Can you explain how you calculate efficiency - is it for example :
Given the amount of heat ( in say joules ) that is released by the burning
a
specific quantity
of gas, find the amount of heat transfered to the water and take the
quotient ?

It is far more complicated than that. In fact, under the calculation method
used, it is even possible to get more than 100% under certain conditions.
However, it does approximate the potential absolute efficiency in
translating fuel energy into useful heat energy that may be distributed
around the system. See www.sedbuk.com for details.

Christian.