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Replacement condensing boiler - GBP 256
(Figures from the Energy Saving Trust)

I dont see how a condensing boiler will save me the entire annual spend
on gas.

Did seem high I must admit.

A jump from 65% to 91% is 26% more efficiency. So one's annual gas
spend would have to be fairly extreme to achieve that sort of saving.
These are not realistic figures, or even close.

On those figures the underlying gas bill would be a very large £896 pa.

If Dribbles favourite figure of 55% is used the underlying gas bill
would be a slightly more reasonable £647 but still much more than my gas
bill (cast iron heat exchanger).

Go the other way of course and the figures get very very silly indeed.
76% - £1553. 80% -£2118.

Only goes to shown you shouldn't trust those who have an axe to grind.

--
Roger Chapman