How do you measure efficiency for a heating system?

A central furnace’s efficiency is measured by annual fuel utilization efficiency (AFUE). AFUE is a measure of how efficient the furnace is in converting the energy in its fuel to heat over the course of a typical year.

Choosing a higher efficiency will increase the cost of your project, but will save you more on energy bills and may qualify you for a tax credit or a utility company rebate.
Choosing a lower efficiency will reduce the cost of your project, but will save you less on energy bills.
We recommend High efficiency (90 to 95%), since in our opinion it provides a good trade-off between costs and savings, but the decision is yours.
The following estimates* for an average U.S. home may also help you decide:

Heating efficiency	Savings on gas costs	Lifetime savings
Regular: under 90%	–	–
High: 90 to 95%	8%	$1,614
Top: 95.5%	11%	$2,235
Top+: 96%	12%	$2,433
Top++: over 96%	13%	$2,817