On 21/11/2011 16:11, Simon T wrote:
OK, I know other people have had this problem and have done a cursory
Google search, but have not been able to shed much light on what might
be the problem in my particular case.

Basically, the central heating round at my parents house refuses to
switch off, even when the thermostats aren't calling for heat and the
timer is off. The pump continues to run and the boiler continues to heat
the water and the only way to get it to switch off is to turn everything
off at the mains.

I suspected it might have been a stuck 2-port valve, but having checked
both of these found that neither one (CH or DHW) were stuck and appeared
to be operating fine.

Their set up is a standard Gravity Fed System with the feed and
expansion tank in the loft.
Glow Worm hideaway boiler (in a small room next to the kitchen)
Danfoss Electronic Timer (in the kitchen)
Danfoss Room Thermostat (in the Hall)
2x Danfoss 2-port valves (for CH & DHW - in Airing cupboard next to hot
water cylinder)
Danfoss cylinder stat (in airing cupboard)
CH pump (unsure of make - in the airing cupboard)

Again, the 2-port valves were most definitely not jammed open, which was
my first guess. In fact when I went round there to check them, I found
my mother had switched the gas off on the boiler (it didn't occur to her
to switch off the power supply), the timer was off, both 2-port valves
were closed, but the pump was still running?

Any suggestions what to try next?

Thanks in advance.


Each of the zone valves has an auxiliary switch which closes when the
valve is open, and is independent of the motor circuit. These switches
on the two valves are wired in parallel - with a permanent live feed on
the grey wires and a switched live (which switches on the boiler and
pump) on the orange wires.

It's almost certain that one of these switches is stuck in the closed
position - even when its valve is closed. If you disconnect the valves'
grey and orange wires from the junction box, and check for continuity, I
think you'll find the problem - one or other switch is stuck shut.
--
Cheers,
Roger
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