Efficiency of old boilers

18 Mar 2007
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United Kingdom
I have an old boiler which is supposed to be 65% efficient. Can anyone tell me where the other 35% goes ?
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Yeah, unfortunately the other 35% serves to warm up your garden, (and maybe you kitchen, or whatever room the boiler is in!)
But if some of the heat lost goes to heat my kitchen then it is not really wasted. So how much of it would get lost up the flue, I have a short metal flue which connects into an internal flue.
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There is a formula to calculate flue losses,but accept the fact that the best part of 35% is being wasted.You may argue that some heat is used from the boiler giving off heat from the heat exchanger/casing etc,but in reality this is a small percentage of the total losses.
Put your hand / face near your flue / chimney and you will soon feel where it goes....

....seriously.... don't do this, but the 35% flies off to heat the sky.
I have an old boiler which is supposed to be 65% efficient. Can anyone tell me where the other 35% goes ?
I`ve got an old boiler - and with a bit o fettlin` I can get 69 :!:
How much of a difference would a service make to an old boiler's efficiency, say if it had not been done in over 10 years and was burning with more than a slight tinge of yellow
that sort of fettling - By a Registered Gas Installer - would be a good idea . There may be hidden safety issues - No jokes here :idea: And a decade without looking at is too long.
More to the point: what does it matter where what part of the loss goes? You've got it, whatever the figures are.
This is what I have pieced together, I hope it's not too wide of the mark:

Modern condensing gas boilers burn with a much richer mixture (less air for a given amount of gas) than old open-flue boilers or balanced-flue boilers. Some modern boilers even have a Lambda sensor to automatically control the mixture and use the minimum air; the less air a condensing boiler heats, the more efficient it can be.

An open flue boiler needs a lot more air in the mixture (and hotter flue gases) to reduce condensation in the chimney. Condensation had to be avoided in the old cast-iron heat exchangers, otherwise they could rust through. Old boiler casings were much warmer than modern boilers; the last balanced-flue boiler I had fitted in 1995 lost about 700W from the casing.
I'd have thought 35% heat loss is a small price to pay for a relable, maintainence free boiler, even a new condensing boiler will struggle to give better than 10% loss and the savings are soon wiped out with repairs, not to mention the near certainty of having to be replaced much sooner than an old fashioned boiler. Also 69% [ after fettlin] is surely warm enough to make anybodies toes curl up

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