Vaillant 835 e combi boiler.. Banging noise & HW issue

[Scotland007]

Evening
I have a Vaillant 835 e combi boiler.
All was fine up until a few weeks back and I noticed a knocking noise on the boiler.
I turned down the temperature for the rads and this got rid of the noise.
But now it's getting abit colder I want to turn up the heat but back to the same issue?.
Also the hot water has started playing up not sure if it's related? But starts off hot for about a minute then gradually goes luke warm, if I turn of tap and wait a few seconds and turn it back on the same occurs.
Any help would be gratefully appreciated.
P. S...
Brought the house in February this year and boiler was serviced before purchasing.

 

[terryplumb]

Best to engage an engineer to examine and diagnose the cause/s.

 

[Scotland007]

Hmmmm is this how the forum works?
It's like asking about car issues and being told to call a mechanic!
I understand that calling a plumber would be the easiest and less stressful solution and if I exhaust all avenues than yes I will have to take out a loan and call one in.
I wouldn't go anywhere near the gas side of things just the water.. I'm an engineer and have replaced parts in the past to overcome issues and wouldn't undertake anything out of my league.
Just enquiring if anyone has had a similar issue or if they could point me in the right direction.
Any help is greatly appreciated
Thanks

 

[ianmcd]

your description of the noise is very vague, would be better if you could post a video of it making the noise, we might be able to help you

 

[DP]

Hmmmm is this how the forum works?
It's like asking about car issues and being told to call a mechanic!
I understand that calling a plumber would be the easiest and less stressful solution and if I exhaust all avenues than yes I will have to take out a loan and call one in.
I wouldn't go anywhere near the gas side of things just the water.. I'm an engineer and have replaced parts in the past to overcome issues and wouldn't undertake anything out of my league.
Just enquiring if anyone has had a similar issue or if they could point me in the right direction.
Any help is greatly appreciated
Thanks

Do you have calibrated flue gas analyser?
If not then you are off to a poor start as you need this instrument to check what the combustion is like, possibly temperature differential.

 

[Scotland007]

Will try and update a video later when on a decent wi-fi as keeps saying error.
Thanks

 

[Scotland007]

No sorry unable to upload a video as security error occurred?? Tried a few times but same result.
To be honest the more pressing matter is hot water. Strange as starts of hot temp goes up to 75 and demand is still there but after a few mins drops down to mid 40's? The demand doesn't go just temperatures drops.
If I turn off the tap wait a few seconds and turn it back on then same again, hot water about a minute then luke warm.
Many thanks

 

[Jajodu]

What's the boiler doing while hot water is running?
Is the flame symbol going out?
Sounds like a circulation issue, blocked secondary heat exchanger perhaps.

 

[ianmcd]

check the pump