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water heater flue as heat exchanger?

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Posted by taylor on December 10, 2007, 9:32 pm
Have any of you inserted a heat exchanger in the center flue of a gas
fired water heater as a method to transfer heat to the tank? I can
imagine thermal expansion issues, conductivity to be addressed, etc,
and wonder if it's been attacked. Thanks,


Posted by daestrom on December 10, 2007, 10:19 pm

Not that I've heard of.  Of course the central flue of gas heaters has a
spiral vane in it that improves the heat transfer to the flue walls.  It
'spins' the gasses as they rise and adds more surface area to collect the

If you remove that, it would lower performance and then your heat-exchanger
would have to do better than the vane did in the first place before you come
out ahead.


Posted by gary on December 11, 2007, 4:14 pm
I think that one potential problem is that if you cool the flue gas
below the boiling point of water, you will get condensation, and
regular gas heaters are not designed for the corrosive effects of the
They do sell some condensing water heaters that are designed  for


Posted by daestrom on December 11, 2007, 11:25 pm

In many a gas-fired hot-water heater, you can *hear* the condensate dripping
into the flame when it first fires up.  The cool tank walls condense water
out of the flue gasses and drips down the flue onto the burner.  Some folks
have thought they had a tank leaking when they hear this.  But a quick
review of the owners manual and the 'fine print' explains it.

This condensing of water vapor out of the flue gasses *does* provide a lot
of heat input to the water, but it raises heck with the tank.  As you say,
most are not designed for the corrosiveness of this condensate and after
seven or ten years, the 'fire tube' will corrode through and you have a


Posted by Mark Schofield on December 12, 2007, 1:33 am
 condensation of flue gasses can also harm the tiles and cement grout of
masonry chimneys.

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