In a properly operating system, when the furnace gets the plenum up to

temperature and the blower comes on, the correct amount of air passes

the exchanger to transfer enough heat from the exchanger to keep the

delta T in the "sweet spot" where maximum heat energy is removed from

the heat exchanger, and the minimum required heat escapes up the

stack. On variable speed blowers, the system is set up for a specific

temperature size across the heat exchanger. On my furnace it is spec'd

at between 30-60 degree F temperature rize across the heat exchanger

with 28000-40000btu output

The bigger model of this furnace is 45-75F at 89-72000 btu output.

My blower is set at about 650 CFM


There is no "sweet spot" for the heat exchanger efficiency. Setting the blower on HIGH blowing more air over the heat exchanger will always be more efficient then lass air.

It's only a sweet spot in terms of comfort and not feeling a draft in your home, not in terms of heat exchanger efficiency.