Boiler starting up when swiched off at timer?

[James 3454]

Hello, Im having an issue with my boiler starting up when it is switched off, the heating has not been used for several months but the hot water was coming on in the evenings as set by the timer. Then 2 days ago it started up in the middle of the night when it was switched off, the only way i could switch it off was by cutting the power at the consumer unit.

This has happened before a few years ago when we had a very cold winter, there is a frost stat that overrides the timer and starts up the boiler to stop the pipes freezing, i had a look at it but it appears to be ok i think, as i turn it up i hear a click at about 20 degrees, i opened it up and above the click there is continuity between the two terminals, and if i turn the dial down again it gos open circuit. The boiler is on as soon as i give it power from the consumer unit, turning the frost stat up and down does not seem to affect it, and its far from freezing anyway.

So im thinking it may be the timer, it looks like a honeywell ST699, i will open it up and check now but i was wondering if there is anything else that will cause the boiler to start up when its switched off at the timer, it was working fine until 2 days ago, im not a plumber so i was wondering if i could pick the brains of the people in the forum?

Many thanks

James

 

[Simon F]

Hi James, the advice will always be to get someone relevantly qualified to take a look.
having said that it is a case of working back to find where the power is coming from to then see where the fault is.
is it y plan s plan, some other setup??? room stat, cylinder stat??? lots of possibilities

 

[JBP Heating]

Could be the contacts in the ST699. Did you notice if either of the lights were on next to the sliders when the fault occurred?

 

[James 3454]

Hi James, the advice will always be to get someone relevantly qualified to take a look.
having said that it is a case of working back to find where the power is coming from to then see where the fault is.
is it y plan s plan, some other setup??? room stat, cylinder stat??? lots of possibilities

Thanks Simon, Ive downloaded the wiring diagram for the timer and will download one for the boiler, the boiler in the attic has 3 cables from downstairs running to a fused switch, then 2 cables that run on to the boiler, im guessing that the 2 that run to the boiler are for the hot water and heating control, and the 3rd that runs to the switch is the frost stat override, but i will investigate further and test to confirm that is the case. I dont know if it is a Y plan or and S plan, i dont know what they are but ill google now and find out. There is a temperature stat in the hallway, im not sure if there is one on the cylinder, ill check. If it is something electrical i can probably trace it myself, though as its happened out of the blue it could be a poor /intermittent connection somewhere which could be a pain to trace. Im assuming im correct in thinking that it has to be something electrical, there is the thermostat, the frost stat, the timer and the boiler and possibly another stat on the cylinder, and something is giving the boiler power when it shouldent be?

Thanks again for your help.

James

 

[James 3454]

Could be the contacts in the ST699. Did you notice if either of the lights were on next to the sliders when the fault occurred?

Thanks, no neather lights on the ST699 are on, the hot water was set to timed once, and the heating set to off when the fault occurred, but ive set them both to off now.

Cheers.

James

 

[James 3454]

Yep there is a cylinder stat, a honeywell L641A1039, i thinking that neither stat should start the boiler if its switched off at the timer though?

Thanks.

James

 

[JBP Heating]

OK, in my experience the light would be on if the timer was misbehaving. Perhaps go back to your frost stat, make sure it is set to around 5 degrees, wait until you see the fault and then try turning it up, see if it still clicks. It could be ok at 20 degrees but not at, say, 10 degrees.

Just trying to think of basic checks that I can recommend to you. Other than that I can only suggest a competent person fault finding the electrics.

 

[JBP Heating]

Just reading back up, looks like you may have already tried this frost stat test.

 

[James 3454]

Thanks, i dont think its the frost stat or the timer, ive disconnected the frost stat and removed the panel on the timer, so both are now open circuit, but the boiler still starts as soon as i switch on the power at the consumer unit. I think the system is an S plan, is has 2 zone valves, but the wiring is nothing like the suggest plan on the spec sheet, i only have 4 wires to my boiler, 2 live 2 neutral (one of which is live) and an earth. Im working on tracing the wires and drawing a circuit diagram to try and make some sense of it. Its getting the power from somewhere...

Thanks

James

 

[heliotrope]

I would be looking at probable fault on zone valve.

Make and model of boiler would be helpful.

 

[James 3454]

Thanks ill investigate them tomorrow, the boiler is a Vaillant but i cant find the model number, ive looked all over it and googled any number i can find on it, ill try ringing vaillant tomorrow.

Thanks for the help guys.

James

 

[JBP Heating]

What zone valves do you have? Can you see the make or describe what they look like?

