313D Psychrometrics and Humidity Control 2

Question 1

What does the Sensible Heat Ratio (SHR) of a cooling system represent?

Accepted Answer

The ratio of sensible heat removal to total heat removal (sensible plus latent)

Answer

SHR = sensible load ÷ (sensible load + latent load), indicating what fraction of total cooling capacity handles temperature reduction versus dehumidification.

Question 2

What happens to relative humidity when air temperature increases while the moisture content stays the same?

Accepted Answer

Relative humidity decreases because warmer air can hold more moisture

Answer

As temperature rises, the moisture-holding capacity of air increases, so with unchanged absolute moisture content the relative humidity (a ratio) decreases.

Question 3

What is the primary function of a humidistat in a residential HVAC system?

Accepted Answer

To sense indoor relative humidity and signal the system to humidify or dehumidify

Answer

A humidistat detects relative humidity and cycles humidification or dehumidification equipment to maintain the set humidity level, functioning like a thermostat but for moisture.

Question 4

How many grains of moisture are in one pound of water?

Accepted Answer

7,000 grains

Answer

There are exactly 7,000 grains per pound of water; HVAC technicians express air moisture content in grains per pound (GPP) of dry air.

Question 5

Under what condition does condensation form on interior window surfaces?

Accepted Answer

When the window surface temperature drops below the dew point of the indoor air

Answer

Condensation forms on any surface whose temperature falls below the dew point of the surrounding air, regardless of a single fixed RH percentage.

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

313D Psychrometrics and Humidity Control 2