Boiler tripping RCD

[tommullin]

Hi All,

I just got home from work and all the lights are off. Checked the consumer unit and main switch is tripped. Only the main breaker is an RCD, all the rest are MCBs so everything goes if the RCD trips. Anyway, went through one by one and found the fault was on the kitchen sockets circuit - and one by one again, found it was the boiler.

Tried switching the boiler on a few times and every time I switched it on, it tripped. Suspected the pump, so I checked it for any damp or smells, nothing suspect looking. Came back downstairs and turned the thermostat down until it wouldn't turn the central heating on to try and stop the pump from coming on (at least I could have a shower!). Powered up the central heating and it didn't trip. Great, it must be the pump, easy fix. So I crank up the thermostat to prove the issue is with the pump, and its working fine, pump is pumping, radiators are hot etc.

So I didn't actually do anything to fix it, I may have nudged some wires when I was checking the pump, but that's about it. But, with the RCD tripping the main switch, I don't really want to risk it tripping out when I'm away and defrosting the freezer etc. Other than swapping out the MCBs for RCDs, is there anything I can do to fault find this one to see what the problem is? I know theres a lot of trades on here, so I was hoping someone with some experience might be able to point me in the right direction or maybe some clues on common faults.

Thanks in advance! Tom

 

[WorkshopChris]

Wonder if running the boiler got some heat into the pump so that when you re tried it the fault had cleared?
Problem is things that go away by themselves can come back by themselves.
If you still suspect the boiler it may be worth turning it off while your out till you are sure.

 

[the snooper]

Think I'd start by checking the wiring to the back of the fused spare that the boiler is wired into and go from there start with simple things like connections first

 

[gaz1]

Tried switching the boiler on a few times and every time I switched it on, it tripped. Suspected the pump, so I checked it for any damp or smells, nothing suspect looking. Came back downstairs and turned the thermostat down until it wouldn't turn the central heating on to try and stop the pump from coming on (at least I could have a shower!). Powered up the central heating and it didn't trip. Great, it must be the pump, easy fix. So I crank up the thermostat to prove the issue is with the pump, and its working fine, pump is pumping, radiators are hot etc.

any chance your system is dual fed from the boiler to both hot water and the central heating from reading that above thats what your stating ?

if refused to work before you switched it to hot water only then it worked your pump is still working to then turn your thermostat higher then run the central heating would quote that you have a dual fed system

 

[tommullin]

Its a hot water tank in the airing cupboard with a central heating pump. The hot water is gravity fed from the tank. I'm not too clued up on central heating so I'm not sure on the specifics of the system, but I can check and answer any questions.

I was thinking either pump or some below operation pressure switch causing the problem? but I'm clutching at straws. I only had the hot water on for about a minute before I tried the central heating again and it worked, so not a long time at all.

cheers for the replies guys!

 

[dobbslc]

I thought the pump would be on with both heating and hot water.
Could it be the 2 or 3 port valve?
The RCD will only trip with an earth fault. NOT overload.
You could have a dodgy pump, stat, valve in there, sometimes with earth faults a few on and offs will "clear" the fault as things heat up.
Did you have everything on that kitchen ring main unplugged? If not you may have a faulty appliance like the gas hob ignited or extractor hood that you can't unplug. There's a thing called build up where you have a bit of earth fault leaking from various places and just turning on one last item will trip out the RCD, if you then switch on in another order it'll look like the fault is somewhere else.
Ie you have 10ma on the kettle and 5ma on the oven, 10ma on the microwave you only need another 5ma's to pop the RCD. Most things will have a small amount of earth leakage.

Another thing is with all the other MCB's turned off all of the neutrals in the house are still connected bad to that RCD. Any faults elsewhere can still trip it out even with circuits turned off.

Phew!
Start with the simple stuff, anything that's either new or has been dropped or blown lamps etc eliminate them from the system.
Have you hung any pictures recently?

 

[Kayos]

If it's gravity circs then the pump only comes on with the central heating, no valves or anything needed

 

[tommullin]

That's very useful information, thanks everyone. Especially regarding the neutrals, I hadn't considered that. I unplugged some stuff, but then I went around one by one switching stuff back on and it was so repeatable when I switched the boiler on I thought it must be that.

Sounds like ive got some stuff to think about and a starting point for investigating.

 

[Wightsparks]

If there is an electric element in the system then 99% of the time (in my experience) it is that

 

[tommullin]

do you mean an immersion heater? It has got one of those but its switched off and always has been. I haven't ever checked any of the wiring but what you can see is a bit of a mess, maybe it could do with a bit of a tidy up.

 

[Wightsparks]

Yes an immersion heater. Less likely if it is switched off but if it is corroded and only the Live to it is switched then you will have a path from the Neutral to earth (the water in the tank) which could trip the RCD.

Also as dobbsic said have a think about what you did last before you had the problem?

 

[DAPPH]

Tom did you sort your problem ?

I'm disabled so can't do what I once was able to do very easily plus my memory is causing me probs
Around Guy Faulks time last year i I had an annoying intermittent problem develop on a combi balanced flue oil fired boiler " Fire Bird " model . It was driving me crazy , as the bills started to mount up with a " qualified service / boiler repair engineer .


The C*** had loosened the oil burner unit on his third visit reckoning it was tight on its shaft and causing too much current draw .
The next day it tripped the breaker yet again . The burner unit was held on by one thread , it wobbled dangerously when I put my hand on it as I used it as a support to help me get down on my kneeling frame thing
I happened to put my hand on the lower circulation pump & found it damp . Took the cover off it was bone dry , took the wiring off it , then tried to fire the boiler up & the fuse tripped again . Undid the cover on the upper circulation pump that sat almost directly above the lower one to disconnect it & water ran out of it .

What had happened ...... the thermal controller / diverter valve that splits the flow of hot water to rads & taps is above the pair of circulating pumps . It was dripping once the boiler was on ,, the drip fell almost directly onto the power cable of the top circulation pump , out of sight it had formed a puddle round the sealing gland . Over time it had seeped into the casing , evaporated on the circuit board & shown up an an earth fault . The pump impeller shaft hole was flooded with water too . not just a couple of small spots of condensation .

£ 14 or so later that afternoon I put on a new diverter valve & used an old circulation pump I've had in my bits filing cabinet for years .. fault cured.

It took far longer to rebuild the burner unit & fuel, pump & tighten down all screws & bolts correctly it , so everything sat dead square on its mounting plates etc . & turned freely .

FireBird appears to be one of the Chinese , " Best export quality " , British government purchased for the " Warmer Home " Scheme . Used in the Green scheme offering free energy efficient replacement oil or gas water heaters for boilers over 30 years old .

 

[normspanners]

Suspected the pump, so I checked it for any damp or smells,

bet it was damp inside ! they normally are !