This issue sounds similar to one that I am currently experiencing with my unvented hot water system. For a few months now, following a service of the cylinder (and boiler), I am experiencing a noise and vibration from somewhere on the hot water pipework that is connected to my unvented Santon cylinder. It has been very difficult to narrow down the exact source of the noise and vibration. I have so far had two visits from a qualified and unvented certified heating and plumbing engineer, (in addition to the first visit where he undertook a general service of the tank, anode, valves and pipework). He has so far: 1. removed the hot water expansion vessel and checked that it doesn't have water in it, refitted it, recharged it to 3.5bar and refilled the hot water system. 2. When the noise returned a week later, he suggested that the two safety valves, (the pressure reducing valve and the pressure relief valve), that are located along the incoming cold fill pipe should be changed for new ones on the basis that he considered that one or both might be faulty as they are original to the installation, (21 years old), and might be causing the vibration/ noise. This has also not solved the problem which has now turned from a hiss and vibration to a fizzing noise when hot water is drawn from the tank.
Subsequently, I have taken my own readings from the expansion vessel to see if the vessel is losing pressure through a hole or fracture and introducing air into the system. I have taken pressure readings when the tank is not pressurised and the main cold water stop cock turned off and water partly drained from the system. So, the first reading I took after pumping up the vessel to exactly 3.5bar ,(which is the correct setting for the vessel). Three weeks later when the noise returned again, I took a second reading with the system unpressurised, and the reading had increased slightly to 3.6bar. I cannot see how this has happened as I would have thought that if the vessel were punctured in some way, the reading should have gone down, not up.
Oddly, the noise is at its maximum first thing in the morning when hot water is firstly drawn. Also, the noise we hear is the same every time. It doesn't sound like a hammer noise, more like bubbles being passed quickly through water and then becoming quiet once the bubbles have dispersed. The noise is much lower or non existent during the day. I have assumed that this is because the hot water in the tank has settled slightly overnight. However, I cannot see how this is happening if the expansion vessel is working, (or am I wrong)?
We have discussed the suggestion that the issue might point to a faulty mixer tap with our engineer, which has been discussed here, but the noise occurs whether we run a mixer or non-mixer tap. We have also discussed the unusual observation that if the mains incoming water is turned off and hot water partly drained from the system, that when the system is re-charged, the noise does not occur immediately, but after about a week of use. When I re-charge the system, I make sure that all outlets, (including wc cisterns), are open and that any air that has been introduced by turning off the cold water mains is fully expelled.
The only solution being offered by our engineer is to replace parts along the fill pipe one at a time until the noise disappears. This has, so far, been an expensive and slow process and has not solved the issue, yet. I worry that the noise could point to a faulty expansion vessel, but as its internal pressure seems to be pretty consistent, I am not convinced that the problem is the vessel.
We are soon to sell our house and don't want to leave a potential problem with our purchasers and are keen to sort it before we leave.
Any thoughts gratefully received.
