I like to look at boiler efficiency in two ways;
Firstly you have your running costs, how much fuel does it use to heat up your home and/or the hot water?
Secondly is the reliability of the machine, will it need expensive repairs and how long will it last before it needs replacing?
This two things are intrinsically linked and surprisingly easy to get right, it's a bit concerning that so many people still get it so bloody wrong. While the fuel usage part can be reduced with decent controls (see controls blog), better home insulation and better glazing the selection of the appropriate boiler for the property is absolutely crucial.
It is so often the case that the selection of the boiler especially in new build and renovation has been done by someone who has very little to no knowledge of what they are specifying or why other than the difference between a combination boiler or a heat only one. All too often in medium to large houses the builder or architect stipulates the largest heat output boiler they can find. This seems like an ok idea on the surface right? Plenty of heat and no risk of it being under powered. Unfortunately if no consideration is given to the actual KW requirements of the property and a boiler is over sized by even a bit there are pretty bad consequences.
So all boilers have minimum and maximum outputs, it is referred to as the modulation "ratio" (although ratio isn't the right word as they don't necessarily modulate in equal increments, but I'm being pedantic). A more accurate description is "minimum turn down" which for most boilers is between a quarter and a tenth of its maximum output. Condensing boilers run best at low temperatures, ideally we want the return temperature to the boiler from the house to be under 55°c as that is the dew point at which condensing starts (or stops, whichever way you look at it).
So back to the hugely oversized boiler, if a house has a demand heat loss of say 20kw (that would be a gigantic house by the way, 5 beds or more with bad insulation) and a 40kw boiler has been fitted with a minimum turn down of 10kw then as soon as the weather isn't that cold or a couple of radiators are turned off then the required amount of heat will drop below the minimum output of the boiler. It then starts overheating and cycling, this makes the boiler run hot and cold in quick succession burning far too much gas and reducing the life span of the boiler hugely (dramatically, I can't stress this enough). Add to that the inevitable crappy on/off controls and you'll see a boiler go into melt-down within five years.
Now that example was fairly kind to the builder (sorry I shouldn't just blame builders, proper professional heating engineers do this just as much and some builders don't) as it was a huge house with high requirements you can see that once it's a normal sized 2-4 bed with only 12kw demand that the boiler would cycle like it was chasing a yellow jersey!
So that's one aspect of efficiency, fuel economy is hugely to do with the selection of the boiler. The second part of reliability is what I consider the lifetime efficiency. If you have a boiler that is twenty percent more efficient than average but lasts for fifty percent of the time then it's clear to see you've backed a donkey.
Reliability as I said earlier can be hugely improved by using the right controls both for the machine and for the property, I'll go into this in much more detail in another post. Although this helps a lot there are other ways to increase your chance of having the same boiler in 10-15 years and that simply comes down to selection. There are many cheap and nasty boilers out there, there are tonnes of expensive nasty ones too and vice-versa. Don't be fooled by advertising, I'm sure you wouldn't be surprised if I told you that the market leaders in the U.K spend vastly more on advertising than the others but their product average at best.
The most simple way to maximise the efficiency of your condensing boiler and reduce fuel use is just turn the flow temperature as low as is comfortable for you. This needs to change on a daily basis as the outside conditions change. Modern weather compensation controls automate this and essentially vary the flow temperature depending on the outside (and often inside) temperature. Because the sensor is outside it can tell the affect the weather conditions will have on the internal temperature before it does. The controller and boiler can then reactively prevent fluctuation of the temperature and not only is it cheaper to run but the comfort levels are more predictable.
If we set our flow temperature below 55°c we will ensure that the return temperature is below that and will be below "dew point" so it will condense and hit its efficiency potential.
A higher flow temperature will often be needed unless the property is very well insulated. We might have a minus outside temperature or the property could have been empty for a while and need to heat up rapidly.
Generally speaking, low and slow is best for condensing boilers. Even if the controls are very basic you can manipulate the boiler and controller by simply turning the boiler down and increasing your running times.
I almost forgot to talk about the actual efficiency ratings, oops...
Boilers in the U.K are rated on the net efficiency of the boiler when in heating mode (this includes when it's a system boiler heating stored hot water) but they don't take into account the hot water efficiency of combination boilers. There are plenty of boilers on the list that don't condense at all in hot water, not a bit
The 0.3% difference in efficiency in heating means they can get placed high but if it's the combi version and you take into account the hot water production then the list is completely different.