Good Morning DIY Friends
Thanks again to all who responded to the initial post on this subject. Hopefully an update on the solution may help others in future...
The Problem
Having conducted two cleaning runs of the central heating system, at the third refill/flush stage the boiler would fire up, but would shut down again in less than a minute. The flow pipe from the boiler would become excessively hot with the return still relatively cool/cold.
The system is a gravity fed S-Plan Plus arrangement, with zone valves for upstairs heating, downstairs heating and hot water.
The Solution
I called in the plumber that services the boiler who believed that the issue was either an air lock or blockage in the heat exchanger. Both scenarios seemed plausible to be honest. He tried opening the internal boiler bleed, but no air or water appeared.
The system is a Honeywell S-Plan model with 3 zone valves for upstairs heating, downstairs heating and hot water. After testing a number of run states it was found that the system would run fine when upstairs heating and hot water were selected, but shut down when the downstairs heating was introduced - bizarre!
This clearly disproved the blocked heat exchanger theory, but how an air lock could exist in a system with common feed and return piping that would affect one zone only was a mystery.
However, with upstairs heating and hot water circulating at low temperature, all downstairs radiators except one were isolated with lock-shields closed. The downstairs heating was then turned on and much gurgling ensued. After much boiler fiddling with on/off and temp control, flow was established to the first radiator and the boiler held up in a stable run state. It was then a matter of introducing one radiator at a time to the downstairs circuit until all were operational and bled.
I suppose this all goes to show that no matter how long you live with a C. H. system (in my case 24 years), or how much work you have done on it in the past, it can still throw up nasty and time consuming surprises.
Phil