 

[oz-plumber]

I would assume it's the PCB.

Have had issues with boilers that the only way to turn them off is to un-plug the unit from the power supply,
even when the boiler is switched off on the front panel the boiler still runs.
You may have the same problem.

 

[James 3454]

The zone valves are Honeywell V4043s, the connection panel to the boiler has 4 connections, Live and Neutral (240V on the live and 0V on the neutral) and 3 and 4, both of which have 240V present, but i suppose if the PCB is faulty then it could be putting voltage on those connections? I really need to find out exactly what boiler i have so i will get onto that now. Thanks again for all your help guys!

Cheers.

James

 

[SimonG]

I'd be fault finding the wiring of the s plan before getting somebody too look at a pcb.

 

[JBP Heating]

I'd be fault finding the wiring of the s plan before getting somebody too look at a pcb.

Absolutely. Electronics on Vaillant boilers rarely fail. Try pushing the levers on the side of the valves. If there is no demand the lever should have some resistance but will move over and then motor back by itself when you let go. If there is demand from somewhere the lever should be loose.

 

[James 3454]

Vaillant are closed until Monday, ive taken a couple of photos if anyone can identify my boiler? Ill go and investigate the wiring on the zone valves now. Thanks Guys!

 

[James 3454]

Absolutely. Electronics on Vaillant boilers rarely fail. Try pushing the levers on the side of the valves. If there is no demand the lever should have some resistance but will move over and then motor back by itself when you let go. If there is demand from somewhere the lever should be loose.

Thanks, ill do this now, im assuming i do it with the boiler running?

Cheers.

James

 

[JBP Heating]

First of all I have to say that there is no way you should be taking the case off of that boiler or doing any work/testing on it unless you are Gas Safe Registered! Or accessing any electrics unless you are competent to do so.

You need to check the valve levers whilst the fault is occurring. So heat and water would be off and boiler would be running like you described

 

[James 3454]

Thanks, With the heat and hot water off and the boiler running both zone valves are springing back and there is tension. After i did this the boiler now seems to be staying off so i think we may have located the problem!! I will leave it with power but switched off at the timer and see if it starts playing up again. I think moving the lever may have got the motor back up and running, dirty contacts on the commutator or warn brushes possibly... ill have a look.

Thanks for your concern, i feel i am competent enough with electrics to do what i am doing, i have studyed electrical engineering and regularly work servicing and repairing electronics, though i realize i am not a household electrician and i do not know how the system has been installed, i feel am completely safe removing the panel and testing which pins are live and referring to a manual to see if they should be live.
Plumbing and gas i do not know what im doing, i have done some basic plumbing, changed a tap, and swapped a radiator out but that's about my limit, i do not really understand how these valves work and im certainly not going to touch the gas side.

Thanks again for all your help, Is it likely that one of the valves will need replacing? i suppose if it happens again i should check one at a time and see which one stops the boiler.

Cheers.

James

 

[JBP Heating]

Yes do that if you can to figure out which one it is. Sounds like a sticking microswitch in which case you will need to run the heat and water to open the valves and maybe the switch will stick again. Valve heads can be replaced.

 

[stevetheplumber]

you realy need to check if its the boiler or the system as said you cant legally work on the boiler but removing the cover isnt affecting the combustion side but you can fault find from the s plan junction box provided it has been wired correctly the grey and orange wires from the valves are the ones to start with
grey is power out from the boiler and it then passes through both micro switches in paralel and comes back on the orange wire to fire the boiler i would disconnect them both and use a meter to see which valve,if either is making the circuit i very much doubt this is a board fault

 

[James 3454]

Thanks Guys, ive been switching the heating on and off the past few days and the problem has not re appeared, so i think im just going to leave it for now, i know what its likely to be should it happen again.

Thanks again for all your help guys, it is very much appreciated.

Cheers.

James

 

[James 3454]

Hello Again. The problem has returned, I tried pushing both the levers on both valves, one was stiff and sprung back, the other was loose. Nether turned the boiler off... I tryed it with the hot water switched on, and then with both the heating and hot water switched on, but its the same, one is stiff and springs back and the other is loose.

So I'm thinking there is demand when there shouldn't be on the one that's loose all the time, but I'm unsure as to why the other one does not go loose even if i switch both the heating and hot water off, am i correct in thinking it should do?

Again many thanks if any one could help me out here?

James

 

[JBP Heating]

Hello again. The lever will be loose when there is demand and the valve has opened.