313D System Installation and Startup 3

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?

Accepted Answer

18 AWG

Answer

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?

Accepted Answer

Moisture or a refrigerant leak is present in the system

Answer

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?

Accepted Answer

Measure static pressure drop across the coil and compare to manufacturer specifications

Answer

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?

Accepted Answer

Remove refrigerant — system is overcharged

Answer

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?

Accepted Answer

Wire insulation temperature rating

Answer

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.

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

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

313D System Installation and Startup 3