central heating not going off

[ian]

We have a fairly modern (about 6 years old) central heating system. Recently it has stopped turning off on the timer and continues even though the room temperature has gone well past the temperature set on the room thermostat. The only way to turn off the heating has been to switch off the system at the mains. I presume there is some kind of control on the system which respondes to the timer and/or thermostat and this has broken down, is this correct? If so is it the small metal box on the pipes near the boiler and can these be replaced easily? Really appreciate anybody's help.

Ian

[The Natural Philosopher]

Yup. Its almost always a stuck motorised valve.

My UFH did this one day..and I came down to find a tropical regime in the rooms it controls..and a sadly depleted oil tank.

The timer controls the motor on the valve - usually via the thermostat, which opens up the flow to water, and also has a microswitch which switches on the boiler and pump.

If you are lucky, it will be just a new head you need..the motor and switch part..otherwise it may be a drain down and spanner job.

[Harry Bloomfield]

The Natural Philosopher formulated the question :

That would depend upon the circuit used. If my clock turns off, there is no way that anything can run. So in my case the problem would indicate stuck on or welded contacts on the clock itself. Another possibility might be a frost stat which as been set way to high, as this ought to be wired to bypass the clock.

[The Natural Philosopher]

Agreed, but most setups in the last 20 years used a motorized valve to switch the boiler/pump, to AVOID stats or clocks welding up due to high starting currents.

[Roger Mills]

True, but not all systems with motorised valves will continue to run the heating - if the valve malfunctions - when switched off at the programmer or room stat. For example, S-Plan systems *will*, but Y-Plan systems *won't* [See

]

So it depends on what type of system the OP has.

[Roger Mills]

This is revelant only to uk.d-i-y but has got cross-posted by some scum-bagging troll!