Energy saving: debunking exaggerated claims

There are people out there trying to sell all sorts of energy-saving solutions to hotels. Some work; others make exaggerated claims and some are plain fraudulent. One of the most powerful weapons against dishonest vendors is the simple scientific concept of an “energy balance”, the well-known principle that all the energy you put into something comes out in some form.

A hotel’s heating system provides the perfect example. When you use fuel in the boilers, you convert the fuel’s chemical energy into heat. That heat has three principal routes out: by conduction through the walls, roof, windows, etc.; in the air leaving the building through ventilation (including leaks from the building fabric) and in the boiler exhaust gases going up the chimney. The sum of thermal energy leaving in the boiler exhaust, convection and conduction equals the chemical energy input as fuel.

U, the useful heat output of the boilers, is set by ventilation loss V and conduction loss W. F, the fuel energy input, is the sum of U, S (heat in exhaust gases) and a small constant loss from the boiler casing, C. If heat transfer is improved, S falls relative to U.

Suppose someone tries to sell you a boiler-water additive or an air-removal system which improves heat transfer in the heating system. They will probably argue that part of the benefit derives from improved heat transfer in the radiators or other heat emitters, but even if true, that is not relevant: if their heat output is increased they will run for shorter periods or at a lower temperature, depending on their mode of control. The building’s demand for heat is what it is: in the diagram, U is locked equal to V+W.

Where improved heat transfer will undoubtedly be beneficial is in the boilers, where it would enable more heat from the combustion process to get into the circulating water. However, bear in mind that the building’s heat requirements would have been unaffected. So improved heat transfer means that the same useful boiler output (balancing the building’s unchanged heat demand) can be maintained with less fuel input. As a result – again on the basis of an energy balance – the amount of heat going up the chimney when the boiler is firing will be reduced, and this will be detectable as a reduction in exhaust temperature. In fact reduced exhaust temperature will be the only directly-measurable effect.

The measurement of boiler exhaust temperature is a standard routine method for testing and quantifying boiler efficiency and it is possible to work out what temperature drop corresponds to a given improvement in efficiency. It turns out that to you need to have started with an exhaust temperature over 650°C to achieve the 30% improvement that some of the dodgier products claim. That’s dull red heat for steel, by the way. Try this on-line calculator which allows you to compare two boiler test results: set everything the same for cases A and B apart from the exhaust temperatures. For sensible before and after temperatures you will conclude that the maximum plausible saving is of the order of 6-7% .

And if someone tries to sell you something that claims to improve heat transfer in your heating system, insist on an independent before-and-after test of boiler exhaust temperature to verify the effect.