So by your question and you only wanting relevant answers ,what are you after.
If you want to know the actuall savings in regards to possibly changing your boiler.
You will have to ask a far more in depth question .
As in what sort of system was in before what controls did it have on it was it a gravity system , was it fully pumped, what boiler was fitted ,where the rads changed for better output rads , has it been changed for a combi , is it used the same length of time .is the room stat set at same temp etc etc .
It is not just a straight forward question as in i now have a condensing boiler and i am saving this amount that will tell you very little on where the savings are actually being made
I was looking for a list if the parameters, not just a generic statement of the it's-much-too-complicated-for-any-one-but-an-expert-to-understand type. Please don't assume that all DIYers are thick.
A google for gas analysers told me that gas analysers measure the oxygen, carbon monoxide and carbon dioxide in the flue gases and the flue gas temperature. Some will also measure the amount of other combustion products, e.g NOx.
I found this on the Kane website and it is self explanatory
Combustion Efficiency v Boiler Efficiency
A combustion or flue gas analyser is used to measure the efficiency of the combustion process within a boiler. This is not the same as the boiler efficiency as it does not take into account, for example, the heat losses from the case of the boiler. So generally the efficiency stated on the rating plate of the boiler will always be lower than the measured efficiency of combustion.