Underfloor heating tripping RCCB when calling for heat.

[robbie77]

Hi

I have a wunda underfloor heating wiring centre that calls for heat and fires the boiler from a switched live that connects to a zone valve.
When the call for heat is made the RCCB trips. I have disconnected the pump and tested and it's still tripping so maybe it's a faulty zone valve or faulty wiring.
It worked perfectly but it has not been run for some time and in that time the power for the wiring centre was moved to a new fused spur on my new extension with a new fuse board with the RCCB. Previously the wiring centre was on the same circuit as the boiler and zone valve ( main fuse board RCD protected). I don't know if it's a coincidence or not.
I put a mutlimeter on the wires that call for heat from the zone valve and as soon as 240v hits the zone valve it trips.
Can someone point me in the right direction for some other testing I can do.

Thanks
Robbie

 

[oldbutnotdead]

Think you've effectively got a borrowed neutral in there. It's not good practise to have one system powered by 2 separate circuits...
Your external live driving the zone valve- is that live also connected to the microswitch on the valve (& thus the pump and gas valve)

 

[robbie77]

Think you've effectively got a borrowed neutral in there. It's not good practise to have one system powered by 2 separate circuits...
Your external live driving the zone valve- is that live also connected to the microswitch on the valve (& thus the pump and gas valve)

Yes. An alternative to this I could run a long extension to the underfloor heating from the same circuit as the boiler - this is what I did before the electrician came! A bit messy really but I suppose a good test or add the boiler on the new extension. I totally understand that it's not good practice.

 

[robbie77]

The underfloor heating has it's own pump too and it's oil fed - not that it makes much difference!

 

[robbie77]

Think you've effectively got a borrowed neutral in there. It's not good practise to have one system powered by 2 separate circuits...
Your external live driving the zone valve- is that live also connected to the microswitch on the valve (& thus the pump and gas valve)

So the only wire connecting the two circuits is the external wire from the UFH wiring centre to the microswitch wiring. Would this still be a borrowed neutral situation?

 

[oldbutnotdead]

Yes. RCDs and RCBOs work by comparing current going up the live and back down the neutral- any imbalance greater than 30mA (usually) will cause them to trip.
So your UFH heating control sits quite happily until it calls for heat, at which time the live from the microswitch connects to the boiler live and the current returns down the boiler neutral not the UFH neutral thus causing an imbalance.
Best way to avoid this problem (and potential danger to persons working on the system in the future) is to run the whole system from one circuit.
Easiest fix is to run a live from the boiler wiring centre to supply the zone valve microswitch. If you do this, make sure you mark the zone valve cover with a big label warning it is fed from 2 different circuits.

EDIT It's never a good idea to use the live feeding the zone valve motor as the microswitch live- you can end up latching the valves open when other circuits call for heat.

 

[robbie77]

You're spot on. Thanks for the explanation.
I ran an extension cable from socket from the boiler circuit to ufh wiring Centre and pump and no problem at all.

I'm going to put the boiler on a fused spur on the same circuit as UFH - this is simple enough to do and hopefully cure it.

 

[Taylortwocities]

I'm going to put the boiler on a fused spur on the same circuit as UFH - this is simple enough to do and hopefully cure it.

That’s how it should have been done from the start!