3 ms·

This Old House on PBS routinely installs systems over 95%. There are systems here listed at 98.5 AFUE https://www.energystar.gov/products/most_efficient/furna

• londons_explore 5y ago
  Fossil fuel systems 'cheat' a little in efficiently metrics by using the 'Lower Heating Value' of burning the fuel. That's the amount of energy the fuel contains if you assume that the resulting water produced in combustion is released as water vapor. However in real boilers, there is no need for this - by condensing that water vapor back into liquid water you can get more energy out. That's the 'Higher heating value'. When measuring efficiency, we always use the lower heating value, whereas we ought to use the higher heating value. Therefore a "98% efficient furnace" is actually more like 88% efficient when considering the higher heating value.