boiler losing pressure - ch pipes not leaking.

[dan_the_plumber]

Can you loose pressure through an unvented coil from a sealed CH system? I always thought the pressure on the main side of the cylinder being 3bar would if anything over pressurise the CH system an discharge via the PRV as the CH system pressure would only be around 1-1.5 as opposed the 3-3.5 in the cylinder? I'm genuinely curious as I would like to know if I'm wrong no discredit to other people's suggestions

Yeah it's a fair shout. But what your talking about there is static pressure. As soon as you open a tap now you've dynamic pressure, which is less by about half.
So when you open a hot tap you may see the boiler pressure (heating circuit) fall and then when the hot tap is closed you may see the boiler pressure rise

 

[SavageW]

Your reasoning is correct.

However, it wasn't clear from the original post whether the system was vented or unvented and it's easy to miss such details when they are buried in a thread of more than 30 posts.

I weren't meaning to sound rude,I was more worried I didn't know something that more experienced guys did. It was the OP's pic with the cylinder an the potable water expansion vessel with the PRV terminating into some sort of plant pot that made me assume it was unvented.

 

[SavageW]

Yeah it's a fair shout. But what your talking about there is static pressure. As soon as you open a tap now you've dynamic pressure, which is less by about half.
So when you open a hot tap you may see the boiler pressure (heating circuit) fall and then when the hot tap is closed you may see the boiler pressure rise

cheers Dan, I wouldn't of even thought of that. I'm new to this an still gaining experience but thanks for explaining