Where on a residential heating system is a "PTO" installed?

The correct choice indicates that a "PTO," which stands for "Pressure Transducer Output," is installed between the furnace and the main trunk supply in a residential heating system. This location is significant because the PTO monitors the pressure within the system, providing critical feedback regarding the performance of the furnace and the overall heating efficiency.

By placing the PTO in this position, it can accurately assess the pressure differential before the air enters the main trunk supply, which is essential for regulating airflow and ensuring that the heating system operates within safe and optimal parameters. This monitoring helps identify potential issues, such as clogs or inefficiencies, early on, allowing for timely maintenance and adjustments.

The other locations mentioned do not fulfill the same function as effectively. For instance, placing the PTO at the furnace outlet wouldn't provide a comprehensive overview of the system's performance, and installing it near the thermostat would not accurately reflect the pressures in the ductwork. Similarly, positioning it at the condenser unit pertains more to cooling systems rather than residential heating systems. Thus, the chosen location of the PTO is critical for monitoring the heat distribution effectively.