This post was from me a couple of years ago, thank you for your help but unfortunately I had no luck fixing the problem...
"Post by steve1000 » Thu Nov 18, 2010 11:02 am
I've had a plumber flush through my 20 year old oil fired central heating system [which has always been reliable] & refill with inhibitor. There is a lot of 'rushing' 'running' 'pouring' water noises sometimes now, around the pump/valve area, when the system is working. It sounds like air to me. The plumber [local, reliable?] has come back, scratched his head & walked away trying to tell me it's normal admitting he has no idea what's causing the noises!
I do have a 10 bar vent device fitted beside the cylinder, which appears to be working; I took it apart & cleaned it, there is an occasional slight hiss as if some air is coming out. The radiators are bled regularly & there is no air in them. I'm a bit stumped; any ideas please as to what may be wrong?"
Things have moved on. The above symptoms recently changed rather dramatically. The heating side is still fine. But when the hot water has been fired up for 30 mins or so there comes a surging noise & hot water pours from the header tank overflow for about 2 mins. I'm using the [expensive] immersion heater now.
One suggestion was a split coil in the cylinder. This presumably wouldn't foul the hot water from the cylinder as the cold tank head is 200mm above the header tank, so any flow would be from cylinder to coil. But would it overflow all the time, whether the pump is working or not?
I've just had a power flush & the plumber's suggestion was a replacement 3 port valve [Honeywell v4073a]. Unfortunately this didn't work. Next morning the hot overflowed after 30 mins. I'm wondering whether the cylinder thermostat might be faulty [the immersion heater has it's own], but I can't see that producing the old symptoms from 2010. Does anyone have any ideas please?