Boiler advice

[blurredman]

I have a 20 year old worcester bosch (ZWB 7-25 R25 HE specifically) combi boiler that was in the house when I moved in 5 years ago.

I haven't been having many problems other than I needed to replace the water pump on the thing just a couple of months after moving in (it had seized solid) and the unit has been fine since but it's complaining a bit now.

The reliability of it working as of recently however is decreasing.
I have no idea concerning boilers, I know they're relatively simple things but I've got no experience.

However, the main problem is that when running the HOT tap, sometimes the boiler shoots very quickly up to beyond operating temperatures much quicker than it feasably should and goes into a state of 'standby' I suppose where it thinks about things after turning off the igniter. Whilst doing this, the boiler is making banging noises, and you can also hear this resonating through the taps (which is still running cold). I have however noticed that at this point the pressure isn't really where it aught to be at all. Manually opening the valve to raise pressure, and waiting until the boiler has had it's 'think about things' the temperature starts to go down on the display, the igniter comes on again as it should and the hot tap runs hot with no noises.

Is this the symptom of a diverter valve not letting in enough water? Is the knocking sound air? If the system is low on pressure and not pushing out this air, then presumably this is the cause for the overheating condition?

I've recently put the central heating on a few times to see if this does the same thing but it appears not - pressure builds up fully, in a normal manner and seems to regulate properly.


Thanks
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CBT: 12/06/10, Theory: 22/09/10, Module 1: 09/11/10, Module 2: 19/01/11
Past: 1991 Honda CG125BR-J, 1992 (1980) Honda XL125S, 1996 Kawasaki GPZ500S, 1979 MZ TS150.
Current: 1973 MZ ES250/2 - 18k, 1979 Suzuki TS185ER - 10k, 1981 Honda CX500B - 91k, 1987 MZ ETZ250 (295cc) - 40k, 1989 MZ ETZ251 - 50k.

[UncleFester]

Limescale and kettling.
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2017 Z1000SX

[Ste]

Sounds like a job for SinghPal™.