Thorn Heating Olympus 38/50B not heating over 18 degrees

[doctorrocket]

The above boiler has been excellent for 25 years.
Recently, the temperature in house started to decline. The recirculation pump has been changed, and a new Honeywell thermostat fitted.
Boiler fires up when the room is below 16 degrees, but cuts out when 18 reached
It keeps coming on throughout the day, but never exceed 18 degrees (The same thing happened with the old thermostat)
The New Room thermostat is set to 25 degrees
It can take up to three hours before it starts
Additional Information:
I closed down the boiler and then went through the To light the boiler procedure
The Pilot light lit immediately
However, according to the instructions, you then turn the boiler thermostat to 5, and the main burner will start.
The Main burner did not start
Anyone any ideas please

[doctorrocket]

Additional Information:
The boiler has only been coming on every few hours.
When it last came on, it ran for approximately 15 mins then went off.
I turned up the boiler thermostat, and it came on again.
It went off 10 mins later

I then turned up the boiler thermostat again, and it came on.
When it stops, the circulation pump stops as well
It appears that when the boiler reaches temperature, it shuts everything off
It seems as if the room thermostat has no influence on the boiler, yet this was the same with the old thermostat (which is why we changed it) as well as the new one, so it should not be the new thermostat

Yet the system has been running for 25 years without a problem, so the wiring I would assume is correct

[Atticus Finch]

Sounds like a faulty boiler thermostat. They are still available from any heating parts supplier for approx £30.

It's a Ranco C77P0105, Heateam Part no C77P0105, GC No 382322

[doctorrocket]

Many thanks for your comments

It is resolved now

It turned out to be, I guess, a stuck hot water section on the three way Honeywell valve.

It was set to 60 degrees , and the water was on temperature.

I turned it to 70 degrees, and the boiler came on as you would expect, as well as the heating

I left it on till the middle of the night, when I woke up as it was quite hot

I turned it back to 60 degrees, and it has been OK ever since