313D Cheat Sheet 2026

The 30 highest-yield 313D facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. Which Manual D concept defines the path through the duct system with the greatest pressure drop? → Critical path
2. What is the purpose of applying a 'safety factor' or equipment oversizing limit in residential load calculations? → To prevent excessive short cycling while ensuring comfort
3. What does a micron gauge measure during the evacuation of an air conditioning system? → Absolute pressure in the system
4. When routing refrigerant lines through a wall or floor, the technician should: → Install a protective sleeve or grommet to prevent abrasion
5. Which document must a refrigerant reclaimation facility provide to verify that recovered refrigerant meets AHRI 700 purity standards? → Certificate of Reclaim (analysis report)
6. Which of the following is a safety control designed to shut down the compressor if the refrigerant pressure on the high side becomes dangerously high? → High-pressure switch
7. Which NEC article primarily governs the installation of air-conditioning and refrigerating equipment? → Article 440
8. Under the NEC, which article specifically addresses wiring methods and equipment for air conditioning and refrigerating equipment? → Article 440
9. What is the purpose of the crankcase heater on a compressor? → To prevent refrigerant migration and oil dilution in the crankcase during off cycles
10. During a nitrogen pressure test on a refrigerant system, the technician should use a: → Dry nitrogen cylinder with a pressure regulator set to the test pressure
11. What is the minimum slope required for a condensate drain line in a residential installation to ensure gravity drainage? → 1/4 inch per foot
12. A residential system is charged by weight per manufacturer specifications, but superheat is still too high after charging. What should the technician do next? → Check for airflow restrictions, TXV operation, or refrigerant distribution issues
13. What is the correct method for charging a new R-410A system that was evacuated and requires a full charge per manufacturer nameplate? → Charge liquid refrigerant through the high-side service port with the system off
14. When a schematic shows a relay coil labeled 'IFR' (Indoor Fan Relay) with its associated contacts shown elsewhere in the diagram, the contacts will be labeled: → IFR with a number suffix to indicate which contact set they belong to
15. In troubleshooting with a schematic, if voltage is present on both sides of a normally closed safety switch, what does this indicate? → The switch has opened (tripped) and is blocking current flow to the load
16. What is the primary purpose of adding superheat to the refrigerant vapor before it leaves the evaporator? → To ensure only vapor enters the compressor, preventing damage.
17. What indoor relative humidity range does ASHRAE recommend for occupant comfort in residential buildings? → 30–60%
18. A new installation has the outdoor unit located 60 feet below the indoor unit. What additional installation step is required for the refrigerant piping? → Install oil traps in the suction line riser every 20 feet
19. Which refrigerant cylinder color code is used to identify R-410A? → Rose/pink
20. What is the function of a filter-drier in a refrigerant circuit? → Remove moisture and contaminants from the refrigerant
21. What does a high-pressure switch do in the electrical circuit of a residential air conditioner? → Opens the control circuit if refrigerant pressure exceeds a safe limit
22. Why is it critical to keep moisture out of recovered R-410A refrigerant? → Moisture reacts with POE oil to form acids that damage compressors
23. What is subcooling in a refrigeration system? → Chilling the refrigerant liquid below its condensing saturation temperature
24. A homeowner reports that their new AC system is making a gurgling noise from the indoor coil. What is the MOST likely cause? → Air or non-condensables in the refrigerant circuit
25. Under the 2021 International Residential Code, what is the minimum required capacity for a condensate overflow shutoff device? → The device must stop the unit before the overflow pan fills
26. Which terminal on a residential heat pump thermostat controls the reversing valve for switching between heating and cooling modes? → O or B
27. In rectangular duct design, what does the term 'aspect ratio' describe? → The ratio of the duct's longer side to its shorter side.
28. Which section of the International Mechanical Code specifies the required access opening dimensions for HVAC equipment installed in attic spaces? → IMC Section 306
29. A technician finds the outdoor unit operating but no air movement from supply registers. The indoor unit is energized. What is the most logical first check? → Indoor blower motor and capacitor
30. 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? → Pour a new, level concrete pad or install a high-impact plastic pad.