Mechanical Systems
Mechanical Systems
The Mechanical domain covers the equipment that heats, cools, moves water, and vents combustion gases. Questions often test whether you can distinguish a maintenance item from a safety defect, and whether you know the sequence for safe operation. You are not required to calculate system capacity, but you must recognize improper installation, venting, and drainage.
Core rules and concepts
- Combustion safety: a natural-draft furnace or water heater needs a clear flue, proper draft, and combustion air. Test for backdraft at the draft hood with a mirror or smoke; do not rely on a flame alone. Vent connectors should slope upward toward the chimney and be mechanically fastened.
- Gas piping: look for a shutoff valve at each appliance, a sediment trap, a proper union, and no gas odor. Leak testing with a combustible-gas detector is a supplement, not a substitute for soap bubbles.
- Cooling: the condensate drain must slope to a receptor, be trapped when connected to a sewer, and not terminate in a crawlspace. The suction line should be insulated. A temperature split outside the manufacturer’s range may indicate airflow or charge problems; report the measurement, not a diagnosis.
- Water heating: the TPR valve must have a full-size discharge pipe, no cap or plug, no reduction, and termination within 6 inches of the floor or at the exterior. A drain pan is required where leakage would cause damage.
- Plumbing: drain lines need slope (typically 1/4 inch per foot for 2-inch and smaller), proper traps, and venting. An S-trap or an unvented trap is a defect. Check supply pressure and visible corrosion.
Worked example
A gas water heater has a TPR valve with a 3/4-inch discharge pipe reduced to 1/2-inch copper and capped at the bottom. The cap is a safety defect: the TPR is designed to relieve excess temperature or pressure, and a cap can prevent discharge. The reduction restricts flow and may not pass the valve’s rated capacity. Report both conditions, recommend removing the cap and restoring full-size piping, and note that the discharge must terminate within 6 inches of the floor or at the exterior.
Traps that cost marks
Operating a furnace with a strong gas smell; calling a rusty flue “cosmetic”; missing a backdrafting water heater; accepting a condensate drain that runs uphill; and reporting “refrigerant low” from a single temperature reading.