High-Limit Cycling in a Closet With a Quiet Blower Tap
An 80,000 BTU/h, 80% AFUE upflow furnace in a closet with a louvered door runs four to five minutes, then the main burner shuts off while the blower keeps running. About three minutes later the burner relights and the cycle repeats. The high limit is a 160°F automatic-reset disc on the plenum. A technician measures 148°F supply air and 68°F return air at the plenum; the nameplate calls for a 35 to 65°F temperature rise. Static pressure readings are minus 0.85 in. w.c. on the return and plus 0.32 in. w.c. on the supply. The filter is new, the evaporator coil is clean, and the blower is on the low-speed tap because the installer wanted a quieter system. A 3-ton coil sits in the same closet. No diagnostic code is stored, the limit resets every time, and the customer describes the system as "short cycling but it always comes back on."
1. Trace the sequence of operation in this furnace and explain precisely why the burner drops out while the blower keeps running.
Read the worked answer
The thermostat calls for heat on W, the control starts the induced-draft blower, and the pressure switch proves once the inducer pulls the correct draft. The hot surface igniter is energized, the gas valve opens after the warm-up period, and the control proves flame through the remote sensor. After the blower-on delay, typically 30 to 90 seconds, the blower motor starts. From that point the plenum temperature climbs. When the plenum reaches the limit's setpoint, the limit disc opens and breaks the 24-volt circuit in series with the gas valve, so the burner shuts off. The blower does not stop, because the fan switch is still closed on a hot plenum and its off-delay keeps the motor running. As the blower removes heat, the plenum cools to the limit's reset differential, the disc closes, the gas valve is re-energized, and the whole cycle starts again. This is not thermostat short cycling. The high limit is a safety device that is being forced to act as an operating control, which means the furnace is overheating on every cycle.
2. Use the measured numbers to identify the most likely cause and state what else you would check.
Read the worked answer
The temperature rise is 148 minus 68, or 80°F, against a 35 to 65°F nameplate range. That is about 23% above the top of the range, which by itself tells you airflow is roughly 20 to 25% low for the firing rate. The static readings confirm it: minus 0.85 in. w.c. on the return plus 0.32 on the supply gives about 1.17 in. w.c. total external static, far above what this blower is rated for, and the return is carrying most of that penalty. The low-speed tap compounds the problem by moving less air than the medium or high tap the manufacturer lists for this combination. I would compare the measured static and rise against the blower performance table, then look for a restricted or undersized return, an undersized return grille, a closet louver with too little free area, or a coil and cabinet mismatch. The door must be on and the coil clean while measuring, and the reading should be taken only after the unit reaches steady state.
3. What correction would you make and how would you verify it? Include what you must not do.
Read the worked answer
Set the blower to the tap the manufacturer's table calls for with this cooling coil and heating input, and correct the return-side restriction, which may mean adding return capacity, enlarging a grille, or restoring free area at the closet louver. Then re-measure. The goal is a temperature rise inside 35 to 65°F and a total external static within the blower table, with the burner running continuously and the closet door in place. What you must not do is jump out the limit, replace it with a higher-temperature disc, or tape the limit closed, since any of those removes the only device protecting the heat exchanger from overtemperature. Because this unit has cycled on its limit repeatedly, inspect the heat exchanger for distortion and cracks and confirm CO air-free is acceptable before signing off. Document the before-and-after rise and static, the tap change, and the return correction on the invoice.