Gross and Net Efficiency

Efficiency, in heating appliances, is not about how quickly your boiler heats up your home. It's a measure of how much of the energy you've paid for is used to make your home warmer and how much is lost through the flue. Efficiency is stated as a percentage but there is a significant difference between gross and net efficiency.
For decades, gross efficiency alone was used and it makes good sense. Gross efficiency looks at the total amount of heat which can be produced by burning the fuel gas (or oil) and looks at how much of that heat the boiler can deliver inside your home; the rest is lost via the flue. Room-sealed, standard efficiency gas boilers can put about 80% of the total energy into your home and lose about 20% via the flue; their gross efficiency is about 80%. (Open-flued, standard efficiency boilers are less efficient.) Before condensing boilers, you couldn't get much past 83% gross efficiency or water would start to condense out in the heat exchanger, flooding your boiler. The 10% which was latent heat was always lost.

Then someone worked out a marketing wheeze. It said, since we can't recover the latent heat let's take it out of the calculation. We'll calculate the efficiency, discounting the latent heat, and call it net efficiency. So a boiler with a gross efficiency of 82% becomes a boiler with a net efficiency of 91%. Fantastic! That looks much better and we can now tell the public our boilers are 91% efficient. Well, where one manufacturer went, all the others had to follow or lose out commercially. So boilers were measured in net efficiency.
A while later someone developed the condensing boiler, a high efficiency boiler. Most of these have a gross efficiency of about 88% and under some circumstances might reach 91%. So, still using net efficiency, most of these are reaching over 97% and under some circumstances reach a magical 101%. Apparently no heat loss at all and even better than perfect. Amazing! In reality, high efficiency boilers are a big improvement but they still lose about 10% of the heat generated from the gas you pay for.

With gross and net efficiency, we feel that the industry should have stuck to gross. Science uses gross. Marketing uses net.
When we service boilers we choose to state the combustion efficiency as a gross efficiency figure since it shows what percentage of the gas you pay for actually produces useful heat. In the end though, as long as we see past the marketing wheeze, it doesn't matter which we use to compare boiler efficiencies as long as we stick to one or the other. We can only compare gross with gross and net with net.

If you want to know more, we wrote an article on another web site which gives more information:


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