Expansion vessels and water hammer

17 Jul 2020
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United Kingdom
Hi Guys,

We've had pretty bad water hammer issues on our hot taps. I installed 2 vessels:


They have fixed the issue, but the refillable vessel (bottom link) has been giving me headaches since it needs refilling every 2-4 weeks. I also don't know what kind of pressure gauge to use for it so we're kind of in the dark about pressure levels and how much air to fill it with, but also how much the pressure in the vessel should be for our given system.

I am looking for:
1. a good pressure gauge that'd work with this particular item
2. how to calculate the amount of pressure required in the vessel. I am guessing it is in correlation to the pressure from the main system. We're using a combi-boiler system.
3. I've always wondered why the hot water has water hammer issues, but not the cold. It sounds counter intuitive since the mains pressure is also high.

I have considered a 12 Ltr tank but I have no place to stick it, hence why I've gone for the smaller absorbers.
Last edited:
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Take the valve out to see if the seating (red & blue) is loose.
Whilst I appreciate that a lot of water hammer can be fixed by using arrestors, you really want to find out why it's happening and sort that. Just because the hammer isn't apparent don't mean that it isn't still happening.

Hammer is usually caused by a concussion being created by a valve(s) that closes too quickly, that then sends a shockwave back down the pipe that creates banging/knocking at any weak/loose areas of pipework. As @Bodd suggest you want to look at the hot side of things and check any valves/taps and see if you can specifically recreate it.

If you find you are having to re-charge the EV every couple of weeks then there is something wrong with it (test the shraeder valve with a little water/spit and see if it bubbles) and it needs to go back.
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We've had pretty bad water hammer issues on our hot taps. I installed 2 vessels:

In all my years I have never used these to fix a water hammer, it has always been a valve/tap/stopcock issue, for me at least.

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