All of the following are examples of ways to improve energy efficiency of heating systems, except

a. closing off rooms that aren’t in use.
b. frequently cleaning the furnace filter.
c. fixing cracks in the walls and floor.
d. opening the front door in winter to filter and clean warm air.

D. Opening the front door in winter to filter and clean warm ai

Explanation:

All of the following are examples of ways to improve energy efficiency of heating systems, except .

A. Closing off rooms that aren’t in use.

B. Frequently cleaning the furnace filter.

C. Fixing cracks in the walls and floor.

D. Opening the front door in winter to filter and clean warm air

for an heating systems it is better to keep the front door shut so that all the heat energy does not escape outside.

when heat energy is within a closed system ,it can still be utilized to keep the inhabitants warm otherwise there will be an exchange with the cold from winter fall outside the air, and the heating system's efficiency will be reduced