Combi tripping RCD in heating mode only

[Markyboy69]

I’ve got a Baxi combi boiler. When it’s in heating mode (not hot water mode) it trips the house RCD. This has been happening for over a year. I’ve had numerous Baxi engineers out and they’ve pretty much replaced everything in the boiler. They told me it was the plug socket so I used an extension lead on another circuit and it still tripped. They told me I must have a plug socket near a heat source so I unplugged everything in the house except the boiler and it still tripped. It trips sometimes after a few minutes or even after a day. When the heatings turned off it doesn’t trip at all (didn’t trip all Summer). My question can this problem be external to the boiler? The boilers insured and under the agreement if they can’t fix it they have to replace it. But Baxi keep fobbing me off with no definitive answer. I haven’t had heating for at least 16 months. But I’m still paying £47 a month breakdown cover. Any ideas? Thanks in advance.

 

[EFLImpudence]

You need an electrician who can find where the fault is.

 

[CBW]

Yes could be external controls. Do you have any motorised valves at all? What thermostat do you have? Which Baxi combi, as they’ve done a few over the years.

 

[Markyboy69]

Baxi Neta tec combi 24 GA and no external valves. The wireless controller is a Drayton digistat and that’s a couple of days old. It started tripping out already 2 days after a Baxi visit.

 

[ericmark]

There are three tools we use.
1) RCD tester
2) Insulation tester
3) Clamp on ammeter

We can test bit by bit, which can be time consuming, or we can have a informed guess which can shorten the process.

If an expert has missed the fault, we would assume not a run of the mill fault, but what we don't know is if the people who visited you were really experts, or minions who follow a set procedure.

So down to the guess bit, what is activated when using central heating, but not DHW? As said any motorised valves, and room thermostat.

But the other possibility is pipes expanding, and this is hard to find, some thing physically moving due to the heat, and causing a short.

As to if isolating radiator by radiator to see if the pipes feeding that radiator are expanding and causing the problem would be rather hard.

 

[CBW]

Baxi Neta tec combi 24 GA and no external valves. The wireless controller is a Drayton digistat and that’s a couple of days old. It started tripping out already 2 days after a Baxi visit.

If Baxi have pretty much replaced everything, then it might not actually be the boiler, more so anything on the rcd circuit. I’ve said it now on a few threads, I attended one once, electrician said it was the boiler, but the fault lay elsewhere on a ring circuit - customer had hung a mirror up in her hallway and had nicked one of the cores.

 

[JohnD]

Boiler related trips are sometimes due to water getting into electrics.

How long is the delay between turning on the heating, and the RCD tripping?

Does it happen day and night?

Do you have a wall-mounted room thermostat?

Does the boiler pressure sometimes drop and need topping up?

Where does the condensate pipe go?

Please show us your Consumer Unit (fusebox) with the cover open so we can read the labels, and a wider pic showing the cables around it.

 

[Markyboy69]

Boiler doesn’t depressurise and there’s no signs of any leaks. The thermostat is wireless and it’s on a stand so not wall mounted. The boiler trips the power at random times from 10 min to 13hrs day or night. But the RCD only trips when the heatings on and at no other time at all. I’ll get a pic of the distribution board when I’m home tomorrow. When an engineer messes with the boiler it doesn’t trip for a couple of days usually but then starts tripping again. Until they fiddle with it again and it might last 2 or 3 days then start tripping again. Its weird.

 

[JohnD]

How certain are you that there is no water leak inside the boiler? It need only be a drip.

Tell me about the condensate pipe.

If it had not been a combi, I would have said look under the circulating pump and the 3-port valve.

 

[Markyboy69]

Condensate pipe comes from the bottom of the boiler, goes straight down for a 1.5ish meters along the wall 2m ish then outside to a trap. I had 9 engineers out all say there’s no leak. There’s no sign of leakage so I’ve only got their word to go on.

 

[Gasguru]

I assume the room thermostat wireless receiver is very close to the boiler and it's wiring is surface mounted and probably fine. That only leaves the diverter actuator in the boiler...a leak on the valve spindle could find it's way into the actuator (that operates at 240v).

 

[Markyboy69]

If Baxi have pretty much replaced everything, then it might not actually be the boiler, more so anything on the rcd circuit. I’ve said it now on a few threads, I attended one once, electrician said it was the boiler, but the fault lay elsewhere on a ring circuit - customer had hung a mirror up in her hallway and had nicked one of the cores.

Problem with that is I’ve had the boiler running on an extension lead run to a different floor on a totally different circuit and it eventually trips the circuit. The RCD didn’t trip once through the whole of spring and summer. As soon as the heating was switched on for winter the RCD starts tripping again. It’s not an instant trip. It can run for a day or just an hour. It’s random. I’m sure if it was a damaged cable other electrical appliances would be tripping the RCD. But only the boiler does it no matter what circuit it’s fed from. All signs point to the boiler but it’s a nightmare trying to find someone who knows what they’re doing. I think I’ll get a sparks in to check the electrical installation before I call Baxi out again. At least I can present some paperwork saying it’s sound to Baxi or get the sparks to sort any faults on the house electrics side. So far I’ve spent over £1600 trying to get this sorted but I still haven’t had heating for 2 winters. It’s madness.

 

[Markyboy69]

I assume the room thermostat wireless receiver is very close to the boiler and its wiring is surface mounted and probably fine. That only leaves the diverter actuator in the boiler...a leak on the valve spindle could find its way into the actuator (that operates at 240v).

I’ve had 15 visits from engineers and if there’s a leak they haven’t found it. There’s no sign of any water that I can see either. I think the only thing they haven’t replaced is the casing and the lead to the plug socket. I’m at a loss. I’m going to get an electrician in to check my wiring in the house. At least that way I can rule it out or get it fixed.

 

[JohnD]

Sometimes there is a damaged cable in the floor that causes a trip when walked on.

I have seen rare examples where a cable has been crossed over a hot pipe ( which softens the insulation) and crushed under a floorboard, damaging the softened insulation.

 

[ericmark]

As @JohnD says, heat seems to be the problem, be it plastic getting soft, or a pipe expanding.

But one starts with the simple first.
1) Have insulation resistance tested,
2) Ensure the boiler on it's own circuit, with it's own RCBO.

One thing we have not considered is a borrowed neutral, with a 30 mA RCD it seems unlikely it would hold in but as yet not seen picture of the consumer unit, so could be 100 mA version.

Also I note some boilers stipulate type A RCD's, but until the last regulation up grade type AC were often fitted.

What I can't understand is why you would wait 2 years to fix this, what are you not telling us.