Central heating won't turn off

[Simon T]

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.

[ARWadsworth]

Have you got a multimeter and do you know how to use one?

[Pete Shew]

Is the thermostat wired or wireless?

Pete

[Simon T]

Wired

[Simon T]

Yes to both!

[Roger Chapman]

The micro-switch on a 2 port valve could be stuck closed even though the valve itself had closed which action should have opened the micro-switch contacts.

[Roger Mills]

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.

[geoff]

In message , Simon T writes

What do you mean by "thermostats"?

What resistance is the temperature sensor showing at the pcb connector?

[Simon T]

Well there's a thermostat on the wall in the hall which measures the room temperature, and there's a thermostat attached to the hot water cylinder which measures the temperature of the hot water that comes out of the tap.

I'll have to check that and get back to you, will next be going round my parents on Wednesday.

[cynic]

Gravity hot water, pumped heating sounds like a "C plan" For the hot water to continue heating the hot water valve must be remaining open. Are you sure the valve is closing when it should? Are you really sure the hot water primary is gravity circulation?

If the system is actually an "S plan" is there a frost stat somewhere calling for heat possibly by failure or possibly by inadvertent alteration.

If you do not have a wiring diagram one should be available on line if you google.

[Simon T]

No I meant its gravity fed system (ie uses a feed and expansion tank), as opposed to be a closed system which is connected directly to the mains. Both the DHW and CH are pumped.

Both valves were in the closed position when I examined the system, I can only imagine they were passing slightly if, as my mother said, the rads were still red hot.

Its an S plan looking at the diagrams posted elsewhere in the thread. Not aware of a frost stat anywhere on the system.

Will do!

[ARWadsworth]

That is the most likely answer.

I went to a job this evening and I just disconnected the orange cables in turn to identify the faulty 2 port valve.

[The Natural Philosopher]

if you are lucky, you can get just the switch.

Otherwise its a new head job..takes 5 minutes to swap out - no plumbing required :-)

[ARWadsworth]

I am not going to mess about swapping the switch at work. The customer was given a price for a new head (Drayton so it's an easy swap). It's a balance between labour and parts cost and I have to put the customer first.

[Simon T]

A quick follow up for those still reading this thread.

Went back round the other day and checked the continuity of the grey & orange wires on both valve actuators and both were fine.

Got continuity when the valve was open and no continuity when the valve was shut.

Switched the power back on, system seems to be working again. System shuts down when the valves are closed.

I'm wondering if my mother buggering about with the programmer might have had something to do with it not working (though the timer was off when I checked it out last time when the pump was still running).

[Roger Chapman]

Well you could get pump overrun for a short time but that would be a few minutes rather than hours so if it was well after the programmer switched off then it will have to be an intermittent fault. Intermittent faults are a pain to trace as they tend to go away just when you think you have got to the root of the problem.

I have had such a fault for years which causes the overheat stat to fire as the bypass circuit can't cope if the boiler keeps firing when all the valves are closed. But it only occurs very occasionally and usually late at night. I think I have finally cured it having found it in action when I was forced out of bed in the early hours by bladder pressure but I am not at my best at 4 in the morning. It was the newest of my 3 two port valves which I have discounted before because it was so new. Since changing the head there has not been a recurrence but it is early days yet.

[Simon T]

Well apparently she seemed to think the heating didn't come on at all the other morning, so I'm thinking faulty electrical switch on the CH actuator.

But as you say, as the fault is intermittent, it may take a while to pin down.

Thanks to all those who responded.

[Dave Liquorice]

Or the mechanicals have got a bit gummed up and there is quite enough torque to operate the switch. It could be gunk in the valve shaft to the wet bit or the rack/pinion that drives the valve open.

[katshkins]

My boiler is connected wireless by nest two days ago upstairs heating was variable once ok once could. Today may boiler even thermostat is off. Is working permanent and only downstairs is heating.

[Cynic]

1. You have resurrected a thread from six years ago.
2. Your post makes little sense - is English your first language?