Overheats

[Winco]

Just moved house. Boiler is Glowworm Fuelsaver Mk2. Previous owner had a service contract; the last service said everything was OK but a system flush was recommended.

CH (up and down stairs) and HW all appear fine, although the boiler will overheat and shut off. After 30 seconds or so, it will re-light and heat again for 30 seconds or so, and shut off again. Heat up, overheat, shut off...This cycling will continue. The return pipe to the boiler is cool during this cycling, whilst the feed pipe is naturally hot. It is as though there is not enough flow through the boiler to accept the heat it is giving out.

A flush is needed I can here you all say...

So, all radiators were removed and thoroughly flushed with a garden hose. The contents of the radiators were black, very black! The rest of the system was drained. The radiators were refitted, and the system emptied and refilled/bled several times, running the pump to ensure the low lying pipes were run through. In one of the refill sessions, I rigged the garden hose to flow through the boiler in both directions. The water flow was good, no apparent restrictions.

The system now has a system cleanser in for the moment.

Despite my efforts, the boiler still performs this overheat cycling. I have set the front panel thermostat on the boiler so that feed pipe is 80 degC.
The pump speed is adjustable but that does not appear to make any difference. The CH is balanced nicely, and the furthest radiator warms as it should even on the slowest pump setting. Setting the pump to fastest speed does not eliminate the overheat cycling.

What else should I do or check?

Could all of the black iron in the water worn away the vanes in the pump? and even on the fastest setting, the flow is not enough?
Could there still be sludge in there? I am considering rigging the mains water feed into the feed from the header tank to the system, temporarily blocking the vent pipe, and flushing the system that way. My mains pressure is 2 to 3 bar.

Any further ideas to stop this overheat cycling?

 

[seco services]

my first check would be the pump. you should hear it when you change speeds.

and i take it you haven't flushed the system yet since adding the cleaner.

 

[Winco]

The pump does sound as if it is pumping more water when I increase the speed setting. Especially so having just refilled the system whilst air is still present.

I have not flushed the system since adding the cleanser. I do plan to drain and flush the system in a few days and perform the 'mains water into the header tank feed' routine. I just wanted to give the cleanser a chance to do its work.

I would have thought there would have been a reduction in the cycling with the flushing carried out to date, and the cleanser unblocking and dispersing any pockets of remaining sludge. But the cycling is the same as it was before I embarked on this issue.

 

[clf-gas]

Pump could still be worn. Check it when you next drain down.

Or is the spindle turning on temp stat with the knob, could be on a lowish setting though showing higher.

 

[Winco]

I will check the pump.

Once the system is up to temperature, and the return to the boiler is hot (there is no room stat!) the boiler will not light, naturally. I can control the point the boiler lights by using the front panel thermostat, so I guess that is OK.