National Home Inspector Exam (NHIE) Heating and Cooling Systems Summary

The filter catches dust before it reaches the blower and coil. A clogged filter restricts airflow, makes the system run longer, can overheat a furnace, and can freeze an AC coil. Changing filters is routine maintenance that inspectors commonly recommend.

Wrong choices

• Higher airflow through the ducts: A dirty filter blocks air; it does not increase it.
• Lower electric bills: Restricted airflow makes the system work harder, often raising bills.
• Carbon monoxide in the water heater: The filter has nothing to do with the water heater.

The heat exchanger keeps burner gases separate from the air blown through the house. If it cracks, combustion gases, including carbon monoxide, can enter the living space. Suspected cracks call for evaluation by a qualified HVAC contractor.

Wrong choices

• Cause the air conditioner coil to freeze: Coil freezing is an airflow or refrigerant issue.
• Make the furnace run more quietly: Noise is not the concern.
• Increase the water pressure: The heat exchanger has nothing to do with water pressure.

Standards of practice require inspectors to operate heating and cooling systems with normal controls, such as the thermostat, and observe them. Dismantling, adjusting, or servicing equipment is outside the scope and is left to HVAC technicians.

Wrong choices

• By taking the furnace apart to see the heat exchanger: Standards do not require dismantling equipment.
• By adjusting the gas valve pressure: That is service work, not inspection.
• By adding refrigerant to test the charge: Charging refrigerant is technician work.

Condensing furnaces pull so much heat from the exhaust that water condenses out of it. The cool, wet exhaust is vented through plastic pipe (often PVC) as the maker allows, and the condensate water is drained away. Older, mid-efficiency furnaces vent hot gases through metal vents or chimneys.

Wrong choices

• A masonry chimney with no liner: Condensing furnace exhaust is cool and wet and would damage an unlined chimney.
• Single-wall metal pipe that ends in the attic: Venting into an attic is never acceptable.
• No vent at all: All fuel-burning furnaces need venting.

A heat pump is an air conditioner that can run in reverse. In heating mode, refrigerant picks up heat from outdoor air (even cold air) and releases it inside. A reversing valve switches between heating and cooling. Electric strips provide backup heat.

Wrong choices

• Using only electric strip heat at all times: Strip heat is backup; the heat pump mainly moves heat with refrigerant.
• Burning gas in the outdoor unit: Heat pumps do not burn fuel.
• Pumping hot water through the ducts: Ducts carry air, not water.

Warm, humid air passing over the cold indoor coil gives up moisture, just like a cold glass sweats. This water drips into a pan and runs out a condensate line. A clogged or disconnected line can cause water damage.

Wrong choices

• To drain refrigerant: Refrigerant stays sealed inside the system.
• To cool the compressor: The compressor is in the outdoor unit.
• To clean the air filter: The drain has nothing to do with the filter.

The AC coil sits above the furnace and makes water when cooling. If its drain pan or drain line leaks or clogs, water drips onto the furnace, causing rust and damage to controls and the heat exchanger. Inspectors report it and recommend HVAC service.

Wrong choices

• A leaking gas line: Gas leaks do not leave water stains.
• A dirty return air filter alone: A filter does not leak water by itself.
• The thermostat: The thermostat has no water.

Each gas appliance typically needs its own manual shutoff valve that is accessible and in the same room, close to the appliance (commonly within 6 feet). This lets the gas be shut off for service or emergencies without turning off the whole house.

Wrong choices

• A shutoff only at the gas meter: A shutoff near the appliance is needed in addition to the meter valve.
• A shutoff that can only be reached by cutting into the wall: Shutoffs must be accessible.
• No shutoff, so it cannot be turned off by accident: A shutoff is needed for service and emergencies.

When a duct separates in an attic or crawlspace, conditioned air blows into an unconditioned space. Rooms on that run get less heating or cooling, energy is wasted, and in cooling season the moist air can cause condensation. It should be reconnected and sealed.

Wrong choices

• It improves attic ventilation as required: Ducts are not attic vents; disconnected ducts waste energy.
• It increases furnace efficiency: It lowers system efficiency.
• It matters only in summer: It wastes energy in both seasons.

Two copper lines connect the indoor and outdoor units. The larger suction line carries cold refrigerant gas back to the compressor and should be insulated to prevent sweating and heat gain. Missing or damaged insulation is a common finding.

Wrong choices

• The smaller liquid line: The smaller liquid line is warm and usually not insulated.
• Neither line: The suction line should be insulated to prevent sweating and energy loss.
• The thermostat wire: Thermostat wire is not a refrigerant line.