Worcester Heatslave 20/25 pressure relief valve

30 Dec 2008
Reaction score
United Kingdom
Irritatingly, after I replaced the expansion vessel, the pressure relief valve has started weeping. Each day, I need to top up the water in the system. There is a drip coming from the pipe that pokes out of the wall of the house, leading away from the pressure relief valve.

I don't especially want to have to drain down the system all over again in this weather. Can I just put a SECOND pressure relief valve on the outlet? (i.e. downstream of the existing faulty one).
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No, there's a drain cock at the bottom of the boiler which is easy to reach. It is, however, 26 years old now!
As you are changing it use it to drain down.
If you close the AAV's very little water is lost.
It is only necessary to change the plastic part which unscrews.
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Are you sure that your new expansion vessel is working? Does it part fill with water when hot? Some times the feed hose gets blocked. If you need to replace the valve, if you close off the auto air vents and relieve the pressure through the PRV, if you have the new one ready to fit, you should be able to change it with only a dribble. This would be no more bother than trying to shoe horn in a second valve. I assume you don't have isolation valves on the flow and return.
E.T.A Can't type as fast as Terry.
Thanks guys. @oilhead - yes the expansion vessel seems to be working properly. The side with the water in it gets hot from top to bottom, and the side with the air in it stays cooler to the touch. If I open the fill valve, I can see the pressure on the gauge rising at a steady rate, but the needle slows down a bit for a few seconds when it hits the pressure that starts compressing the diaphragm, so I'm assuming that side of things is all working. The water is definitely being lost through the relief valve, as there is a small puddle outside the house under the pipe leading away from the relief valve and I can see a regular rip from it!

Terry, I'm interested in the idea of only changing the plastic bit, as it looks like the valve body is screwed directly into the boiler itself, and I don't fancy having to re-seal that in a hurry, if I don't fully drain the system down! I've attached a couple of photos of it.


Using pipe grips on the black bit between the red plastic and brass. Unscrew anti clockwise. There's a sealing washer on the flange.
The red cover may bend a little but its only cosmetic. It is only the rubber boot over the spring that causes the drip.
Screwfix sell the prv
Just an update guys. Many thanks. I did the job last weekend and all is good now. Pressure gauge has stayed out all week, and the overflow pipe from the PRV is no longer dripping.

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