HVAC Systems and Controls Flashcards

An air-side economizer in fixed dry-bulb mode should use outdoor air for free cooling when:

Answer: Outdoor air temperature is below the changeover setpoint and cooling is needed

Dry-bulb economizers open outdoor dampers when outdoor air is cool enough to offset the cooling load.

What is the primary purpose of a freezestat on an air handling unit?

Answer: To shut down the fan or close outdoor dampers when air entering the coil nears freezing

A freezestat, typically set near 38°F, protects water coils from freeze damage.

An enthalpy economizer differs from a dry-bulb economizer because it considers:

Answer: Both temperature and moisture content of the air

Enthalpy is total heat content, which accounts for sensible and latent heat.

In a VAV system, the duct static pressure sensor that controls the supply fan is typically located:

Answer: About two-thirds of the way down the longest main supply duct

Placing the sensor downstream ensures adequate pressure reaches the far terminal units.

A VAV box with hot water reheat typically opens its reheat valve when:

Answer: The zone needs heat and airflow is at its minimum setting

Reheat is used after the box has throttled to minimum airflow and the zone is still too cool.

The minimum outdoor air damper setting on an air handler exists primarily to:

Answer: Provide required ventilation air for occupants

Minimum outdoor air satisfies ventilation codes such as ASHRAE 62.1.

Most commercial buildings are controlled to a slight positive pressure in order to:

Answer: Reduce infiltration of unconditioned outdoor air

Slight positive pressure limits drafts, moisture, and dust infiltration through the envelope.