Final answer:
To ensure an air conditioner works efficiently with an old furnace, it is essential to replace the furnace filter. Additionally, air conditioners are typically judged by their efficiency in transferring energy from a cold environment relative to the work output.
Step-by-step explanation:
When installing an air conditioner on an old furnace, one key step you need to consider is replacing the furnace filter. Furnace filters trap dust and other pollutants, ensuring the air circulating through your home is clean. Over time, these filters can become clogged and hinder the efficiency of both heating and cooling systems. To maintain optimal performance and air quality, these filters should be changed or cleaned regularly according to the manufacturer's instructions.
The quality by which air conditioners are often judged is their energy efficiency, specifically the amount of energy transferred by heat from a cold environment, compared with the required work output (option d). This measure reflects how effectively the air conditioner is using electricity to cool the space, which is a critical consideration for both environmental impact and operational cost.