Condensing
The real difficulty here is when people ask for 1 representative efficiency % when there are so many variables. The main issue comes down to the fact that the amount of condensing, and in turn efficiency, is not linear. Also, do you want daily figure? What is the out side temperature for that day? Yearly? How was the weather that year? How much was it condensing/ How big are the radiators? And so it begins. All these factors are addressed in our article 'How condensing adds efficiency'.
Another variable
Would you like to know how much efficiency condensing adds? Or, how much a condensing control strategy like weather compensation adds?
These are two different answers ofcourse. Controllers that promote condensing dont just add efficiency by increasing the condensing effect. They also can increase efficiency by lowering boiler cycling (see modulation ratio and boiler cycling),
Another issue here is that the genral conception is that a controller that promotes boiler condensing (a modulating controller) #link to what is a modulating controler# only adds efficiency by causing condensing. However this is all part of a much bigger puzzle ..
What controller are you coming from?
For example if using a toy controller this will condense much more than that boiler with basic on off control..
Which lead me on to cycle rate
Stop and starting has it's own losses. Particularly is the fan also has a post purge along with the pump.. this will chuck hot air outside
How well was their boiler able to cope with the load I.e. what's the boiler size compared to the property and how good is the modulation ratio..
What you may get from a lab test result will only reflect 1 set of circumstances from all these variables.
