HVAC System Design and Operation Flashcards

Under the ASHRAE 62.1 Ventilation Rate Procedure, what is the breathing zone outdoor airflow (Vbz)?

Answer: Rp × Pz + Ra × Az

Vbz adds a per-person rate times population to a per-area rate times floor area.

What sensor is most often used for demand-controlled ventilation (DCV) in high-occupancy spaces?

Answer: Carbon dioxide (CO₂) sensor

CO₂ levels track occupancy, so outdoor air can be adjusted to match the people actually present.

Why are commercial buildings usually kept at slightly positive pressure?

Answer: To reduce unconditioned infiltration and the moisture problems it causes

Slight positive pressure limits infiltration of untreated, humid outdoor air through the envelope.

In a PID control loop, which term removes steady-state offset?

Answer: Integral

The integral term builds up error over time until the process variable reaches setpoint.

What does an optimal start control strategy do?

Answer: Changes the equipment start time based on space and outdoor conditions so occupancy setpoints are reached just in time

Optimal start learns how long recovery takes and starts the system as late as it can while still hitting setpoint by occupancy.

What is a typical design friction rate for the equal friction duct sizing method in low-pressure supply duct?

Answer: About 0.08–0.10 in. w.g. per 100 ft

Low-pressure commercial supply duct is commonly sized near 0.08–0.10 in. w.g. per 100 ft.

Which PMV range does ASHRAE Standard 55 commonly use as the acceptable thermal comfort range?

Answer: -0.5 to +0.5

PMV between -0.5 and +0.5 corresponds to roughly 10% or fewer occupants dissatisfied (PPD ≤ 10%).