Ok problem you seem to have is that some of the sludge,scale and crap has found its way to the heat exchanger......
Funnily enough my dads was the same last year, it took me 2 power flushes to solve it.....
Second time round is when i bought the pump attachment for my machine and flushed it directly from the boilers pump....this shifted quite a few lumps of crud (yes i was sad enough to put the drain water through a filter to see what came out)
Yes 5 rads can be done in half a day but i would still allow a whole day because i dont just turn up, power flush, take money and tip my hat goodbye....(I dont base my prices on hourly rate, i have set prices with a minimum price no matter how many rads you have....reason for this is chemicals, the price i paid for my machine, my experience and just because......which is why i would be with you all day to solve your problem)
I dont diagnose a power flush unless i am pretty certain this will solve their problems......
I dont know your plumber but if i am 100% sure that the system is dirty enough to be causing the problem my customer has then i always offer to do a second power flush free of charge except for the chemicals which i ask them to pay for...
So far i have only ever had to do this once and that was a micro bore system....
The intermittent side of things are circumstansial....if the crud sits in a place where it doesnt block some of the heat exchanger then all is well, if it moves to a place where it partialy blocks it then it will possibly kick the overheat stat in
Although with my dads what would happen is the crud would block the exchanger and cause a noise like a jet engine (he described it) and then loud banging noises through the boiler..
The exchanger would get blocked under the right conditions and cause the heat exchanger to be starved of water causing said exchanger to flash steam and cause the banging.....
Hope some of this dribble helps you....
everything you have said makes perfect sense and rings completely true. thanks for that. the guy who is performing the flush certainly seems to know what he is on about and he has tested everything else first.He has told me if I had a new combi that a power flush should be done just like what you have said, so I know that he isn't trying to get me to have it for the sake of it.
From everything I have read, its seems that a power flush is worth a try. In terms of hours on the job, when I have spoke to him, he has made it clear that he wants to do it properly and not just fit the job in.
Its funny what you say about the well known gas supplier - slightly off topic - a friend at work says that they have had a leaflet through saying they are now offering interest free credit on a new boiler with fitting, I doubt the leaflet explains that they charge 'x' amount more in the first place.