My fifteen year old Worcester Bosch keep locking out and losing pressure. My plumber has replaced a few parts, but I feel that it is approaching the end.
Its replacement will need to run seventeen radiators in thirteen rooms over three floors, and including two bathrooms. Firstly, what size boiler should I go for, and secondly, which make?
Firstly regardless of boiler exchange I would be looking at the
system volume if you have 17 radiators + pipework.
Even a new boiler
might need some extra help here.
If you have a sealed system boiler/combi, the internal expansion vessel is unlikely to accommodate system volumes much above 110 litres.
(Manufacturers Instructions will give maximum system volume dependant on temperature).
Volumes exceeding that figure will need an additional expansion vessel somewhere on the primary pipework.
If not, you will always be running excessive system pressure and push near to (or beyond) the pressure relief valve opening point (3 Bar).
On an older boiler it's not uncommon for expansion vessel diaphragm/schrader valve leakage.
The problem you are describing is indicative of just that issue (or a physical water leak somewhere - pipework/radiators/pressure relief leaking)
and the low pressure cut-off within the boiler is protecting the boiler (usually around 0.5 Bar).
So, test expansion vessel pressure (without water in the boiler) - usually 0.85 - 1 Bar, check for obvious system leaks, calculate system volume (> 110 litres?).
After all that - three floors/17 rads would be a great time to look at zoning the heating circuits.
My 2p's worth...
Dave.