Boiler keeps firing Although controller is OFF

[topgazza]

I've read a few threads about this but none seemed to help....

So, I have a Potterton Netaheat boiler a BG controller in an unvented system. Last night my wife noticed that the boiler would fire up every few minutes, for a few minutes even though the controller had both HW and CH off. the central heating thermostsat was also "off". However the expansion rad was too hot to touch and the water out of the tap was almost boiling which was a real worry as i thought it would have cut out earlier. No radiators were hot other than the expansion.

The motorised valve lever next to the boiler has lots of resistance and returns to its normal position. The boiler thermostat didnt stop the boiler when it was reduced until is was all the way to minimum. But a while later it turned the boiler off when turned down from 4 -3. Could that be where the problem is but I don't understand why a stuck boiler thermostsat would fire a boiler when controller is off. I can control the off/on by switching off at the wall as a fail safe measure of course but any advice appreciated please

 

[stem]

The boiler will get its 'run' signal from your external controls ie thermostats, programmer, motorised valve(s)

For example motorised valves contain a small microswitch that when the valve is opened by the external control thermostat, this is what switches on the boiler. It is not uncommon for these microswitches to stick 'on' even when the valve has closed. If you have basic electrical skills and a multimeter, it's not difficult to test.

But you will have to give more information about your system to get further advice. For example do you have a 'C' 'S' or 'Y' plan. If you don't know the difference there are lots of diagrams on the internet that show the difference.

 

[topgazza]

Thanks for replying.

I think its an C plan by the looks of the various schemes. Its a pumped not gravity system as its unvented. One thermostat i cant see is the cylinder one but its not been adjusted. I guess in terms of thermostats there is the boiler, room and cylinder. From memory there is a two port valve in the cupboard along with the usual pump and the usual unvented bits and bobs.

Given that the hot water was scaldingly hot and the expansion rad was the same I would hazard a guess that its the hot water thermostat either on the tank or the boiler one...but of course could be any of those related components. I used to be an electrician so I'll get my trusty Fluke out later and check that microswitch in the valve

 

[Agile]

The HW valve may be powered on or jammed on.

So that should be your first place to look at.

Tony

 

[topgazza]

I'll give it a "wiggle" and see if its free or needed to be freed up. If it is I'll probably get it replaced as a precaution. I am checking some local unvented G3 plumbers so if i can't do a simple DIY fix I'll get one of the boys in

 

[ServiceNumpty]

Something doesn't make sense, you mention only one zone valve, plus an expansion rad. Are you sure its not a Gravity Fed Hot Water with pumped central heating via the heating Zone Valve.

 

[Agile]

Is your motor valve a two pipe or three pipe?

Just noticed you said the lever is free to move. So probably not jammed but the switch may be broken.

 

[topgazza]

Apologies.... its two motor valves both two pipe. One is partially tucked behind the unvented tank, its accessible OK but not in plain sight. But for sure i have an unvented system. I did a couple of fine adjustments with a hammer and tweaked the valves a few time to free anything up and to check they moved and low and behold everything seems to be working ok. I'll keep an eye on it of course but thank you all for your advice and taking the time to reply.

Much appreciated

 

[topgazza]

So BG engineer came out as part of the Homecare I have. Good guy, been before and he checked everything and to be fair it was working OK when he was there which is never ideal. he checked the valves and made sure they were free. Adjust the stats including the boiler stat but they weren't far out from what he expected. he did say that if it happens again he would change "everything" meaning valve heads, controller and possibly tank stat. Everything's been fine for a week but I woke up at 02.00 hearing the boiler firing even though water was timed off and heating room stat was turned right down. Expansion rad was boiling hot again and boiler was again merrily burning gas heating water I didn't need.

So call put in again and the same guy is coming out which is handy. Be nice if the fault is there when he arrives but no guarantees of course. given the symptoms would you expect him to replace the cylinder stat, controller, hot water valve ? I have sympathy as the old "no fault found" is likely to be the case by the symptoms are clear when it does play up.

 

[topgazza]

Turned out to be the microswitch in one of the valves.... as expected. Switch seemed to be working OK when metered but the fault was intermittent and he changed the valve head and everything is fine.

Thanks for everyones help here..... as always