313D System Installation and Startup 3 — Questions and Answers
Question 1: When sizing a thermostat wire for a new residential split system requiring a common (C) wire, what minimum gauge is recommended for runs up to 100 feet?
- 20 AWG
- 18 AWG (Correct answer)
- 16 AWG
- 14 AWG
Correct answer: 18 AWG
18 AWG thermostat wire is the industry standard minimum for residential control wiring up to 100 feet, providing adequate conductivity for 24V control signals.
Question 2: After a new system installation, a technician performs a standing vacuum test and observes the micron gauge rise from 300 to 1,500 microns over 15 minutes with valves closed. What does this indicate?
- The system is properly dehydrated and leak-free
- Moisture or a refrigerant leak is present in the system (Correct answer)
- The vacuum pump is functioning correctly
- Nitrogen from the pressure test is still off-gassing
Correct answer: Moisture or a refrigerant leak is present in the system
A rising micron level during a standing vacuum test indicates either a leak allowing air/moisture in, or residual moisture boiling off inside the system.
Question 3: What is the correct procedure for verifying proper airflow across an evaporator coil during system commissioning?
- Measure static pressure drop across the coil and compare to manufacturer specifications (Correct answer)
- Check that the blower wheel spins freely by hand
- Verify supply register temperatures feel cold
- Confirm the filter is less than 3 months old
Correct answer: Measure static pressure drop across the coil and compare to manufacturer specifications
Measuring the static pressure drop across the evaporator coil and comparing it to the manufacturer's chart confirms whether actual airflow matches the design CFM.
Question 4: During startup of a new R-410A system, the technician finds subcooling is 25°F and superheat is 4°F. What adjustment is indicated?
- Add refrigerant — subcooling too low
- Remove refrigerant — system is overcharged (Correct answer)
- Increase blower speed to raise superheat
- Replace the TXV — it is hunting
Correct answer: Remove refrigerant — system is overcharged
High subcooling (target ~10–15°F) and very low superheat together indicate an overcharged system; refrigerant should be removed.
Question 5: A residential air handler is installed in an attic with an ambient temperature of 130°F on a summer day. Which wiring component requires special attention during installation?
- Thermostat wire color coding
- Wire insulation temperature rating (Correct answer)
- Conduit fill percentage
- Connector torque spec
Correct answer: Wire insulation temperature rating
Standard NM cable is rated for 60°C (140°F) ambient; in extreme attic heat, the installer must verify wire insulation is rated for the actual ambient temperature.
Question 6: What is the minimum slope required for a condensate drain line in a residential installation to ensure gravity drainage?
- 1/8 inch per foot
- 1/4 inch per foot (Correct answer)
- 1/2 inch per foot
- 1 inch per foot
Correct answer: 1/4 inch per foot
A slope of 1/4 inch per foot (2%) is the minimum required by most codes and manufacturers to ensure self-draining condensate lines without clogs.
Question 7: Which startup procedure step is performed LAST before switching a new system to cooling mode for the first time?
- Verify crankcase heater operation
- Open refrigerant service valves fully (Correct answer)
- Confirm electrical disconnect is rated for equipment ampacity
- Check thermostat wiring connections
Correct answer: Open refrigerant service valves fully
Service valves must be verified fully open immediately before startup so refrigerant can circulate; closed valves cause immediate compressor damage from lack of refrigerant.
313D System Installation and Startup 3
When sizing a thermostat wire for a new residential split system requiring a common (C) wire, what minimum gauge is recommended for runs up to 100 feet?