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Updated 01 November 2020   Boiler Services Ltd 2009-2020

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High Efficiency Gas and Oil Boilers — what gross and net efficiency mean

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In general use the terms water vapour and steam are not accurately defined. Steam may be the scalding, invisible gas just at the spout of a boiling kettle, or the hot mist of water droplets which forms immediately beyond the invisible gas. Many of us use steam to describe the condensation of water droplets on a cold bathroom mirror and we use it to describe the cloud of water droplets coming from the flue terminal of a high efficiency boiler.

More accurately, the cloud or fog or mist that comes from the flue terminal of a high efficiency boiler (particularly on a cold day) is made up of tiny droplets of liquid water suspended in the air. These either evaporate to produce invisible water vapour or drop to the floor as slightly larger droplets.

For the purposes of this article we will use the terms water vapour and steam interchangeably  to describe water which is in a gas state before it turns into the cloud of suspended droplets in the air outside the building.

When a boiler burns gas (or oil or solid fuel) it produces hot gases which include water vapour. The gross heat energy (total heat) from the combustion gases has two components; about 90% is sensible heat (heat which can be sensed or felt) and about 10% is latent heat.

Latent heat is the energy which keeps the water vapour (produced by combustion) in a vapour state so that it passes out of the flue and doesn’t all condense inside the boiler. If you cool the flue gases enough, the water vapour condenses to liquid water.

Conventional boilers would recover as much of the sensible heat as possible, via the heat exchanger, and pass it round the heating system. They could not recover the latent heat without damaging the boiler by flooding it with water it was not designed to handle.

When gross efficiency figures were used it was hard to reach over 83% efficiency. At least 17% was lost via the flue, some as sensible heat since the flue product was still very hot, and some as latent heat.

The manufacturers decided that since they couldn’t recover the latent heat they would discount it in efficiency comparisons, so a net figure (gross input heat minus the latent heat component) was used. Using net figures (which discount latent heat) made boilers look more efficient. A boiler with a gross efficiency of 83% had a net efficiency of about 92%…

Then along came high efficiency boilers (also called condensing boilers). These boilers condense much of the water vapour back to liquid water (which they are designed to dispose of without damaging the boiler). They do this by lowering the temperature of the flue gases.

This releases some more of the sensible heat (heat which can be sensed,or felt) but it also releases the latent heat which is no longer needed to keep the water in the flue product in a vapour state. The additional bit of sensible heat and the newly released latent heat are passed into the central heating system water and used to warm the house.

Still using net figures, and now with the recovered latent heat, it was suddenly possible to have boilers producing over 100% efficiency. This sounds like nonsense because it is.

Logically, boiler manufacturers should use gross figures. They tell us exactly how much of the input energy is useful to us and how much is lost. Unfortunately this would make the boilers now look less efficient, so the manufacturers still use net figures.

For natural gas, dividing the net efficiency figure by 1.11 will give you the gross efficiency figure.

When we carry out flue gas analysis we find that most room-sealed, conventional boilers (not chimney type boilers) are about 78% - 82% gross efficient (18% - 22% of your money is wasted).

Using the same combustion gas analysers, most high efficiency boilers are about 90% gross efficient (10% of your money is wasted).

Changing from a reasonable, old, room-sealed boiler to a new high efficiency boiler will save you about 10% - 15% of the money you spend on gas. Figures claiming savings of 30% or more seem not be be physics but magic.

Old open-flued (chimney type) boilers are less efficient, sometimes much less efficient. Part of the reason for this is that they use warm, room air to dilute the flue gases going up the chimney; this is room air that we’ve already spent money heating. Changing an old, open-flue boiler will save more money, maybe 25% of what you spend on gas, and the new boiler will be an inherently safer design too.

How much money a new boiler will save depends largely on fuel prices. A 15% saving on a gas spend of 1,200 per year would save 180. If gas energy prices rise sharply, the saving with a new, high efficiency boiler would be higher too.

Still, shorter showers, slightly smaller baths and slightly cooler houses could save more.

A shower from a combi boiler (at 2018 UK gas prices) costs about 1.40 for each hour. Three people having a 15 minute shower each day will cost you 380 per year, just for the gas to heat the shower water. Reducing your shower time to 10 minutes would save a third of that; if you’re really frugal and can shower in 5 minutes you’d save two-thirds, but most of us don’t manage that. Electrically heated showers use much less water, but at 3 times the price to heat, so the cost per hour is similar. However, because of the lower flow rate with electric showers we’re not tempted to stay in them as long.

Just lowering the temperature of our homes from 22 to 21 would save us about 8% of the gas or oil or electricity we use to warm our homes, and let’s not forget insulation.

Loft insulation, cavity wall insulation, double glazing (or even triple glazing) all save energy. Loft insulation and cavity wall insulation are remarkably cheap too. Sometimes loft insulation and cavity wall insulation are available with grants in the UK which may make them free of charge to some people.

Loft insulation and cavity wall insulation may have a pay-back time as short as 3 years; if you are lucky enough to get an insulation grant which makes them free of charge, the payback is immediate!

If you want to read another explanation of technical terms, including gross and net efficiency, here’s the link:

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