Invented cylinder system. How do I know how much hot water is left inside and what temperature?

[BlueLoo]

Shall I increase bypass valve to .4 or .5?

Doesn't work like that mate.
You can set them a number of ways, some by calculation, but really, mostly by feel. (Ie heat).

If you don't know what it was set too originally, most plumbers start at 0.2 bar and work from there.

 

[Jonny Vee]

Doesn't work like that mate.
You can set them a number of ways, some by calculation, but really, mostly by feel. (Ie heat).

If you don't know what it was set too originally, most plumbers start at 0.2 bar and work from there.

It was at the maximum I well remember it. I rang him down and told me no worries it is the expansion valve and put it to the minimum. We did not get any noise for the rest of season but this here noise is back when heating goes on and off.

 

[BlueLoo]

It was at the maximum I well remember it. I rang him down and told me no worries it is the expansion valve and put it to the minimum. We did not get any noise for the rest of season but this here noise is back when heating goes on and off.

It isn't an expansion valve. CH systems don't usually have them. (They use a tank).
Your plumber is talking out of his arse. Which i find odd because i thought his pipework and layout to be quite good in the photo.

My thoughts are that you set this to 0.2 and that should be ok without digging deeper into its correct setting setup (easy enough but you need to be able to understand the system).
Setting it at 0.2 will also stop your bypass from being mostly open which would explain the low heat on the tank.

All depends on the pump though. Can you post a picture of the pump?
And the boiler, what boiler do you have?