First things first - I'm not a plumber so please forgive me for using any terms incorrectly and sorry for the long post 
I have a Potterton prima 60c hanging on the wall in the airing cupboard on the top floor of a townhouse. It was installed around 13 years ago.
About 3 months ago I found the pilot had gone out and after talking with my excellent corgi plumber, he pointed out that the overheat safety switch had tripped. It was reset and the boiler seemed fine for a few days.
Since then the switch has tripped at an every increasing frequency with the boiler kettling before switching off. Nobody told me how loud this is so when I got woken up by it I thought the house was falling down!
My corgi plumber popped in to have a quick look between jobs. The pump seems to be working ok (and is only a couple of years old), the boiler thermostat is set to low and was replaced new only 6 months ago. He did notice that my expansion tank was quite low in water and because it's only a short way from the boiler and pump, he surmised that it may be drawing air into the system. He made a quick adjustment and the tank now has plenty of water in it.
The problem is that it's still happening. Now it doesn't even last a few hours without kettling and shutting off.
To test if it was because of a weak pump, I turned the pump up to setting 3. It seemed to last a little longer but still kettled and cut off. Interestingly the thermostat on the side of my cylinder is set to 60 Celcius but when I measured the temperature of the water at the hot tap after the boiler shut off, it was 70 Celcius. Maybe it's just the running of the pump after the boiler shuts off that keeps the water heating in the cylinder.
When the boiler is running, if I turn the cylinder thermostat down I can cause the boiler to cut off so I assume that sensor is working. Also, if I turn the boiler thermostat down after it has been running for 10/15 minutes I can cause the boiler to cut off that way too. So I assume that is ok (should be it's only 6 months old).
Do any of you clever people have any ideas based on what I've written here?
I've booked my plumber to come back again but I'm struggling to understand what can be causing this.
Many thanks.
Other info:
Central heating is off at the moment so the water is just circulating around the cylinder and boiler. It's been like this since spring. CH is still working ok, tested it today.
I've bled the air valve at the cylinder - there was a little air in it but I thought that was probably because the whole cylinder was replaced a few months ago.
I have a Potterton prima 60c hanging on the wall in the airing cupboard on the top floor of a townhouse. It was installed around 13 years ago.
About 3 months ago I found the pilot had gone out and after talking with my excellent corgi plumber, he pointed out that the overheat safety switch had tripped. It was reset and the boiler seemed fine for a few days.
Since then the switch has tripped at an every increasing frequency with the boiler kettling before switching off. Nobody told me how loud this is so when I got woken up by it I thought the house was falling down!
My corgi plumber popped in to have a quick look between jobs. The pump seems to be working ok (and is only a couple of years old), the boiler thermostat is set to low and was replaced new only 6 months ago. He did notice that my expansion tank was quite low in water and because it's only a short way from the boiler and pump, he surmised that it may be drawing air into the system. He made a quick adjustment and the tank now has plenty of water in it.
The problem is that it's still happening. Now it doesn't even last a few hours without kettling and shutting off.
To test if it was because of a weak pump, I turned the pump up to setting 3. It seemed to last a little longer but still kettled and cut off. Interestingly the thermostat on the side of my cylinder is set to 60 Celcius but when I measured the temperature of the water at the hot tap after the boiler shut off, it was 70 Celcius. Maybe it's just the running of the pump after the boiler shuts off that keeps the water heating in the cylinder.
When the boiler is running, if I turn the cylinder thermostat down I can cause the boiler to cut off so I assume that sensor is working. Also, if I turn the boiler thermostat down after it has been running for 10/15 minutes I can cause the boiler to cut off that way too. So I assume that is ok (should be it's only 6 months old).
Do any of you clever people have any ideas based on what I've written here?
I've booked my plumber to come back again but I'm struggling to understand what can be causing this.
Many thanks.
Other info:
Central heating is off at the moment so the water is just circulating around the cylinder and boiler. It's been like this since spring. CH is still working ok, tested it today.
I've bled the air valve at the cylinder - there was a little air in it but I thought that was probably because the whole cylinder was replaced a few months ago.
