HVAC Equipment: Heating, Cooling, and Ventilation Units
Cooling and heat pump equipment
Know the difference between split systems (outdoor condensing unit plus indoor coil), packaged units (all components in one cabinet with supply and return ducts), and mini-split/VRF systems. Heat pumps add a reversing valve, defrost control, and auxiliary or emergency electric resistance heat. The balance point is the outdoor temperature where heat pump capacity equals building load; below it, supplementary heat carries the difference. Defrost terminates on time-temperature or demand logic; a stuck defrost board shows up as ice on the outdoor coil and low suction pressure.
Furnaces and combustion
Read the rating plate for input and output Btu/h and temperature rise. AFUE describes seasonal efficiency; an 80 percent furnace vents in metal flue pipe, while a 90 percent condensing furnace vents in PVC and produces condensate that must be trapped and drained. Temperature rise equals output Btu/h divided by (1.08 times cfm). Gas input derates with altitude, roughly 4 percent per 1,000 feet above sea level, which matters across Arizona elevation ranges. Combustion air, venting, and clearances are code items frequently tested.
Metering devices and coils
Fixed-bore or piston metering devices are matched to the condenser and require matching indoor/outdoor combinations; TXVs modulate to hold superheat and are the modern default. Check that the piston or TXV kit matches the refrigerant and tonnage. Airflow, charge, and duct static all interact: a dirty evaporator coil or restrictive filter drops airflow, drops suction pressure, and can freeze the coil.
Worked example
A furnace rated 80,000 Btu/h output moving 1,200 cfm: temperature rise = 80,000 / (1.08 x 1,200) = about 61.7°F. If the measured rise is 45°F, suspect excess airflow or underfiring; if it is 80°F, suspect low airflow, a dirty filter, or overfiring.