Boiler tripping RCD

[Dibergio]

NYE the RCD on the consumer unit trips, and after resetting keeps tripping.

Electrician visited on New Year’s Day (RCD had stopped tripping) and identified a number of issues, and one likely fault in the ring main.

He came back on Monday, fixed the fault and replaced the consumer unit so we now have two RCD’s.

This evening one RCD tripped which has central heating, cooker, downstairs lights and sockets. Turned everything off, unplugged everything and after gradually turning it all back on again it seems like the boiler is the culprit.

Around 10 mins after the boiler firing up the RCD trips.

When I only turn on the hot water it doesn’t though.

Boiler in question is a Potterton Suprima 60.

I’m not handy, so guess I’m really looking to understand if I may have missed anything?

And I assume I should be getting a heating engineer out rather than sparky again?

 

[blup]

Did he carry out an EICR and did that identify the boiler circuit as tested, with or without limitations?

And why did you need a new consumer unit?

Blup

 

[Gasguru]

The RCD monitors the current flowing in the Live and Neutral conductors and in a perfect system those currents would be equal.
However, due to various reasons there is always a little leakage of current to the Earth conductor...once that reaches a threshold (typically 30mA) the RCD will trip as it's assumed to be a fault condition.

It sounds like the electrician didn't really try to find the underlying fault and rather than measuring the leakage current just assumed that by splitting the board (and having 2 RCDs) that would solve the problem.
What were the other issues with the installation?

It's quite possible you have leakage elsewhere in the electrical installation/appliances and when the heating operates it's just sufficient to raise the leakage to the RCD threshold ie. there may be nothing wrong with the heating system.

The only reason operating the heating over the hot water could cause a trip would be say a damp zone valve or faulty cabling from programmer/room thermostat etc.
Post a few pics of the zone valve/valves etc and you might find something obvious such as a leaking valve.

Really you need an electrician that can measure leakage current and isolate where the fault is....99% of heating eng.s won't have the equipment and will be guessing unless there's clear evidence eg. water logged zone valve etc..

 

[Dibergio]

Thanks for the replies.

The electrician measured leakage on all of the downstairs sockets, but didn’t to my knowledge do it for the kitchen appliances.

he found leakage between two downstairs sockets, disconnected them and then ran a new cable. We then discovered that all the upstairs sockets were on the same circuit as downstairs - apart from the kitchen.

we have a nest thermostat downstairs but heat link is upstairs in airing cupboard - is that where the valves you refer to should be found?

I’m going to double check on the heating by turning off all the other circuits and appliances and turning on the heating and see if it still trips (sure I’ve tried that already but will try again). Would that point to boiler being the culprit?

 

[Dibergio]

Just tried again. Every circuit on RCD turned off except for kitchen sockets, all appliances off apart from boiler.

turned on heating and RCD trips after 5 mins.

does that point towards boiler/heating being culprit?

with hot water when I put that on override, the immersion tank comes on - but the boiler also fires and immediately turns off. I’m confused at what should do what there.

 

[Gasguru]

It sounds as though he only measured leakage on the electrical circuits and didn't then check with the appliances connected and operating.
If you have switched off all the other circuits (the MCBs are off) and only the heating system is powered and the RCD trips that would narrow it down to the heating system.

The immersion heater is the electric element that heats the cylinder when the boiler fails...normally it should be off at the switch on the wall.

Run the boiler in hot water mode (ensure there is a demand from the cylinder by running off hot water if necessary) and see if it trips.
Then run just the heating and see what happens.
The zone valve(s) etc are normally next to the cylinder.

 

[ianmcd]

This evening one RCD tripped which has central heating, cooker, downstairs lights and sockets.

You need a better electrician and a replacement central heating pump

 

[Dibergio]

Ok this morning both the heating and hot water has tripped the RCD, within a minute or two.

So should I start with a heating engineer?

 

[JohnD]

When you say "immersion tank" you mean "hot water cylinder." This is heated by the boiler, though it probably also has an electric immersion heater that should normally be turned off, but can be turned on if the boiler is out of action, like today. The immersion heater is nothing to do with the boiler or its controls.

RCD trips are more often than not caused by watery appliances that have a leak.

Boilers and central heating pumps have water in them, and electricity. So your description points in that direction.

Sometimes you can see damp, or limescale, or green stains, where water has dripped.

Another common cause is an eroded electrical heating element, as in an oven, but that does not match your description

To isolate suspect appliances, you have to unplug them, switching off is not enough. Nor is MCB enough as it is only single pole.

A boiler may have a switched FCU on the wall, which, if reasonably modern, is probably a DP isolator.

Unless you continue to get trips when the boiler is isolated, I'd start with a heating engineer.

 

[Dibergio]

Sorry yes all appliances were unplugged or isolated.

Got a heat engineer coming this afternoon - quick phone diagnosis and he suggested zone valve or pump which seems to tally up with you guys

 

[kidgreen61]

Pump or mid pos/zone valve fault.

 

[Dibergio]

Guy today tested the zone valve (3 way), boiler, and pump.

valve is ok. No sign of water leaking anywhere. Boiler looks ok.

Still unable to isolate the fault from the boiler, pump or electrics.

turned off the pump and got the boiler on, fires up runs for a minute then bangs. Didn’t trip switch but then turns itself off.

turned pump on at very low pressure and still trips.

checked resistance of pump and getting something like 20 ohms when it should be 200, so we are going to replace that on Tuesday and see if it fixes the problem.

he gave it a 50/50 chance of being successful.

 

[ianmcd]

look at #7 I have already told you that it is the pump

 

[Dibergio]

look at #7 I have already told you that it is the pump

pump was replaced and it is still tripping.

engineer said his last guess was replacing the pcb in the boiler, despite it all looking ok on the surface.

electrician coming tomorrow to double check the RCD, and potentially isolate the fault...

 

[rbranco]

can you run an extension lead to some other sockets in the house not on the same RCD then run the boiler controls off that, then see if the tripping moves

With the main boiler circuit properly isolated.
I would check for leakage between L-E and N-E on the boiler main circuit, problem is you may need to bridge your programmer relays as the circuit on the L will not be complete until everything is in a running configuration.
Do the same on immersion heater and also from L/N to the cylinder to see if there is any breakdown of the element.
Also check L/N of the boiler to the main house E to see if there is any leakage though the water bonds, and again the same on the controls as move them about.
Is your immersion heater on the same circuit as the boiler?
Whoever did my house (old wire board) decided ages ago to put the boiler and immersion on different fuses which was a good idea.