I just came across an interesting article that quantifies some of the
results from switching to a variable speed furnace blower motor.
https://www03.cmhc-schl.gc.ca/b2c/b2c/init.do?language=en&shop=Z01EN&areaID 00000045&productID 000000450000000014
The paper is called "Effects of ECM Furnace Motors on Electricity and
Gas Use" and is put out by the "Canadian Centre for Housing Technology".
Some interesting points:
--Gas consumption actually goes up slightly due to the fact that you
don't have the "waste" electrical head from the motor helping warm the
air. By the same token, you save some money cooling because you have
less unwanted motor heat.
--Net savings are highest in a well-sealed and insulated house.
--Based on coal-fired electricity, net reductions in GHG emissions due
to an ECPM motor are over 900 kg of CO2 a year in all cases (over 1,800
kg of CO2 a year for houses with air conditioning).
--For houses which currently do _not_
run the fan continuously, most
could improve air quality through circulation with an ECPM motor with no
increase in utility bills. (But also no substantial decrease.)