Help please boiler won't turn off.

Noticed that it was getting rather hot in the house.
I have programmable Horstmann room stats one upstairs and one downstairs both set at 20 but noticed the bottom one showing 21.5 and the rads very hot and boiler running. I also turned down my hot water cylinder stat but boiler kept running.

I turned the hot water and central heating settings to off on the boiler programmer but the boiler still wouldn't cut out.

In the end I turned it off at the electrical socket which eventually stopped it.

What would be the problem please?

Thanks

You will have a couple of 2 port valves near your hot water cylinder, one of them will be stuck on (educated guess).

Thanks for the prompt reply.
Out of interest, why doesn't setting the heating to off on the programmer turn it off if the valve is stuck on?

Thanks

The experts can answer that as it puzzled me when I had the exact same problem recently but mine was on the hot water side of things. Turned out to be the two port valve stuck and constantly calling "more"

The clock and thermostats open and close the motorised valves.
When the valve opens it activates a micro switch in the valve which signals the boiler to fire up.
If the valve is stuck open then the switch will also be stuck on causing the boiler to keep firing.
It is also possible for the micro switch fail and to stick on.