I am sorry that you find that posting by Enb54 of any use.
He is someone with plenty of time to read the manual and then to just rephrase what it says again. He has never seen one and is not qualified to work on them.
Most of the rest of us are gas qualified and WE do work on them and some of us are very familiar with your model.
Unfortunately you are not very good at answering a simple question with a simple answer without bringing in additional and sometimes confusing information.
Your boiler is a very simple piece of equipment compared with an Airbus 320.
Tony
He directly answered the question posed, I asked how does the system work, he answered logically, from a technical point of view, but in language that I understand. You just slagged him off, and didn't answer anything, like the 4 engineers employed to repair two boilers that have failed. The engineers just say Ravenheat boiler? pile of crap! If they are so rubbish, then someone must have experience of fixing them, or is the markup by rubbishing them, and fitting a new boiler for profit? Where that boiler when it breaks down after the warranty runs out another pile of crap?
If my boiler is so simple, and so obviously full of common faults, then why run a thread to 5 pages? With contrite and conflicting information, that has often been incorrect, or someone disagreeing with another, or someones 'opinion'
Seems like an electrical engineer (as myself), has the logic to decode logical fault finding guides, published by the manufacturer, but gas registered engineers guess, and swap parts ad-hoc, and slag off others that have an opinion. As has been my real world experience, as well as on-line experience.
I put your post down to over-exuberance, I just want closure on this, it's not nice to be slagged off twice in a thread, where I just ask a question.
In my business, I'd be sacked for swapping parts ad-hoc, theory random decisions, the device doesn't do xyz, it needs part a. end of fixed.
Not it needs this..oh no..thats wrong..oh it needs this then..nope..oh well this must be wrong..
BTW one of my mates designed the wings for the new Airbus..so a rather inappropriate comment..if you have nothing positive to say, then don't say it.