The boiler is used in an open vented system. It used to control well modulating to reduce gas input in response to temp nearing setpoint. Now it will get near its set temp but instead of modulating as it did, it cuts out due to over temp? Any ideas

Pat Brannigan, September 2011

OK carefree. I will check the NTC sensors (what does NTC stand for?) to see if they have similar resistance at the same temperature. There is no set-up pots on the board that I can see? anyway I'll check the sensors.What resistance should I expect at say---- 20 deg C?
Regards

Pat Brannigan, October 2011

If it's a system boiler, the only influence over temprature and modulation are the ntc's and possibly PCB, most likely ntc's

Carefree, September 2011

Hello Carefree. Maybe I didn't explain myself clearly, my fault. The boiler is used as a heat source for a top up to a large solar tank, and rads as normal. It is not a combi boiler. Does this make a difference? thanks for your input. Pat.

Pat brannigan, September 2011

I'll assume this is in Hot water mode seeing as it's still warm(ish), if it is, the hose (on the flow from the main heat exchanger) has been modified due to the original internally deteriorating. This breaks down and blocks the filter in the tee below the hose.

The new hose is different material and has a white ID stripe along it.

The solution is change the hose (cleaning the connecting pipework throroughly), remove, clean and refit the filter. If the particles are small enough, they may have passed through the filter into the DHE which may also need cleaning, you won't be able to tell until the hose and filter are properly clear.

Carefree, September 2011