The ideal test is 10 minutes from cold, but are more often tested when warm, you have non user adjustments to fire them up on high or low regardless of what the boiler would do in normal mode, it is sad to say that most engineers do not know how to test condensing boilers, when I asked the question not too long ago at an 'engineers' meet, most said they serviced in the same way as the old ones, this cant be done, and I was told by one that 'I just hoover the innards like I used to with the old ones'
, condensing boilers are more efficient when they are working harder, this is why you cannot just put a boiler in, (regular condensing, with tanks), it has to sized correctly, not oversized as happened previously. Some of these boilers have aluminium heat exchangers, but NOT condensers, the condense comes from the flue which is plastic, and runs through a plastic trap/drain, I have seen them run in copper, which is a poor poor job, worcester bosch have an aluminium heatex, but is gauranteed for 10 years providing that the water quality is correct, again, most dont have the equipment or dont know how to use it properly, and so the water in the system is left acidic/alkaline which will damage an aluminium heatex in 2 - 7 years, aluminium transfers heat better, steel holds the heat better, thats the main difference. The cost for a proper job, with the correct boiler etc isnt cheap, but with a 5 years manufacturers gaurantee, then maintenance contracts at £12.42 per month (with us anyway), it isnt overly important.