Vaillant Ecomax boiler pressure gradually dropping daily

14 Jun 2016
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United Kingdom
Hi guys,

I've had problems with my boiler dropping pressure since the last few months. Particularly after having the heating on. It wouldn't go up and over 3, but would go down to 0. So I've been topping it up daily. Alongside my living room radiator that had a leak from a nut on one side - clearly down to corrosion/rust around the base the nut tightened into.

Regarding the rad. I isolated all the rads one night, wrapped tissue paper around each rad's pipes and topped up boiler pressure. It still dropped to 0 and there was no sign of a rad leak. So this indicated an issue with the boiler as well. I found out it could be a fault with the expansion vessel, either needing re-pumping or replacing.

I had a guy come and check. He showed me that there was some water leak from the expansion vessel relief valve. So he advised it needed changing. I got the part and he changed. He also applied ptfe tape to the leaking rad. Anyway, since the fix, I still notice a gradual drop in pressure on a daily basis. I topped to 1.1 last Monday [6th]. This morning it's down to 0.7. This is all without turning the heating on.

So, is there any other part that maintains pressure? I heard something about a rubber diaphragm. Other than that, what else could be the issue... leak somewhere else? How can I investigate?

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