On Sep 15, 6:43*am, Michael B wrote:
When I observed this, here in Louisville, high efficiency furnaces
were extremely expensive. The guy had run supply and delivery
duct from the bedrooms, they only got heat when the other unit
was running, and indeed, they stayed warm too, giving indication
of the extent of heat to be recovered with an old unit.
If I had been doing it, my efforts towards heat recovery would
have been on a water heater, which typically has more loss
because of its design. I've seen some interesting arrangements
with vehicle exhaust tubing and computer fans.
Here, the January average temperature is 30 degrees, with infrequent
excursions into the single digits.

On Sep 13, 9:31*am, wrote:

On Sep 13, 8:12*am, Phil Again wrote:

On Sat, 13 Sep 2008 04:25:59 -0700, Michael B wrote:

.... and the exhaust from the first was directed to
the fire box of the one above. That one sent its output to a couple of
bedrooms rather than having 'dueling fans'.
Indeed, that second one recovered considerable heat from the first one,
and the exhaust from it was still warm enough to maintain good venting.

*Michael:

Do I presume correctly the second furnace was not connected to the gas
line, and no fire in the firebox? *The only heat in the 2nd firebox came
from the exhaust of the first?

Wasn't there a problem with chimney back-drafting in winter? *Or did this
happen down South where you don't get temperatures below 10 degrees F?

I was left with those questions and others too. *It sounds like the
second furnace was just being used as a heat exchanger to recover some
additonal heat from the exhaust of the first furnace. * I was also
confused by the ending statement: "Rather than modifying the existing
unit, you might consider alternatives. " * Is he actually suggesting
this double furnace arrangement as a sound alternative? * And if so,
I'd say that while not modifying the existing furnace itself, he is
making a huge change in the overall heating system.

It doesn't sound very practical to me. * In addition to having to
somehow rig in a second furnace, you also have to consider that the
second furnace, which presumably is acting as a heat exchanger only,
has it's own blower. * How much does it cost in electricity to run
that versus the amount of extra heat being recovered? * Then, factor
in that to gain that additional heat, you are pulling air through
ducts from the conditioned living space, heating it only slightly,
then sending it back. * With typical duct work, you might very well
give back much of the energy being gained due to heat loss as it's
being moved around.

It would seem to me that from any practical sense, if you want to get
more heat out of a gas furnace, just buy a high efficiency one.

By the way, the existing furnace was a hot water system unit, built
during the days that it was more expensive to meter the gas than
for the actual gas. My, things have changed.