Checking the Charge Quiz

Ten questions on superheat, subcooling and verifying a refrigerant charge. Pick an answer to see whether you are right and why. Related reading: The Code: charging.

1. On a system with a fixed orifice (piston) metering device, which method is normally used to check the charge?

   1. Subcooling
   2. Superheat
   3. Amp draw only
   4. Sight glass

   A fixed orifice cannot adjust, so superheat changes with the charge. Charge to the target superheat from the manufacturer's chart or a target superheat calculator.

2. On a system with a TXV, which method is normally used?

   1. Superheat
   2. Subcooling
   3. Suction pressure only
   4. Discharge temperature

   A TXV holds superheat steady on its own, so superheat no longer reflects the charge. Charge to the subcooling on the data plate or in the installation instructions.

3. Target superheat for a fixed orifice system is based on:

   1. Indoor wet bulb and outdoor dry bulb temperature
   2. Outdoor wet bulb and indoor dry bulb temperature
   3. Supply air temperature only
   4. The compressor's amp draw

   Indoor wet bulb reflects the load on the evaporator and outdoor dry bulb reflects the condensing conditions. Together they set the target superheat.

4. Before adjusting the refrigerant charge, what should you confirm first?

   1. Correct indoor airflow and clean coils
   2. The thermostat brand
   3. That the outdoor unit is level
   4. The age of the system

   Low airflow or dirty coils change pressures and temperatures and make a correctly charged system look wrong. Fix airflow first, then check the charge.

5. Roughly how long should a system run before charge readings are reliable?

   1. 30 seconds
   2. 2 minutes
   3. 10 to 15 minutes
   4. 2 hours

   Pressures and temperatures need time to settle under a steady load. Readings taken right after startup are misleading.

6. High superheat and low subcooling on a TXV system most likely point to:

   1. Overcharge
   2. Undercharge
   3. Too much indoor airflow
   4. A dirty condenser

   With too little refrigerant the evaporator is starved, so superheat rises, and little liquid is stacked in the condenser, so subcooling falls.

7. High subcooling and high head pressure on a TXV system with a clean condenser and normal airflow most likely point to:

   1. Undercharge
   2. Overcharge
   3. Low indoor airflow
   4. A weak run capacitor

   Extra refrigerant stacks up as liquid in the condenser, which raises both subcooling and head pressure.

8. Where should the line temperature for superheat be measured?

   1. On the liquid line near the condenser
   2. On the suction line, close to the service valve where the suction pressure is read
   3. On the discharge line
   4. In the return air

   Measure the line temperature close to where the pressure is taken, with a clamp or probe that is insulated from the outdoor air.

9. When a system has been evacuated and is being charged from empty, the most accurate method is to:

   1. Charge by suction pressure
   2. Weigh in the factory charge plus the line-set adjustment, then verify
   3. Charge until the suction line sweats
   4. Charge until the compressor amps match the nameplate

   A scale gives the exact amount. Superheat or subcooling then confirms it once the system is running.

10. Superheat well below target on a fixed orifice system with normal airflow usually means:

    1. Undercharge
    2. Overcharge
    3. A restricted filter drier
    4. A very high indoor wet bulb

    Too much refrigerant floods the evaporator, so the vapor leaving it barely warms above saturation. Very low superheat risks liquid reaching the compressor.