I have learned from past experience that you are best to keep the system as near as the existing when replacing unless the customer states an interest in converting, then you can discuss with them the pros and cons of it.
For one the existing boiler is obviously a heat only, and is far smaller than your standard combi boiler and would not fit in the existing space, this then means resituating the boiler elsewhere, getting your feeds moved and finding somewhere to run your condensate
Like I said previously I would have been interested in putting in an indirect unvented unit but limitations regarding the PRV and also the fact that I would need to alter the pipework even then to probably accomodate the existing boiler to have the unvented to run sufficiently, because Im not sure if you could run an unvented off of convection as these systems are usually pumped and controlled with motorised valves (another addition that would be required while I'm ripping the floors up, for what seems like a convoluted solution)
I could go through all that and possibly convince the customer that it's the way to go (though I would probably be working for British gas then) and while I'm going through all this work they would probably be scratching theirs heads thinking "surely he could have just kept the system the same and just replaced the cylinder"? And when I have said "oh no you can't get the same cylinder as what's in, they don't exist anymore" that's when customer jumps onto the internet and finds one and then suspects you are ripping the a**e out of the job.
At the end of the day if you keep the system the it is, your not going to confuse abyone. They're happy because they know how to work it and also because IT WORKS. I'm not opposed to converting systems but in this case best to leave well alone. The system is so old, that pressurising the pipework could be a bad move in itself as someone stated previously
To explain in more detail, when the boiler heats the water inside it, the water expands do it's density becomes a little lower than the slightly cooler water in the pipework above. Put another way, the water at the top of the system becomes heavier than the water below so the cooler, heavier water falls down the return pipe into the boiler, and the hotter lighter water rises up the flow pipe to the cylinder heat exchanger, where it gives up its heat, cools and flows back down the return pipe to the boiler. It gets re-heated and the sequence continues until the cylinder reaches boiler temp and equilibrium is achieved. Circulation then stops.
As you can imagine, the motive forces moving the water are microscopic so large pipes are needed. 28mm copper is considered the minimum.
Thanks very much for your explanation, when you had said convection previously I was thinking that it was along those lines but then thought bloody hell it must takes ages to heat haha which would explain why it would benefit from larger pipework, thanks again for the help and explanation. Another thing learned
