I would guess every boiler has a sweet point where it is at most efficient. This may be when throttled back, or flat out, or anywhere between, so like driving a car, if we know that sweet point we could set the building management computer to keep the boiler at it's sweet point. Maybe starting to heat a room slightly earlier or latter, or slower or quicker to keep the boiler as long as it can at that sweet point.
Same as when we drive a car, there is a speed at which on that incline it does best MPG.
However most of us don't have building management so we can't keep the boiler on that sweet point. So just like the car with the different driving conditions with Urban etc. So the boiler will preform better or worse depending on conditions under which it is used, so using the eBUS or using the return water temperature must affect the boilers efficiency. Also using a boiler which is too big or too small.
If I was to get the years gas bill for this house, it should show how much power is required, and the more years then the more accurate the result, but simple fact that it has never failed to heat the home must show it is too big. I will guess that is in order to get enough domestic hot water? However the two big hot water uses both use electric to heat the water, dish washer and washing machine.
But gas has according to Boyle's Law and Charles' Law has both heat and volume so to get 100% power from the gas the expansion of the gas must be used, as with power stations where first turbine is gas, then next two or three are steam, even then they don't capture all the power.
I am told you can get boilers with Sterling engines that turn the last bits of power into electric and use the feed back tariff. I would hope these are the most efficient or there would be little point doing it. This must mean to get maximum efficiency there must be some sort of engine, I am told mini generators with the waste, well not really waste as it is used, but that heat is used to heat a block of flats.
I saw in Mount Pleasant on the Falklands, the heat from the generators not only heated to accommodation but also warmed the incoming water to get it warm enough so the chemicals in the water treatment plant would work, I note at home shower from gas works A1 in the winter, but in the summer it can go cold, then hot again, reason is the shower head is not allowing enough flow, as warmer input water means thermostat using more cold and less hot water, so heater closes down, have to open hot tap a little to maintain flow.
I am sure the kettle boils faster in the summer.
So if all rooms can be maintained at 22°C all year round the boiler is too big, we should expect to have to allow some rooms to cool in order to have enough energy to maintain rooms used at 22°C. Or supplement heating with a gas or electric fire.
Like a car with Urban and combined etc with different MPG, a boiler would need to have minimum, half throttled back, and maximum efficiency figures before they really mean anything.
