The floor below my hot water tank is showing signs of some moisture. It seems to be directly below this thing..

I put a tin foil tray under it last week and noticed another wee dribble today. I’ve a maintenance contract with BG so will be putting a call in to them anyway.

Tell tale for the pressure relief valve in an unvented system??

Tundish is the word I was looking for

https://www.google.co.uk/?gws_rd=ssl#q=tundish+pressure+relief+valve

A tundish

Edit – too slow!

Some kind of overflow with the break acting as a visual indication that overflow is occurring ?

have you got a megaflo ? If so, you prob need to recharge the airspace

It’s a Powerflow2000!! Sounds fantastic.

As for the previous post, google translate has given up.

Aha
http://www.renewable-energy-centres.co.uk/downloads/dl/file/id/42/powerflow_2000.pdf

WATER DISCHARGE
If water is occasionally being discharged
as the water is heated, this would be
likely to indicate that the Expansion
Vessel (F) needs recharged. In the event
of it occurring, switch off all power
supplies to the cylinder, re-charge the
vessel as below.
If water is continually being discharged,
firstly check with a gauge that the
pressure allowed through the Pressure
Reducing Valve (A) does not exceed 2.1
Bar. If it does exceed 2.1 Bar the
Pressure Reducing Valve should be
replaced using a replacement supplied by
McDonald Engineers. Next, check the
charge in the expansion vessel and
recharge as below. If the pressure is okay,
check which valve is discharging and
replace with a replacement supplied by
McDonald Engineers.

RECHARGING EXPANSION VESSEL
To recharge the expansion vessel, close
the mains stopcock and open a hot tap.
Connect a pump with gauge to the air
inlet on top of the vessel and charge to
2.1 Bar (30 PSI).

Bike pump?

Normally a schrader fitting, so yes.

Cool.

So, what’s happened – has the big ball on top of the boiler filled up with water or something? Why?

The diaphragm loses air like bike tyres that are not used. Shock pumps also perfect for this.