313D System Installation and Startup Questions and Answers

Question 1

During a new split system installation, the technician notes the liquid line is 3/8" and the suction line is 7/8". The line set length is 50 feet. The manufacturer's instructions state the outdoor unit comes pre-charged for a 15-foot line set. What is the most critical next step for the technician regarding the refrigerant charge?

Accepted Answer

Add a calculated amount of refrigerant based on the additional line set length and diameter.

Answer

Most outdoor units are pre-charged for a standard length, often 15 feet. When the line set exceeds this length, additional refrigerant must be added. The amount is calculated based on the manufacturer's specification (e.g., ounces per additional foot) for the specific liquid line diameter. Failing to add the correct amount will result in an undercharged system, leading to poor performance and potential equipment damage.

Question 2

A technician is performing a deep vacuum evacuation on a newly installed residential air conditioning system. After achieving a vacuum of 400 microns, the technician isolates the system from the vacuum pump. The micron gauge reading slowly rises and then stabilizes at 1,500 microns. What does this indicate?

Accepted Answer

The system is leak-free but still contains moisture.

Answer

When a deep vacuum is pulled, moisture in the system begins to boil and vaporize. If the system is isolated and the vacuum level rises but then holds steady (e.g., between 1,000 and 2,000 microns), it signifies that the remaining trapped moisture has finished vaporizing, raising the pressure. A continuous, unending rise in pressure would indicate a leak.

Question 3

Which of the following is a critical reason for installing a P-trap at the base of a long vertical suction line riser in a split-system air conditioner installation?

Accepted Answer

To ensure proper oil return to the compressor.

Answer

During operation, a small amount of compressor oil circulates with the refrigerant. In a vertical suction line, gravity can work against the flow of this oil back to the compressor. A P-trap at the base of the riser collects oil, which is then pushed up the riser in slugs by the refrigerant vapor velocity, ensuring the compressor remains properly lubricated. Lack of proper oil return can lead to compressor failure.

Question 4

During the commissioning of a new residential AC system with a TXV metering device, which charging method provides the most accurate results and is recommended by most manufacturers?

Accepted Answer

Subcooling method, measured at the liquid line service valve.

Answer

For systems equipped with a Thermostatic Expansion Valve (TXV), the subcooling method is the manufacturer-recommended procedure for accurate charging. The TXV's job is to maintain a constant superheat, so trying to charge by superheat will be inaccurate. Subcooling measures the amount of liquid refrigerant backed up in the condenser, which directly indicates whether the system has the correct refrigerant volume.

Question 5

A technician is installing a new condensing unit on an unlevel gravel bed. What is the proper first step to ensure long-term reliability of the unit?

Accepted Answer

Pour a new, level concrete pad or install a high-impact plastic pad.

Answer

Condensing units must be installed on a solid, level surface to ensure proper compressor lubrication, fan operation, and cabinet drainage. An unlevel surface can cause premature compressor and fan motor bearing failure. The best practice is to install a dedicated, level equipment pad made of concrete or a durable, weather-resistant plastic.

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

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

313D System Installation and Startup Questions and Answers