Diagnostic help for Santon unvented HW cylinder

15 Nov 2009
Reaction score
United Kingdom
Santon Premier Plus 210 unvented hot water cylinder with
Santon combination cold water pressure reduction/expansion valve.
External expansion vessel precharged to 3.5 bar

Cold water is dripping continuously from the combination valve (through tundish). It's not the P&T relief valve.
The expansion valve section of the combination valve has been replaced with no effect.
External expansion vessel pressure is OK.
Have tried re-charging cyclinder by following procedure:
Close off feed to combination valve
Open lowest HW tap until flow stops
Open T&P relief valve until "gurgling" stops
Re-open feed to combination valve until HW tap runs cleanly

Drip from combination valve starts immediately the feed is re-opened.

Am I right to think that the pressure-reducing section of the combination valve must be leaking? Or could it be something worse (heat-exchanger? - gulp). Is there any way to figure this out short of swapping out the combination valve?

Incoming mains pressure is about 7 bar. The installation is about 3 years old.


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If you could measure the pressure in the cylinder that would be useful.

Whilst the pressure reducing valve may be leaking the most likely senario is a leaking joystick tap or shower mixer letting mains pressure into the hot supply.

This is particularly likely when your mains pressure is so high!

Its best if the cold supplies to the house ( apart from kitchen cold tap ) are fed from the reduced pressure connection on the valve assembly on the unvented.

That's a clever idea. Thanks.

There is a separate adjustable PRV with built-in pressure dial on the supply to the cold taps, which is set at 3.5 bar static pressure. I got the 7 bar figure by backing off this valve as far as it would go. I just tried to test your theory by closing this valve until the dial showed zero pressure. This succeeded in shutting off the cold taps, but cold water is still dripping from the combination valve at the same rate as far as I can judge.

By the way, "dripping" is a bit of an understatement: about 7 litres per hour is watering the garden at the moment...
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Then you need to measure the pressure insode the cylinder.

The PRV may just be leaking or the pressure may be excessive.


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