NHIE exam practice question · HVAC
The limit switch is the furnace's temperature safety. It senses the temperature in the heat exchanger area and interrupts the burner if it climbs beyond the safe range, which protects the exchanger from thermal stress and cracking and prevents an over-temperature condition reaching the ducts. What usually drives the temperature up is inadequate airflow — a clogged filter, a failed blower, closed registers or restricted returns — so a furnace that repeatedly trips on limit is reporting an airflow problem rather than a switch problem. That is why the correct response to short cycling on limit is to find the restriction, and why a bypassed or jumpered limit switch is one of the more dangerous defects an inspector can find: it removes the protection while leaving the underlying restriction in place.
The limit switch monitors temperature in the heat exchanger area and shuts the burner down when it rises beyond the safe range, protecting the exchanger and preventing over-temperature air reaching the distribution system.
Forced-air furnaces include a high-limit switch that interrupts the burner on over-temperature. Repeated limit tripping usually indicates restricted airflow from a dirty filter, blower fault, closed registers or inadequate return, and bypassing the switch is a serious safety defect.
Too Hot, Burner Off. And when it keeps happening, the switch is not the problem — the air is not moving.
Look for a jumpered or bypassed limit switch. It is a rare find but among the most dangerous, since it removes over-temperature protection entirely.
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