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Build your own condensing boiler/furnace?

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Posted by theCase on February 24, 2007, 9:11 pm
Has anyone built their own condensing furnace/boiler?   Not the whole
boiler/furnace but the exhaust condensing part.

Yes it may be difficult, but should be a possibility of a competent do-
it-yourselfer.  I would envision something along the lines of:

1) have the current exhaust vent into some corrosion resistant heat
exchanger.  Maybe just a stainless steel section of pipe surrounded by
2) have a blower take suction from the heat exchanger and exhaust to
the outside via PVC.
3) have some kind of wash system to spray down the interior of the
stainless steel portion to remove the corrosive condensates.

Obviously much attention would be needed automating such a device and
ensuring it's safe operation, but it could be a fun(!) project.

I'm posting here as opposed to Alt.Home.Repair as this forum appears
to have a higher signal-to-noise ratio (ditto on alt.hvac, those guys
are nuts)

Attn: Nicksans.  You're a numbers guy, any idea what kind BTU's could
be recovered from the phase change from gas exhaust to liquid

Posted by nicksanspam on February 25, 2007, 9:47 am

I've often thought about surrounding a woodstove pipe with moving air to
make a condensing chimney that extracts most of the heat from the stove,
with a well-insulated firebox. How can we avoid leaks and corrosion? Make
the inner pipe pressure less than the outer pipe pressure with a high temp
draft blower at the exhaust end? This may be a good idea, since a lower
temp chimney will have less draft. Replace the inner pipe periodically?

Sounds complicated.

About 15% for wood, people say. A condensing chimney might work better with
damp wood. The ASHRAE HOF says the composition of natural gas depends on
geographical location, with 70-96% methane, which has a (non-condensing)
lower heating value of 21,495 Btu/lb and a (condensing) higher heating value
of 23,875 Btu/lb; 1 to 14% ethane, with 20,418 and 22,323 LHV and HHV; 0-4%
propane, with 19,937 and 21,609, smaller amounts of butane, pentane, and
hexane, as well as 0-2% carbon dioxide, 0-1.2% oxygen, and 0.4-17% nitrogen.
They say local gas utilities are the best source of current gas composition
data for a particular area.


Posted by Steve Shantz on February 26, 2007, 10:31 pm

God forbid I be critical of you... You are usually a great source of
boundless creativity, but IMHO, it seems to me you have screwed up big
time, promoting the use of wet wood to improve the efficiency of a
condensing fireplace exhaust system.  Wouldn't Creosote formation
become a huge problem?  My understanding of fireplaces would suggest
that wet wood and accompanying poor combustion will result in creosote
formation, which will condense in your ultra efficient system and
cause all sorts of problems and some very serious fire and disposal
hazards.  Even dry wood would be prone to creosote formation with the
very low chimney temperatures of your idea, especially at low firing
rates that are quite commonly used at night.  Would it not be better
to invest in a very efficient stove, and let the exhaust do what it
has to do?

As for gas heat, where your idea seems to have quite a bit of merit,
your calculations would suggest at best, a gain of about 10% by
condensing all of the water vapor.  Is this correct?  How about a long
coil of 3/8" SS tube inside the chimney, with water flowing inside the
tube, counter current to the exhaust?  Fin-tube would be even better,
if one could line the fins up so the gases would flow between the
fins, and not just over the edge.

One of your most loyal fans!


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