As described on my page that is dedicated to Broag Avanta Remeha repairs, they are a bit different.
http://www.boiler-breakdown-repair-london.co.uk/broag-avanta-remeha/broag-avanta-remeha-repairs.html
One of the more difficult jobs I have done in the past few years, and easily the hardest one this year, was not so long ago in south east London on the Ladywell and Brockley border. For starters, it was not a consistent fault, but one of those “sometimes, sometimes not” cases. That usually means that when I am there, it works fine and about 3 minutes after I come home I get a call that it stopped again.
The only thing you can do in a situation like that, is start at the beginning and work your way through to the end. It is a bit of a long winded process, but it tends to bring the solution for awkward problems.
First step is to see if everything is connected properly with the right kind of pipe and no obstructions anywhere. This particular Avanta Remeha was the combi version, and the biggest model at that. Nothing is guaranteed in this life, but in general it is more likely that the installer cut some corners when you find the smallest models of a range. This one seemed to confirm the rule, and it all looked pretty decent. The boiler sat nice and square on the wall, flue was correctly sealed where it went through the wall, gaspipe was sufficiently large and all the pipework was straight and neatly soldered. We all know you should judge a book by the cover, but if it all looks like good workmanship, it usually is.
The next steps are to check if the electrical supply is correct and the system clean. It proved to be the case too here.
With all that verified, and the controls with a combi being all inside the boiler apart from the roomthermostat, it is now proven that the boiler itself is at fault. These boilers, like virtually all combis, have a pressure gauge that tells you if there is enough water in the system. When you run the central heating for a few minutes, the rising temperature causes the pressure to go up which in turn proves the gauge is working. So far, so good; now turn the boiler to maximum output, and a quick test on the meter to see who much gas is used. This is the easy check to see that the boiler is running at the rate it was designed to work at. By doing this, you can also tell whether or not the gasmeter is functioning correctly.
With all the “rough” work out of the way, it is time for the more intricate parts of boiler repair diagnosis. Time to get the flue gas analyser out and to get a grip on the CO2 and CO levels at minimum and maximum output. They had only drifted a tiny little bit since the service last year, so this was unlikely to be the problem. After they were both re-tuned to exactly what the book says, the fault still persisted as I expected. By removing, measuring and verifying as much as I could, it turned out that there were 4 parts that were not quite as they should be, but not really broken either. Luckily, the parts for Avanta Remehas are hugely expensive, so it wasn’t a financial disaster to get the boiler repaired.