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Understanding why Nett and Gross
figures are used Unfortunately, there are no
layman’s terms for explaining this. The figures used
when testing efficiencies to BS and EN standards are
in nett values. Therefore, the formulae used to
calculate efficiencies, using the test results
obtained, give us nett efficiency figures.
Furthermore, it is the nett efficiency figure that is
used by both BS and EN standards to give the appliance
its efficiency class.
However, Magiglo, along with many
manufacturers has always quoted gross input figures.
This is largely because it is more relevant to the
end-user as the calorific value (CV) quoted by gas
suppliers on a bill are usually gross figures.
Unfortunately this often leads to some confusion for
our customers. Gross and nett efficiencies are
directly related the gas input figures used; in this
case the nett CV figures are used and hence a nett
efficiency is given. For an explanation on the
difference between gross and nett CV figures, see the
notes headed Calorific Value.
It should also be noted that the
output figures are actually neither gross nor nett as
output can only be measured as a pure figure.
In order to convert between Nett
and Gross for either efficiencies or gas inputs,
please see the details below.
Natural Gas
Gross efficiency = Nett efficiency x 0.900
Nett efficiency = Gross efficiency / 0.900
Net Input = Gross Input x 0.900
Propane
Gross efficiency = Nett efficiency x 0.920
Nett efficiency = Gross efficiency / 0.900
Net Input = Gross Input x 0.920
Calculating Gross efficiency In order to calculate the gross
efficiency, use the following calculation.
Natural Gas
Gross efficiency = Max Output /
Max Input (Gross) / 0.9 x 100
Propane
Gross efficiency = Max Output /
Max Input (Gross) / 0.92 x 100
Calorific Value
The Calorific Value (CV) of a gas is
the amount of heat given out when 1 cubic metre of gas
is burnt and can be expressed in gross or nett values.
A distinction is made between:
Gross calorific value: where the
water produced by combustion is condensed and thus the
energy therein is used.
Nett calorific value: where the
water produced by combustion is assumed to be in vapour state and therefore the energy therein is not
used.
The gross CV is higher than nett
CV.
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