Gas furnaces are measured in British Thermal Units (BTUs). BTU is the amount of heat required to raise one pound of water one degree farhrenheit. So if we have a 80% efficiency 100,000 btu furnace, we really only get about 80,000 btus supplied to the home, the other 20,000 btus of heat are going out the home. This is heat energy that we pay for and is going out the home not ever to heat up the living space only the chimney as it exits the home. The higher efficient furnaces will recapture thе heat frоm water vapor bеfоrе thе vapor iѕ emitted оut thе flue/exhaust pipe. This is achieved via the secondary heat exchanger. This in turn is more efficient using all the heat possible which will save money in the years to come.