I've been pondering the above topic in regards to my circa 20 y/o Camray oil burner. I assume boiler efficiency is calculated as calorific value of fuel burned versus how much of that actually ends up as hot water. Googling suggests my boiler is probably only at about the 70% mark.
But is that really such a big deal depending on where the lost heat goes? My boiler chucks out enough waste heat to keep the utility room warm (it has no radiator) and even the toilet and shower room leading off that doesn't get too cold to sit at the throne if I leave the door open although I open up the TRV in there for half an hour before having a shower in this weather.
So really the only truly wasted heat in winter time is that which goes out of the flue. I concede that in summer if I'm just heating the immersion tank then waste heat is more of an issue although a relatively minor one as not much oil is being burned.
So how much of the "wasre" heat of a low efficiency boiler like mine goes out of the flue and how much goes into the house anyway to contribute to general heating?