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 and 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, even the most efficient room-sealed boilers couldn’t get much past 83% gross efficiency. The lost 17% of the heat was required to ensure that the flue gases stayed at over 100°C. This was necessary to prevent the water vapour produced by combustion from condensing back to liquid water and flooding the combustion chamber.

Part of the lost 17% (about 10% of the total heat produced by combustion) was being used solely to keep the water from combustion as steam, as opposed to liquid water. This is called latent heat. The engineers knew that, if they could cool the flue gases and allow the water to condense back from the gas state to the liquid state, they could save that latent heat. The problem was, they couldn’t work out how to do that without flooding the boilers.

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. So boilers were measured in net efficiency.

Eventually, someone worked out how to prevent the condensed water from damaging the boiler. That gave us condensing boilers, high efficiency boilers. 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 prefer 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 whether we use gross or net 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.

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