A homeowner's central air conditioning stops cooling on a hot July day. The thermostat is set correctly and the system is running, but the house stays warm. The most likely cause that costs nothing to fix is:
AThe compressor has failed and needs replacement
BRefrigerant has leaked out of the system
CA severely clogged air filter has caused the evaporator coil to freeze over
DThe outdoor condenser unit has overheated and shut down
A clogged filter restricts airflow across the evaporator coil. Without enough warm air flowing over it, the coil drops below freezing and ice forms, blocking airflow completely — making the system appear to have stopped cooling. The fix is free: turn the system off (or to 'fan only') to let the ice melt, replace the filter, and restart. This is one of the most common causes of apparent cooling failure and is entirely preventable with regular filter changes. Compressor failure and refrigerant leaks are more expensive problems but less common and would not be fixed by changing a filter.
Question 2 Multiple Choice
How does a heat pump provide heating, and why does it become less effective at very low outdoor temperatures?
AIt burns fuel to heat a heat exchanger, just like a gas furnace, but uses less fuel per BTU
BIt extracts heat from outdoor air using a refrigerant cycle — this becomes less effective as the temperature differential shrinks in very cold weather
CIt generates heat through electrical resistance, which becomes inefficient at low temperatures due to increased resistance
DIt recirculates indoor air through a heat exchanger, losing efficiency when the indoor-outdoor temperature difference is large
A heat pump does not generate heat — it moves heat from outside to inside using a refrigerant cycle (the reverse of air conditioning). Even cold air contains heat energy, and the refrigerant cycle can extract it. But as outdoor temperatures drop toward freezing, there is less heat available to extract and the work required to drive the cycle approaches the heat delivered — making it less efficient. Below about 35°F, most systems activate a backup resistance heater. This is the key distinction from a gas furnace, which generates heat by combustion regardless of outdoor temperature.
Question 3 True / False
Air conditioning works by generating cold air and pumping it into the living space.
TTrue
FFalse
Answer: False
Air conditioning removes heat from indoor air — it does not generate cold. The refrigerant absorbs heat from indoor air at the evaporator coil (inside), then releases that heat to the outdoors at the condenser coil (outside). 'Cold' is simply the absence of heat, and an AC system creates it indirectly by extracting heat and moving it outside. This is also why AC dehumidifies as a byproduct: condensation forms on the cold evaporator coil as moisture is removed from indoor air.
Question 4 True / False
A severely clogged air filter can cause the evaporator coil to freeze over, making the system appear to have stopped cooling even though the unit is running.
TTrue
FFalse
Answer: True
This is one of the most important practical consequences of neglected filter maintenance. The evaporator coil must have warm indoor air flowing across it to absorb heat effectively. A clogged filter drastically reduces airflow. Without enough heat input from room air, the refrigerant in the coil drops below 32°F and moisture in the air stream freezes on the coil. The ice layer further blocks airflow, compounding the problem. The fix — replacing the filter and letting the ice melt — costs nothing, but the failure looks identical to an expensive mechanical breakdown.
Question 5 Short Answer
Why is replacing the air filter the single most impactful routine maintenance task for an HVAC system?
Think about your answer, then reveal below.
Model answer: The filter sits in the return air path, and every cubic foot of air the system conditions must pass through it. A clogged filter restricts airflow throughout the entire system — the blower motor works harder (increasing electricity use), heat transfer at the coil is reduced (decreasing efficiency), components wear faster, and in extreme cases the evaporator coil freezes or the heat exchanger overheats. No other single maintenance item affects all of these simultaneously. The filter is also the cheapest consumable in the system, making the cost-to-benefit ratio of regular replacement extremely high.
The cascade of failures from a dirty filter — increased energy use, reduced efficiency, accelerated wear, coil freeze-up — illustrates why preventive maintenance is economically rational. Each downstream failure is more expensive to repair than the filter itself costs to replace.