CH pump tripping RCD - advice?

[Kyrt]

Hi folks, looking for some advice:

This morning when the heating/hot water was scheduled to come on the RCD tripped. I traced the problem to the ring circuit, and then isolated various parts until I identified the fault in the central heating and hot water system - there is a fused switch to isolate it. It is a gravity fed system with cylinder, pump and 2 motorised valves in the airing cupboard, with a Glow-Worm Ultimate 60FF boiler.

I disconnected the thermostat, it still tripped. Then I disconnected the programmer from the wall plate, it still tripped. Likewise the CH and HW motorised valves. Eventually after disconnecting the live wire at the pump the RCD no longer tripped. The programmer was still disconnected at this point, but subsequently I can confirm that with everything else connected, connecting the pump is what causes it to trip. So, to me it looks like a faulty pump but before I call a plumber I just want to check it cannot be anything else - like leakage to earth from the flue fan which when combined with the pump is enough to trip the RCD? I don't know, I am really not an expert on this!

Secondly I am a little confused about why the pump would trip the RCD when the programmer is disconnected - is the live wire to the pump not a switched live from the boiler or programmer? Is it maybe the boiler still calling for pump (overrun?) even when the programmer is disconnected?

Thirdly what could actually cause the pump to fail like this? A blockage? The pump has been working fine, albeit the house is about 15 years old, and there is no evidence of water leakage around any of the components or electrics. Is it possible to isolate the pump and open it up to take a look or is this a really dumb idea for a DIYer?

Edited to add: job for plumber, or electrician?

 

[EFLImpudence]

Just to clarify so the correct wiring diagram can be determined:

You say it is a Gravity System but you have two Zone Valves.


Do you have two heating zones?

 

[JohnW2]

You say it is a Gravity System ...

The OP appears to be describing a gravity-fed but pumped (i.e. 'traditional') system.

but you have two Zone Valves. Do you have two heating zones?

I would have presumed two separate MVs, one for CH and the other for water heating (which is conceptually the same as I have, albeit I have multiple CH zones), rather than a 3-port valve.

Kind Regards, John

 

[EFLImpudence]

Just checking.

I would have presumed two separate MVs, one for CH and the other for water heating

That's what I thought, that's not a Gravity System but S Plan.

 

[JohnW2]

I would have presumed two separate MVs, one for CH and the other for water heating

That's what I thought, that's not a Gravity System but S Plan.

As I said, the OP is not describing a "gravity system" but, rather, a (pumped) "gravity-fed (i.e. not pressurised) system". That's probably why he wrote "gravity fed system" and not "gravity system"

Kind Regards, John

 

[EFLImpudence]

Yes, but I wanted to make sure. It affects when the pump is working.

 

[Kyrt]

Yes sorry for using confusing terminology it is gravity fed in terms of the cold storage and header tanks in the loft, the pump serves the central heating and heats the water in the cylinder via 2 separate motorised valves - one for hot water and one for rads.

 

[theprinceofdarkness]

Central heating pumps have two compartments, the half furthest from the pipes have the "motor" coil in it. Then there is a thin brass separator, then the pump rotor. When its working the motor coils cause a magnetic field to sweep around the housing. The rotor then spins , trying to chase the field around and around. Sounds as though your separator has a hole in it allowing water to get into the dry side of the pump.
Frank

 

[EFLImpudence]

From what you say, it seems that pump does indeed have some earth leakage.

However, it could be that it is accumulated leakage.

For example, say the RCD trips at 27mA, the pump may have 5mA - with 23mA elsewhere on the ring.
It could be that it is this 23mA that has been increased by a new fault somewhere else.

You could rewire the heating to an extension lead and plug it in to a circuit on another RCD.



That the RCD tripped with the programmer also disconnected means that it is tripping when the pump is not being asked to run.

 

[Echo the husky]

A simple insulation test of the pump will soon tell you.

 

[SimonH2]

Bear in mind that you don't need the live to be connected for it to trip the RCD. A the current from other loads causes a voltage drop in the neutral wire - which means that a fault to earth can cause some of that neutral current will pop over to the earth wire, and the RCD trips.
We had the same problem at Mum's a few years ago. There was a drip which dropped onto the pump connection box, and with the orientation of the pump, it filled the box up.