Depending, exhaust temp might not be _that_ high. For one thing,
depends on sensitivity of your digits.
For many, 150F is considered limit for "sustained" contact. For a flue
pipe, that's pretty cold. To recover much more heat from the exhaust,
you'd better be prepared to deal with condensate, corrosion probs, etc.
Were I you, I'd suggest obsessing on all the many ways your house and
its occupants waste energy. Get a good clock thermostat, and see just
how much setback you can handle. Electric blankets work great, a/r, I'm