27 Jul 2021
Reaction score
United Kingdom
Hoping someone can help who has experience of unvented water tanks and boiler issues? It’s a long post… heads up!

We have recently just had a new system boiler installed (Ideal Logic +) and I noticed that the relief pipe outside my house was dripping water within a few days. The heating engineer who installed it came to the house and straight away said the pressure on the boiler was too high- sitting at just above 3. He checked that there was no issue with the loop pipe and then said he reckoned it had something to do with the unvented water tank, but he didn’t have much experience of them so I should find someone else.

Another engineer came out tonight, had a quick check of the loop and then said he reckoned we needed a new water tank, costing c.£1600. I’m obviously wanting to explore all other avenues before this, but don’t know where to start. I just feel like something must be wrong if we didn’t have the relief pipe dripping before the new boiler was installed.

Some things to note:
- Pressure on boiler up at 3 even when heating and hot water off
- Pressure reduces slightly when a tap is turned on
- Apparently when the filling loop is fully closed, the boiler pressure continues to rise as if it is being filled elsewhere (the first engineer said this)

Having googled a few things, I’m wondering if it could be something to do with the expansion vessel on the water tank, or would this only be the case if heating was on?

Also, I haven’t tried bleeding the radiators albeit, surely one of the engineers would have suggested this?

Thanks in advance
Sponsored Links
Hi Amanda.

From what you describe it seems that the unvented cylinder has a pin hole in the heating coil.

So the cold/hot water inside your tank is filling up the heating system. Pressure reducing on using the hot is an indicator as the two systems should be completely seperate.

Did your installer powerflush the heating system, (exactly what he should have done) sometimes it can highlight any weaknesses in the system causing issues like this.

You could test this yourself, turn off the water to the hot water cylinder, and open a hot tap, leave it open for the duration. Monitor this for a few hours (suggest overnight) if as suspected the cylinder is leaking your heating pressure should slowly reduce to zero.

If it continues to rise or stays the same with the tank isolated your problem lies elsewhere.

Hope this helps, others may have other suggestions to test, far cleverer then mine.
Thanks so much for coming back to me.

I don’t know if he did power flush- assuming he didn’t!

Silly question but… how would I turn the water off to the cylinder?
There will be a tap or valve somewhere near it on the incoming cold main. If you follow the pipe back from where it enters the bottom of the cylinder (may be marked "cold inlet"), you will see said tap or valve.
As said earlier, if before you go to bed, turn that valve off, depressurise cylinder by turning on hot tap, you will see if boiler pressure has risen overnight, if so, it's new cylinder time.
Sponsored Links

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local