in my opinion this is a very bad idea for too many safety reasons. if
you think you can economically and safely re-engineer the furnace
better than they already designed it for its 80% efficiency, i believe
you may be dangerously incorrect. it would make more sense to safely
replace the furnace again with a highest efficiency model with the
variable speed blower and whatever other options you desire. you will
want to do a payback analysis on your upgrade costs versus the energy
savings.
easier things to do instead: i would use insulation everywhere it may
be installed and zone thermostat controls for your ducts. and, be sure
your piped drinking water flow in winter does not require this crawl
space heat.
consult your hvac installer to explore any options of blower speeds
that may exist in the present furnace, and discuss heat anticipator
settings of zone thermostats.

peter wrote:
I have several rooms in my house that I don't have to use, so I close them
off and shut off the heater registers in those rooms to save energy.
However, this causes the remaining registers to become very windy and noisy.
I believe the reduction in air flow also increases the furnace output
temperature and therefore increases heat loss while the air is traveling in
the heating ducts in the crawlspace. Also air is lost due to leakages in the
heating ducts -- my crawlspace is warmer than outside by at least 10
degrees.

Is it generally possible to lower the heat output of a furnace? This is what
I have (replaced last year):

http://216.122.22.11/FetchDocument.a...3-559b84978136

I see three burners inside. I can either pull out one of the burner, or use
the shut-off valve to reduce the gas supply. I imagine removing one burner
would be more efficient since the remaining two burners will operate at
their designed setting.

But I'm not so sure the blower speed can be change. I don't mind rewiring
anything if I know it can be done.