Boiler Tripping RCD

[Rich2323]

Hi Guys,

After some advice! I have a gravity fed system with a Vaillant boiler. Over the summer I did a flush with fernox cleaner, drained and added inhibitor. I also rehung two radiators and tested CH and all worked perfectly.

Since coming to turn on CH for the cold weather the radiators were only getting a bit warm with the two furthest away stayed cold. HW was only warm too. Then a few nights ago the boiler was kettling and all rads were cold.

Called a boiler mechanic and he told me to open up my pump to see if it’s full of muck...it wasn’t. When I put it back together the boiler just fired up To 75degrees and then straight down again when in CH mode. In HW mode nothing happened, just the odd click. After a few minutes it tripped my 63A RCD. Left it off and next day I called the same boiler mechanic and he came round...turns out the boiler and pump were dry as there was a blockage in my header tank. Hey presto, blockage cleared and water flowing through the system and all radiators working, boiler sounding healthy. Mechanic left and two minutes later it tripped again. He told me to ring a sparky as it could be over sensitive RCD but having read up about this issue that doesn’t seem likely. I rung a sparky and he said it sounds like the pump is knackered but boiler mechanic said it was fine.

I’ve tested it on just CH and just HW and it’s tripping on both. If the system starts from cold it usually trips after five minutes. If I then flick the RCD back on in trips immediately. If I turn off the boiler the RCD doesn’t trip so it’s defintely to do with the heating.

Anyone have any ideas? The pump seems to be in use with both CH and HW. The CH gets hot within a few mins so the pump is working efficiently I think but I’m not an expert. Everything I read on the internet leads to a whole catalogue of potential issues...any help would be massively appreciated, thanks.

Rich

 

[flameport]

The usual suspects are electrical items which contain water - so the pump and zone valve(s).
If a zone valve is leaking water this should be obvious, as there will be water in the actuator (box on the top of the valve).

Pumps are more difficult, as any leak may not be obvious from outside.
If the pump is separate from the boiler, it can be run on it's own by switching the system off, removing the wires from the pump and temporarily connecting if using flex and a plug.

The other item is the boiler itself, but presumably the boiler person examined the internals and confirmed it wasn't leaking inside?

 

[Rich2323]

The usual suspects are electrical items which contain water - so the pump and zone valve(s).
If a zone valve is leaking water this should be obvious, as there will be water in the actuator (box on the top of the valve).

Pumps are more difficult, as any leak may not be obvious from outside.
If the pump is separate from the boiler, it can be run on it's own by switching the system off, removing the wires from the pump and temporarily connecting if using flex and a plug.

The other item is the boiler itself, but presumably the boiler person examined the internals and confirmed it wasn't leaking inside?

Hi Flameport,

There’s no obvious signs of leakage, perhaps I spilt a small amount of water into the actuator when cleaning the pump but i was very careful so it seems unlikely.

The pump is a Grundfos. Regarding running it on it’s own using flex and a plug, if it was faulty would this process still cause my RCD to trip? I don’t need to have the boiler system on? Or would the pump just push around water still but just not heat it? Can I also do this process with the actuator if it doesn’t turn out to be the pump?

The boiler guy didn’t unfortunately, he took the money and left so it’s still a possibility sadly. I fear it is to do with the boiler as it only seems to trip when the boiler gets to around 50C...it will run for a few minutes and trip, then trip immediately when I reset the RCD which tells me there would be and issue in the boiler.

 

[dilalio]

The heating engineer identified the starvation issue, resolved it and left the system working... he deserved his payment.

If there are further issues, now apparent, the dry-running of components such as pump, valves and boiler are likely to have had an effect.

Try the fly lead to the pump and manually open the valve(s) with the system cold (boiler off). If the RCD continues to trip then you need a new pump.

 

[Rich2323]

Just tried with pump only and pump is fine. Turned on the boiler and after ten mins at about 54 degrees it tripped again. I’m stumped.

 

[Rich2323]

Just removed the switch from wall and found this...looks like the switch red light indicator has burnt out and taken the wire with it and scorched the wire insulation. In the state shown in the photo the boiler is staying on...at the moment!

 

[Rich2323]

Photo

 

[flameport]

That burnt wire may have been the cause, although not necessarily.
The dust/dirt on the wires won't be helping either.
The screws on the top edge as shown in the picture are live even if the switch is off.

The whole switch needs to be replaced, and power to the circuit (or house) must be turned off first (using the RCD that trips).

A neon is optional, and the fuse in the new one will need to be changed to a 3A type, as they are supplied with 13A.

 

[dilalio]

Photo

Looks like the likely suspect. Change it and then next step is to see if it cures the problem. You could also, in the meantime take the fly-lead from the pump and use that to power the boiler - with a 3amp fuse in the plug of course!

 

[Rich2323]

Replaced the switch and all worked great. The washer on the ball cock valve was knackered and dripping outside so I turned the water off, bought a new ball cock valve but I stupidly forgot to turn the water back on before I turned the boiler back on for ten seconds or so. Whatever happened seems to have caused the original problem to return and the boiler and pump seem to be running dry. Tried to blow in to the pipe in the header tank but no luck.

Pipes are tingling so guess there’s crap running through them.

 

[DP]

I take it the boiler is the early ( some 25 year old or something similar) 2 door Square gauge model
No water in the boiler should not fire the burner I would have thought
Both the VC compacts and earlier beastie that looks like Sine 18 woukd not fire the burner if there is no primary circulation or primary is not purged of air.

How can you have gravity circulation on a boiler where pump is the main driving force to get the boiler to work.

Get yourself a digital multimeter, earth leakage is dead easy to find
If you are all thumbs reading circuit diagrams, get a heating engineer who knows how to use test equipment. An electrician familiar with house wiring may also struggle if he/ she cannot follow boiler circuit diagram