New boiler will consume capital, old boiler consumes revenue. Inefficient boilers are financially more attractive than buying a short-lived new one with all the additional expense involved - unless of course the old one has already thrown its hand in.
And you won't be able to warm your boots on a well insulated "efficient" one
I would expect to achieve a net efficiency of around 90% on a Eurostar, as long as someone has not marmalised the baffles previously. If new baffles are required at any point, they may be difficult to obtain.
A 'steamer' would give around a 97% net efficiency, so as the yanks would say, "Do the Math!"
I hear where you're coming from, and at the current low low price of heating oil I expect a new one isn't justified, but I wondered if a 25 year old one is running at something silly like , say, 60%?
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