National Home Inspector Exam (NHIE) · Home Inspection (All Domains)
National Home Inspector Exam (NHIE): Heating and Cooling Systems Concepts
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Overview
This chapter covers the equipment that heats and cools the house: furnaces, boilers, heat pumps, and central air conditioners, along with their venting and ducts. The inspector operates the system with normal controls, looks at what is visible, and reports conditions that affect safety or comfort.
Fuel-burning equipment needs enough air to burn cleanly and a vent that carries exhaust gases outside. Under common model codes, a natural-draft vent connector typically rises at least 1/4 inch per foot toward the chimney, and single-wall metal connectors are kept about 6 inches from combustibles. Backdrafting, rust, scorching near the burner, or a flame that jumps when the blower starts can point to a cracked heat exchanger and carbon monoxide risk.
Air conditioners and heat pumps move heat using refrigerant. In cooling mode, the larger, insulated line is the cool suction line and the smaller line is the warm liquid line. Cooling makes water, so condensate must drain properly, and units above finished space typically need a secondary drain or a pan with a shutoff switch.
Key concepts
Combustion air
Fuel-burning appliances need a steady supply of air to burn fuel completely. A furnace closed into a small room without enough air can produce carbon monoxide and spill exhaust into the house.
Memory tipSmall closet, no air openings = possible combustion air problem.
Vent connector slope and clearance
Under common model codes, a natural-draft vent connector typically slopes upward at least 1/4 inch per foot toward the chimney or vent. Single-wall metal connectors are typically kept at least 6 inches from combustible material.
Memory tipExhaust must always travel uphill to the chimney.
Source: International Residential Code, Fuel Gas chapter (ICC) · as of 2021
Heat exchanger
The metal chamber inside a furnace that keeps burning gases separate from house air. A crack can let exhaust mix with the air you breathe. Clues include rust, soot, a flame that wavers when the blower starts, and high carbon monoxide.
Memory tipFlame dancing when the blower turns on = refer to an HVAC technician.
Refrigerant lines
Split air conditioners and heat pumps have two copper lines. In cooling mode, the larger suction line carries cool vapor and should be insulated; the smaller liquid line carries warm liquid. Missing or damaged insulation is a common finding.
Memory tipBig line cool and wrapped; small line warm and bare.
Condensate disposal
Cooling equipment produces water that must drain to an approved place. Under common model codes, units where overflow could damage finished areas typically need a secondary drain, a pan, or a float switch that shuts the unit off.
Memory tipAttic air handler? Look for the pan and the backup drain.
Source: International Residential Code (ICC) · as of 2021
Temperature split
The difference between return air and supply air while cooling. A split of roughly 14 to 22 degrees F is a common rule of thumb for normal operation; readings far outside it suggest a service call.
Memory tipToo small a split means it is not cooling well.
Often tested
- Carbon monoxide and venting issues are safety items; report them first.
- Check vent connectors for slope, clearance, rust, and loose joints.
- Look for condensate leaks, missing pans, and clogged drains.
- Operate equipment with normal controls only; do not force it in extreme weather.
- Recommend service by a licensed HVAC technician for suspected defects.
3 sample questions
Verified practice questions from this unit.
Question 1. A dirty furnace filter most commonly causes:
- ① Lower electric bills
- ② Carbon monoxide in the water heater
- ③ Higher airflow through the ducts
- ④ Reduced airflow and lower efficiency
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Answer: ④ Reduced airflow and lower efficiency
Key point: Dirty filters reduce airflow and efficiency
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.
Question 2. A cracked heat exchanger in a gas furnace is a concern mainly because it can:
- ① Let combustion gases, including carbon monoxide, mix with the house air
- ② Increase the water pressure
- ③ Cause the air conditioner coil to freeze
- ④ Make the furnace run more quietly
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Answer: ① Let combustion gases, including carbon monoxide, mix with the house air
Key point: Cracked heat exchanger = carbon monoxide risk
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.
Question 3. During a typical home inspection, how does the inspector check the heating and cooling equipment?
- ① By operating it with normal controls, such as the thermostat
- ② By adding refrigerant to test the charge
- ③ By adjusting the gas valve pressure
- ④ By taking the furnace apart to see the heat exchanger
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Answer: ① By operating it with normal controls, such as the thermostat
Key point: HVAC is tested with normal operating controls
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.