Boiler not starting or shutting down correctly

GlowWorm

I have a problem with my Glowworm 38cxi boiler continuing to burn after it is programmed to switch off:

1. The time comes when the heating is programmed to switch off. The thermostat (a Honeywell CM67) sends a signal to the boiler and the boiler switches off correctly.
2. Approximately 30 seconds later, the boiler starts again: the boiler ignites, the green light on the front of the boiler turns on and the fan and pump start.
3. This often happened in the past, but the boiler would then stop after 10-20 seconds. But now, the boiler continues to run:
a. The green light remains on
b. The gas meter shows that gas is being consumed (so it's not just the pump and fan continuing to run until the temperature inside the system has dropped).
c. However, the display panel on the front of the boiler shows the water pressure in the system (currently 1.1) as it does when the heating is off - and not the temperature of the water inside the heating system as it does when the boiler is heating normally.
d. Some heat is delivered to the radiators, but much less than when the heating is operating normally during the programmed hours.
4. If I open one of the hot water taps, the boiler operates more vigorously and delivers hot water correctly. If I then close the tap, the boiler stops all activity and returns to the correct state that it is supposed to be in when the central heating is off (i.e. green light off, no pump or fan activity, no heat delivered to the radiators, display panel showing the pressure inside the system).

Sometimes a reverse problem occurs when the central heating is programmed to come on:

1. The time comes when the heating is programmed to come on. The thermostat sends a signal to the boiler and the boiler ignites correctly, the green light comes on and the temperature inside the system is displayed.
2. Approximately 10 seconds later, the flame appears to go out (even though the thermostat correctly displays the flame symbol, indicating that it is sending a signal to the boiler to operate (as the programmed ambient temperature has not been reached):
a. The green light switches off.
b. No perceptible heat is delivered to the radiators.
c. However, the fan and pump continue to run.
d. The display screen on the front of the boiler displays the temperature inside the system (and not the pressure, as it should do if the system had turned off correctly).
3. If I open one of the hot taps, the boiler ignites and delivers hot water correctly - and also begins delivering heat to the radiators. When I close the hot tap, the central heating continues to operate normally until the programmed time for it to switch off.

If the programmed ambient temperature is reached during the hours when the central heating is set to run, the thermostat correctly sends a signal to the boiler to stop and it does stop normally. If the ambient temperature later drops below the programmed level, the thermostat correctly sends a signal to the boiler to start and it ignites and runs normally. So the problems appear to occur only at the times when the boiler is first programmed to come on - and then when it is programmed to shut down.

Does anyone have any idea what might be causing these problems? Any advice much appreciated!

macman

Faulty frost 'stat sensor?
Where is the boiler located in the house?
Why not eliminate a programmer fault by disconnecting it and using the built in timer, if fitted? If still faulty I'd suspect some sort of PCB fault.

GlowWorm

macman: Thank you for your helpful reply. The boiler is located in a single-storey extension to the (ground floor) kitchen - i.e. quite an exposed part of the house. I have a plumber coming in a few days to do some unrelated work and will ask him to try the fault-finding methods you recommended.