This (or direct method as per BS) is a simple method for calculating the efficiency, which gives an approximately correct value, but with a wide tolerance. It is good enough to reveal the general picture routinely in small-to-medium oil — and gas-fired boilers, which use fuels with nearly constant CV levels that are amenable to a fairly accurate flow measurement. This procedure can be misleading in case of coal firing where the fuel varies constantly in CV and density. This method is certainly not good enough for acceptance test in boilers of any meaningful size, as the procedure involves measuring the steam and fuel flows that may not be accurate enough.