Boiler Efficiency question...

[Brother D]

Looking round my apartment complex coming home and the amount of heat being wasted out from all the boilers is staggering. Thinking about it the gas/fluid exchanger; you heat a gas, this heats water, this is then piped round, and then this heats the internal air space (and raditates) via radiators.

Would it be completely impractical to have a gas heat exchanger around the flue to capture this waste heat? I.e. concentric pipes going around the flue to prevent any possiblity of exhaust gases entering the room, with a simple fan blowing internal air across it and back into the internal space during winter times? I mean this is how mobile heating fan units work anyway.

Just a thought please shoot down on obvious things I have overlooked.

Edited to add

Humm (Damn it, just remembered on a balanced flue the fresh air is drawn on the outer pipe and exhaust goes out the middle), so you would need an intake pipe and a seperate exhaust flue (with heat exchanger).

Edited by Brother D on Tuesday 7th December 18:25

[herewego]

I think you have just invented the condenser boiler and will shortly receive the nobbly prize for services to the planetarium.

[Brother D]

Humm, not quite, I'm talking about the hot exhaust gasses from a condensing boiler...

[cpas]

I think they need to retain a certain amount of heat to escape from the flue. If you exchanged too much it would sort of stall the escape of gasses.

[Gingerbread Man]

Google 'flue gas heat recovery'

[fido]

Not to mention the heat in the hot water that goes to drainage, and it goes on and on ...

[herewego]

Brother D said:

Humm, not quite, I'm talking about the hot exhaust gasses from a condensing boiler...

A 20kW condensing boiler will be losing about 2kW through the exhaust. You could blow the exhaust through an additional heat exchanger to extract some of this heat but the exhaust contains some acids so the exchanger would need to be made of an acid resisting material which is quite expensive. It will also need to be drained in the same way as the condensing exchanger because further condensing will occur in there. I think the exhaust temperature will be quite low so the heat would not be useful to the central heating system but could be a separate heater like having a fan heater mounted over the boiler.
Whether you could do this an a DIY add-on I don’t know, it depends if you can get a suitable stainless steel exchanger.
Maybe a manufacturer could be persuaded to increase the overall efficiency of a boiler in this way given the right incentive.