DIY Doctor UK DIY Forums

My boiler is a Worcester Greenstar 40CDI Conventional, installed about 3 years. All but one of the 19 radiaors in the house have TRVs fitted. The system is controlled by a Danfoss programmer and a Danfoss R/F thermostat located in the hallway. Heating and hot water are programmed to come on at 6am, off at 9am and then on at 3pm and off at 10pm. The thermostat is set at 20C.
Recently, heating has stayed on continuously, regardless of the fact that the programmer is at OFF and the thermostat setting is below the ambient temperature. Although I can turn down the TRVs on individual radiators, the one without the TRV is continuously very hot. The green light on the boiler control panel confirms that the boiler flame is lit. This light can be made to go out by turning down the temperature control on the boiler (usaully set at 4 in its 1 - 6 range) and then turning back up - but at some point it comes on again of its own accord.
Please give me your ideas as to what could be the cause and what is the resolution to my problem.

I presume you have an open vented system with a traditional hot water cylinder.

If so, I would head straight for the three port valve (or two port if you have an "S" plan) as this controls power to the boiler. With the power off, operate the manual lever to see if the gearing action is smooth because if it isn't the valve may be jammed preventing the boiler from shutting down.

This sounds like you either have a frost thermostat that is switched on (not likely, as the weather hs become warmer), or your wireless room thermostat has lost its binding (therefore it is not controlling the boiler). Because you have lost your Boiler Interlock, the boiler cannot turn off until it overheats.
Just turning the thermostat on the boiler down means it trips out earlier.
I don't know the Danfoss receiver as well as a Honeywell thermostat. On those, where the light shines on the receiver to show that it is switched on (ie roomstat is calling), you can press the lamp to crete an override.
Check the batteries in the thermostat.

Plumbbob, MrEnergySaver, thanks for your replies.

Plumbbob. the system is not open, but sealed , with an expansion chamber. I don't know if this changes your advice? Also, I am not as technical as you clearly are and couldn't follow your explanation. Can you clarify in layman's terms, please?

MrEnergySaver, I don't think it can be the space thermostat, as this would only be the case if the programmer was ON, whereas the problem is occurring when the programmer is OFF, i.e. not calling for heat. Could the frost thermostat (which I think will be component within the boiler) be stuck in the ON position?