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Energy bills have been increasing. There are still ways to reduce this.

Boiler Energy Efficiency


This article is about how to make your boiler more efficient. It could be as simple as turning a few knobs or clicking a few buttons. While this article focuses mostly on the boiler, you cannot have an efficient boiler by itself does not translate to an efficient system. So we will touch on the efficiency of outside components in separate articles. The parent article will be system efficiency.

A domestic boiler is required to carry an energy label as required by European legislation. This labelling is generally in the format of an simple A-G or similar scale. However due to changes the scale is undergoing changes. The latest scale is going to be a simple colour coded A-G.

Boiler Efficiency basics

Efficiency in its most basic state is simply saying of every £1 you spend how much £s worth of heat you get out. ie a old permanent pilot light boiler that is ‘G Rated’ gives 50p of heat for every £1 you put in. Therefore it’s efficiency is 50%. Whereas an ‘A Rated boiler gives approx 90p of heat for every £1 you put in. The main difference is the technology used. In older boilers more heat gets wasted out of the flue (straight to the sky), whereas in newer boilers less heat is wasted through the flue and put into your house instead.

Boiler Power and Flow temperature

Boilers are usually sized in terms of KW(metric) of output or sometimes older way in terms of BTUs(imperial) . This is known as the power of the boiler. In Combination boilers the power level of the boiler applies to the Hot Water, whereas the heating power is usually lower ie a 36kw combination boiler will usually output 36kw in Hot Water mode, but 30kw in heating mode. This is usually in the specification.

Flow temperature is the temperature of the central heating water that comes out of the boiler, and is regulated by flow temperature setting. The higher the flow temp the higher the output of the radiators.

The power is comparable to engine size of a car, and the flow temp is equivalent to the car speed.

Boiler Sizing

Sizing a boiler correctly is one of the keys to efficiency and reliability. Oversizing will cause you to have a more expensive boiler with higher long term maintenance costs, whereas a very undersized boiler will not be able to cope with system demands. It’s usually wiser to oversize than to oversize, however things are not so simple. To correctly size a boiler, you need to work out the heatloss of the property and also factor in the ability for you to make improvements down the line. The heatloss in KW will decide your heating requirements. Then if you have a combination boiler the flowrate and temperature required for hot water requirements will decide the hot water KW. For example you may need only 12kw on the heating but 31kw on the hot water.

Modulating boilers

Modern ‘A rated’ Boilers have the ability for the heating requirements to be adjusted both manually and automatically ie 31kw combination boiler provides 24kw of heating. This can be adjusted down to 12 kw. Also to note, regular/system boilers are also able to be adjusted ie a 30kw system boiler can be adjusted down to 12kw.

Finally…how to optimise the boiler

Armed with above knowledge. You can adjust your boiler power maximum to output only maxiumum what is required and also adjust your flow temperature such that your radiators heat output is lower. What this will do may feel like your system is working not as well, however you will end up with a more energy saving efficient and reliable system. Your heating system will take longer to warm up, but your home will warm up more evenly and give your thermostat(s) enough time to sense the temperature. This ultimately will decrease your energy bills.

Advanced optimisation

The most efficient setup would be to use modulating controls ie OpenTherm controls with a modulating boiler ie OpenTherm capable boiler. In this instance the room thermostats act as sensors and tell the boiler exactly what flow temperature to operate at and the boiler will also modulate it’s power level down to what is required.

Boiler Specific Optismation

Following is a list of boiler models/ ranges and how to optimise / adjust them. Please note this information is provided for knowledge and would normally be carried out by heating engineers. We cannot accept responsibility should a fault or problem arise as a result of these adjustments. You make these adjustments at your own risk.

Vaillant Ecotec Optimisation

Additional Resources

Gross and Net Efficiency

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