Heating comes on when control clock is "off"

[abenn]

My central heating, installed by BG about 10 years ago, and maintained by them on contract, has a master clock on which I can set what times I want the heating on and off. It's set to be on from 7:00 to 22:30 every day, and off through the night.

For some time now the heating has been starting up at various times in the night, when the clock says it should be off. Engineers have made five visits, and their first reaction is always that one of the motorised valves is sticking open -- though I don't yet understand how its signal is able to override the clock. To date they've oiled the valves, put new actuators on, replaced the timer unit and, if I remember correctly, replaced the control board inside the boiler. On the last visit, the engineer installed a plug in the lead to the actuator he thought was the culprit, so I could isolate it next time the fault occurs, and see if the boiler then stops (thus proving the problem is with that valve).

Last night the boiler came on at 3:15, so I unplugged the actuator, but the boiler and pump kept going. So I switched off mains power to the system. When I switched it back on again this morning it's working okay again, as it always does!

Any ideas, please, that I can suggest to the next engineer that comes? Though I don't know how the control system works, it seems logical to me that if the problem is caused by a sticky valve, that would only stop it switching off at night, not make it come on in the middle of the night. So, what else could it be? There's a white box about 6" x 4" into which the actuator and pump leads go, which I thought was just some kind of distribution board -- could that have a part to play in the saga?

 

[picasso]

What boiler do you have ?

 

[abenn]

Thanks for taking time to read my post.

The label on the front of the boiler says "British Gas 330". The timer is Horstmann ChannelPlus H37XL, and the box that the valve actuators, receivers and pump connect into is Danfoss. The pump is British Gas Scottish Gas Multihead, and the actuators (1 for hot water tank, and 1 each for two zones that the heating's divided into) are various brands. The two room thermostats (one for each zone) are British Gas RC1 wireless-operated, with WR1 receivers at the valves.

 

[jackthom]

I'd be looking for a way to safely disable the horstmann timer when this happens next time to see if it's somehow supplying a signal that it shouldn't be.

Ideally a check should be made on each of the zone outputs with a multimeter when the fault occurs but that's never going to happen in the middle of the night so at least removing it's power might help narrow down the problem.

 

[picasso]

Get the next engineer to check the primary thermistor, its used for frost protection (I think) and brings the boiler on when it goes out of calibration.

 

[abenn]

I've gone downstairs once or twice when this has happened, and all the lights on the timer are off, which should indicate that it's not supplying an "on" signal to the system. Actually powering the timer down, without losing power to other parts of the system, is beyond my competence level, so I'll have to leave it to the professionals. And getting an engineer to carry out checks at 3 o'clock in the morning is not on

As I mentioned in my opening post, they have already replaced the timer once though, and the present one is a slightly different model from the original, though it did fit straight onto the backplate. Now there's a thought -- is removing it from its backplate a simple and safe thing for me to do (I'm competent in diy electrics), and would that narrow down the problem as you're suggesting?

Just seen picasso's suggestion about the primary thermistor -- sounds like a possible culprit, so I'll mention it next time the engineer comes.

 

[jackthom]

Sorry, must admit I missed the bit about having already replaced the timer so it is unlikely to be the culprit.

 

[andytw]

I would be inclined to ask the engineer to wire a mains light bulb across pins 2 and 4 of the boiler connection block. These are 'Switched Live' and ''Neutral'.
When the boiler comes on at a wrong time then it is just a case of checking whether the light is lit or not and refer to the following conditions.

Light is Lit - Boiler has been told to fire by an external source, troubleshoot programmer and valves.

Light is Out - Boiler has decided to fire up by itself, throubleshoot the boiler

andytw

 

[abenn]

Thanks for that suggestion andytw.

One thing I can't get my head around is the interaction between the boiler and pump. When running normally, the pump comes on before the boiler fires up, and shuts down after the boiler has. If the boiler fires up of its own accord, is there some feature of the control system that will automatically start the pump running? Whenever I'm having this problem, the pump is always running -- I think that's what wakes me up, rather than the boiler itself.

 

[picasso]

If your boiler is wired up correctly the boiler pcb controls the pump.

 

[andytw]

Sticking valve maybe rather than the actuator ??
Don't think this would explain the intermittency you have, though.

//www.diynot.com/forums/viewtopic.php?t=206606

andytw

 

[abenn]

Sticking valve maybe rather than the actuator ??
Don't think this would explain the intermittency you have, though.

//www.diynot.com/forums/viewtopic.php?t=206606

andytw

All the BG engineers have gone straight for a valve. They've removed the actuator, tested the valve by hand, and oiled them, but each time they've reported that they feel free.

But, if it were a sticking valve, or faulty actuator, I can understand how that would stop the heater from turning off when the clock tells it to. But I can't understand how it would make it come on again in the middle of the night, some 3-5 hours after it successfully turned off -- I usually go to bed about 1 hour after the clock's turn-off time, and know it's off because the house is getting cold. In doing my final rounds I also verify that the clock lights are off and the boiler, valve, and pump lights are all off.

Sounds like my boiler is wired correctly picasso, since the pump always seems to correspond to the boiler firing.

So, logic seems to suggest that it's the boiler's pcb that's the culprit, specifically an anti-frost thermistor if it has one (my boiler and its drain are both indoors, not in the garage).

 

[Tipper]

My recollection (now retired) is that make and model of programmer can develop a fault which resembles that of a faulty valve motorhead. Ie it fires up the boiler/ pump at random times or fails to do it at all! (It's actually caused by a flat memory battery iirc but the battery can't be replaced.)

You could try changing the programmer but they are a bit pricey...£100+?

Alternative is to bypass the programmer by wiring the system direct to a plug/socket. That should prove whether it is a faulty programmer.

 

[abenn]

Thanks Tipper. If by "programmer" you mean the bit that I'm calling the "timer" (where I program what time I want it to come on and switch off), one of the engineers replaced that about a year ago. Or is the programmer something else?

 

[D_Hailsham]

Where is the boiler located?