Overflow from drain vent?

[koubliac]

I live in a Victorian terrace with a loft conversion inc bathroom. The bathroom toilet/basin/shower have a drain pipe that exits the house and joins the main external drain pipe. My problem is that there is an overflow pipe outside the bathroom that is leaking (started a few weeks ago, just a few drips at first, but now steadily dripping every 2 seconds). I have taken the wall off to check the pipework. The overflow pipe does not appear to come from the toilet/basin/shower but from the top of the drain vent in the roof? It's tough to see exactly where as the pipes are so restricted. Is this an expected configuration? If so what would cause an overflow from the drain vent?

I have checked this overflow is not coming from the cold water tank (it has it's own overflow).

 

[JohnD]

is there a boiler? or a WC cistern?

 

[koubliac]

No, the boiler is on the ground floor and the wc cistern overflows elsewhere.

 

[JohnD]

is the boiler pressurised, or does it have a feed & expansion tank?

 

[koubliac]

The boiler (Glowworm 18hxi) overflows into the washing machine drain on the ground floor (completely separate), so that can't be it.

 

[twgas]

The boiler (Glowworm 18hxi) overflows into the washing machine drain on the ground floor (completely separate), so that can't be it.

thats the condensate, there should be a header tank in loft or a PRV with a discharge pipe to outside.

 

[JohnD]

is the boiler pressurised, or does it have a feed & expansion tank?