HVAC Cheat Sheet 2026

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

1. What is the maximum fill level (by weight) for a recovery cylinder? → 80%
2. What is the purpose of vacuuming a mini-split system before charging? → Both B and C — remove air/moisture and verify leak-free
3. What troubleshooting method uses the process of elimination by testing components from 'most likely to least likely'? → The probable-cause method
4. What does a 'chilled water coil' use instead of direct refrigerant expansion? → Cold water (typically 44-46°F) circulated from a central chiller
5. What is the purpose of a disconnect switch near an outdoor HVAC unit? → To provide a local means of de-energizing the unit for safe service work
6. Which recovery method is fastest for recovering large amounts of liquid refrigerant? → Liquid recovery first, then vapor
7. What does 'ASHRAE 90.1' establish that BAS designers must follow? → Energy efficiency standards including HVAC controls requirements for commercial buildings
8. What is the most common cause of a clogged condensate drain line? → Algae and biological growth in the drain line
9. What is the purpose of a Variable Air Volume (VAV) box in a BAS-controlled system? → To modulate airflow to individual zones to maintain zone temperature setpoints
10. What type of coil is used in an air handler for the heating mode in a heat pump system? → The same evaporator coil, which acts as a condenser during heating mode
11. What is the function of a 'hard-start kit' (potential relay and start capacitor) on an air handler? → It is not used on air handlers — only on compressors
12. What is the primary advantage of a two-stage or variable-speed gas furnace? → Improved comfort and efficiency by operating at lower capacity most of the time
13. What unit is used to measure the cooling capacity of HVAC equipment in the US? → BTU/hr or tons
14. What is the purpose of indoor coil subcooling measurement? → To determine if the refrigerant charge is correct when using the TXV charging method
15. What is a low ambient kit and when is it needed for air conditioning systems? → Controls that allow a cooling system to operate properly at low outdoor temperatures
16. What is the purpose of the condensate drain pan in an air handler? → To collect water condensed from humid air on the evaporator coil
17. In HVAC, what does the term 'throw' refer to when selecting supply air diffusers? → The horizontal distance the supply air travels before its velocity drops to 50 FPM
18. What causes a compressor to 'trip' on its internal overload protector? → Overheating of the motor windings due to excessive current or poor cooling
19. What is 'infiltration' in the context of building load calculations? → Uncontrolled air leakage into and out of a building through gaps and cracks
20. What causes higher-than-normal wet-bulb temperature at the air handler inlet? → High outdoor humidity and high indoor latent load
21. What unit is used to measure airflow volume in HVAC duct systems? → CFM (cubic feet per minute)
22. What does a high-pressure switch in a refrigeration system do? → Shuts down the compressor if refrigerant pressure exceeds a safe limit
23. What is the correct refrigerant superheat at the evaporator outlet for a TXV-equipped system? → 8-12°F (at the outlet of the coil)
24. What safety hazard does 'liquid slugging' pose to a compressor? → Hydraulic damage from incompressible liquid entering compression chambers
25. What diagnostic information does measuring 'outdoor ambient temperature' provide during refrigerant charging? → It's required for both superheat and subcooling charging charts to determine target values
26. Which federal agency regulates the sale and use of refrigerants under Section 608? → EPA
27. What is the boiling point of R-410A at atmospheric pressure? → -51.4°F (-46.3°C)
28. What is 'demand-controlled ventilation' (DCV) and what sensor drives it? → Outdoor air ventilation modulated based on actual occupancy, driven by CO2 sensors
29. What is a 'non-condensable' gas in a refrigerant system, and why is it harmful? → A gas like nitrogen or air that raises head pressure and reduces efficiency
30. What does the term 'balance point' mean in heat pump system design? → The outdoor temperature at which the heat pump output equals the building's heat loss