[Stoves] Water heating fuel efficiency formula

Crispin Pemberton-Pigott crispinpigott at gmail.com
Thu Oct 3 11:20:43 MDT 2013

Dear Ron and Andrew and Lanny

RON >This is the first time I can recall seeing the number 2.3 MJ/kilo.
This must be associated with some initial moisture?  

It has to be the heat loss for a 'typical' exhaust temperature.

The HHV-1.32 MJ/kg is often given as the LHV value though it is not really
calculated correctly. The correct 'common' or 'average' value would be
HHV-1.38 but there is not use protesting it because the other method is so

2.3 would include some heat in gases above 100C. The HVAC and boiler
industry in the US uses 150 C as the minimum useable exhaust temperature so
the 1.32 does not apply in that case.

An  'efficiency' is a ratio. There are many efficiencies in a stove.

The system efficiency is perhaps the best description for the work
done/energy theoretically available.

Another efficiency is take (the work done/(energy theoretically available x
combustion efficiency)) where combustion efficiency really means the % of
heat that was not lost to unburned gases.

Yet another is the potential heat x the combustion efficiency considering
also the mechanical losses to arrive at the actual heat available from the
fire. This can be used to determine the actual firepower for space heating
and cooking considerations. If the heat gained by the pot including the pot
material and what is inside it is divided by that number, you get the heat
transfer efficiency. That can be useful for tuning the stove's performance.

If you want a measure of fuel efficiency, and to be able to predict the fuel
consumption in future, you want the system efficiency which is also the
easiest to calculate. That tells you how efficiently the fuel resource is
being applied to the 'work' of cooking.


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