313D Fundamentals of Air Conditioning Systems 5

Question 1

What causes a compressor to 'short cycle' in a residential air conditioning system?

Accepted Answer

Oversized system or faulty thermostat causing rapid on/off cycling

Answer

Short cycling typically results from an oversized system that satisfies the thermostat setpoint too quickly or a malfunctioning thermostat, leading to excessive starts and stops.

Question 2

Which type of metering device maintains a constant evaporator outlet superheat regardless of load changes?

Accepted Answer

Thermostatic expansion valve (TXV)

Answer

A TXV uses a sensing bulb to modulate refrigerant flow and maintain a nearly constant superheat at the evaporator outlet across varying load conditions.

Question 3

A residential condensing unit's condenser fan motor is running but the compressor is not. Which safety device has most likely opened?

Accepted Answer

High-pressure switch or internal thermal overload

Answer

A high-pressure switch trips when discharge pressure exceeds the set point, or an internal thermal overload opens when the compressor windings overheat, both shutting off the compressor.

Question 4

What is the primary difference between a single-stage and a two-stage compressor in residential AC?

Accepted Answer

Two-stage operates at full and partial capacity; single-stage runs at full capacity only

Answer

A two-stage compressor can run at a lower capacity stage during mild conditions and switch to full capacity during peak demand, improving efficiency and comfort.

Question 5

What does an elevated wet-bulb temperature at the evaporator inlet indicate?

Accepted Answer

High indoor humidity or high sensible heat ratio

Answer

A higher wet-bulb temperature at the coil inlet indicates more moisture (latent heat) in the air, increasing the total load on the evaporator.

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

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

313D Fundamentals of Air Conditioning Systems 5