How to calculate financial and carbon payback after insulating

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Assuming the condensing boiler is in condensing mode all the time then the likely improved efficiency is going to be from an original value of around 65 to 70% to and improved efficiency of about 95%.

So the calculation for a specific property is simply a percentage calculation.

If the annual fuel bill for the boiler is say £1,000 and we assume the boiler is 65% efficient then the bill would be, if the boiler were 100% efficient – 1,000 × 65/100 = £650.

So if we assume the new boiler is 95% efficient in condensing mode then the revised annual bill would be 650/0.95 = £684.00

An annual saving of £316.00.

There will also be a further saving if the combi boiler is replacing an indirect storage cylinder – the saving would be based on heat lost from the cylinder which in itself would be dependent on the thickness of insulation. (See thermal insulation).

So if the cost of the supply and installation of the new boiler was £3,160 then this would be a 10 year financial payback.

To determine carbon dioxide savings:

The saving of £316 then may be converted back to kWh from the fuel bill simply for gas and electricity.

This is not quite so straightforward for oil because this is normally charged by the litre so assuming that l litre of oil provides 11.9 kWh per litre the kWh may then be calculated.

The conversion factors of energy to carbon dioxide emissions are as follows (source: Carbon Trust and Defra):

Energy conversion to carbon dioxide (gross calorific value basis).

Example: 1,000 kWh of gas will produce 0.185 × 1,000 = 185kg of carbon dioxide.


This article is provided by Ambthair Services

We provide air conditioning design and consultancy, specialising in studios and low energy systems.

Contact us »

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