HVAC Troubleshooting 3 — Questions and Answers
Question 1: A technician finds high suction pressure and high discharge pressure with low superheat on an R-410A system. What is the most likely problem?
- Refrigerant overcharge (Correct answer)
- Restricted metering device
- Refrigerant undercharge
- Failed compressor
Correct answer: Refrigerant overcharge
Both elevated suction and discharge pressures combined with low superheat are classic signs of an overcharged refrigerant system.
Question 2: An oil furnace fails to fire and the reset button on the primary control trips repeatedly. What should be checked first?
- Thermostat calibration
- Fuel supply and oil filter condition (Correct answer)
- Flue pipe connections
- Heat exchanger cracks
Correct answer: Fuel supply and oil filter condition
A clogged oil filter or empty fuel tank is the most common reason for repeated primary control lockout, as the burner cannot establish flame without adequate fuel.
Question 3: A packaged rooftop unit produces insufficient cooling. Subcooling is 5°F and superheat is 18°F. What does this suggest?
- Refrigerant overcharge
- Refrigerant undercharge (Correct answer)
- Faulty condenser fan motor
- Dirty condenser coil
Correct answer: Refrigerant undercharge
Low subcooling combined with elevated superheat indicates a refrigerant undercharge, reducing system capacity and efficiency.
Question 4: A gas furnace produces a yellow, flickering flame. What is the most common cause?
- Improper gas pressure
- Insufficient primary air to the burner (Correct answer)
- Cracked heat exchanger
- Faulty ignition transformer
Correct answer: Insufficient primary air to the burner
Insufficient primary air mixing with gas causes incomplete combustion, producing a yellow, sooty, or flickering flame instead of a clean blue flame.
Question 5: After replacing a compressor, the system has high head pressure and high amp draw. The technician suspects what?
- Non-condensables (air) in the system (Correct answer)
- Low refrigerant charge
- Overcharged oil
- Faulty expansion valve
Correct answer: Non-condensables (air) in the system
Non-condensable gases like air raise head pressure significantly because they cannot condense, adding to the pressure burden on the compressor.
Question 6: A split system AC cools well but the homeowner notices water dripping from the indoor unit. What is the most likely cause?
- Refrigerant overcharge
- Clogged condensate drain (Correct answer)
- Faulty TXV
- Undersized ductwork
Correct answer: Clogged condensate drain
A clogged condensate drain prevents water from leaving the drain pan, causing it to overflow and drip from the air handler.
Question 7: A heat pump compressor is noisy and the amp draw is high. Suction and discharge pressures are nearly equal. What has likely failed?
- Reversing valve
- Compressor valve plates (Correct answer)
- Crankcase heater
- Filter drier
Correct answer: Compressor valve plates
Worn or broken compressor valve plates allow refrigerant to bypass, equalizing suction and discharge pressures and increasing amperage as the compressor struggles.
HVAC Troubleshooting 3
A technician finds high suction pressure and high discharge pressure with low superheat on an R-410A system.
What is the most likely problem?