Which statement best describes the characteristics of a single duct constant volume HVAC system?

Which statement best describes the characteristics of a single duct constant volume HVAC system?

• A. Ducted supply, ducted or plenum return, fixed flow to each space regardless of load.
• B. Variable supply air to meet zone loads based on occupancy.
• C. Separate primary and secondary air paths with independent control.
• D. Two-stage cooling with reheat in each zone.

Answer: (A)

Single-duct constant-volume systems deliver a fixed amount of supply air to all zones from a central air handler. The key feature is that the airflow to each space is constant, not adjusted to the cooling or heating load. Zone temperatures are then controlled by devices that add or remove heat within each space (often reheat in the zones) rather than by changing the amount of air delivered. Because the supply air flow is fixed and distributed through a single duct to zones, the system can have a ducted supply with a ducted or plenum return, matching the description of fixed flow to each space regardless of load. The other options describe different arrangements: variable supply air to meet loads (typical of VAV systems), separate primary and secondary air paths with independent control (dual-duct or other multi-path configurations), or two-stage cooling with reheat in each zone (a different control strategy that isn’t the defining trait of a single-duct CV layout).

Single-duct constant-volume systems deliver a fixed amount of supply air to all zones from a central air handler. The key feature is that the airflow to each space is constant, not adjusted to the cooling or heating load. Zone temperatures are then controlled by devices that add or remove heat within each space (often reheat in the zones) rather than by changing the amount of air delivered. Because the supply air flow is fixed and distributed through a single duct to zones, the system can have a ducted supply with a ducted or plenum return, matching the description of fixed flow to each space regardless of load.

The other options describe different arrangements: variable supply air to meet loads (typical of VAV systems), separate primary and secondary air paths with independent control (dual-duct or other multi-path configurations), or two-stage cooling with reheat in each zone (a different control strategy that isn’t the defining trait of a single-duct CV layout).