Mechanical Systems
Heating and cooling basics
Know the difference between forced-air and hydronic systems, and the sequence of a gas furnace: thermostat calls, inducer or draft fan, ignition, gas valve, flame proving, blower. Inspect what is visible: flame color and shape, rust or scale in the burner compartment, the flue, clearance to combustibles, the filter and the condensate path. A natural gas flame should be blue and stable; a yellow, lifting or sooting flame points to incomplete combustion and warrants an HVAC technician.
Venting and combustion air
Type B vent needs 1 inch of clearance from combustibles; single-wall vent pipe needs 6 inches. Appliances in a small closet need combustion-air openings or an outside air duct, and return air cannot be drawn from the same space as the appliance. Check for backdrafting at the draft hood with a mirror, smoke or flame - spillage is a safety hazard, not a cosmetic issue.
Cooling and distribution
The suction (larger) refrigerant line should be insulated. Condensate must drain by gravity, with a trap, to an approved receptor rather than into a crawlspace. A properly charged system shows roughly a 15–20°F split between supply and return air. Disconnected, crushed or unsealed ducts in attics, garages and crawlspaces are common defects.
What you cannot see
You cannot verify a heat exchanger without dismantling the unit. Describe visible rust, flame roll-out staining, or a flame that changes when the blower starts, and recommend evaluation. Never state that a furnace "is cracked" from appearance alone; report the evidence and defer.