Condensing Furnace Lockout on an 8°F Morning
A 2009 two-story house has a 100,000 BTU/h, 92% AFUE condensing furnace in a second-floor closet, vented with 2-inch PVC intake and exhaust that pass through the rim joist and terminate side by side under a 3-foot roof overhang. Each pipe ends in a downward-facing 90-degree elbow about 14 inches above the patio slab. There are 4 inches of snow on the ground, the outdoor temperature is 8°F, and a 15 mph wind is blowing across the terminations. On a call for heat the inducer runs, the hot-surface igniter glows, the burners light and burn for roughly three seconds, then the flame drops out. The furnace retries three times and locks out; the LED flashes the code for 'pressure switch open.' The filter is new, the condensate trap is full of water, and the drain runs to a floor drain. The PVC flue crosses a cold attic where a section sags between two hangers. The owner says the furnace ran all last winter without a problem.
1. What does the sequence of events tell you about the ignition system? Rank the three most probable causes of the lockout and name the measurement that would confirm or eliminate each.
Read the worked answer
The burners light and stay lit for about three seconds, so the igniter, gas valve, flame sensor, and control board are all functioning. What fails is the proving circuit: the pressure switch made to allow ignition, then opened, which dropped the gas valve. That points at the pressure the inducer develops, not at ignition.
Marginal or blocked inducer pressure. A partially iced or debris-blocked intake termination, or flue gas recirculating from side-by-side terminations under the overhang, changes collector-box pressure once the heat exchanger warms and the plume builds. Confirm by teeing a manometer into the pressure switch hose and reading actual pressure against the switch rating on its label (commonly about -0.6 to -1.0 in. w.c. on this size of two-pipe furnace), watching for a sudden change at dropout.
Water in the flue or a blocked condensate path. The sagging attic section traps condensate and restricts the flue; a partly plugged trap or drain backs water into the collector box and loads the switch. Confirm by checking trap flow and the vent slope back to the furnace (about 1/4 inch per foot).
Failed switch or plugged sensing port/tubing (spider webs, kinked hose). Confirm by comparing measured pressure to switch rating while the inducer runs.
2. Evaluate the vent terminations and the attic vent run. What requirements apply, and why do they matter in these weather conditions?
Read the worked answer
Three problems compound. First, height: terminations 14 inches above a patio slab are close to grade and below the expected snow line; the manufacturer's installation instructions, which the mechanical code makes enforceable, typically require the exhaust to be at least 12 inches above grade plus anticipated snow accumulation. Drifting snow can bury or partially block a downward-facing elbow.
Second, recirculation: both pipes terminate side by side at the same elevation, directly under a roof overhang, in a crosswind. The installation manual normally specifies a vertical or horizontal separation between intake and exhaust (often exhaust above intake) and warns against locations where the plume can recirculate. Recirculated flue gas dilutes combustion air and disturbs the flame, which is exactly the pattern seen here - a flame that lights and then drops out. The sub-freezing temperature and wind make the plume heavier and more likely to hang under the overhang.
Third, slope: PVC vent pipe must slope back toward the furnace, roughly 1/4 inch per foot, so condensate returns to the trap. The sag in the attic creates a low point that collects water, freezes, and restricts the flue. Any one of these can cause the pressure switch to drop; together they explain why the furnace worked in milder weather and fails now.
3. Write the inspection report language for this furnace. What should you state as fact, and what should you refuse to do?
Read the worked answer
Report what was observed and tested, not a diagnosis. Useful wording: 'Furnace did not remain operating. On a call for heat the burners lit and extinguished after approximately three seconds on three consecutive attempts, after which the control locked out on a pressure-switch fault. Vent terminations are side by side under a roof overhang approximately 14 inches above the patio, below the current snow line. The PVC flue has a low spot in the attic with no slope back to the furnace. These conditions are consistent with flue gas recirculation, a blocked intake, or trapped condensate, but the cause was not determined.' Then recommend evaluation and repair by a licensed HVAC contractor before further use, and specifically request combustion analysis (CO in the flue and in the supply air), pressure-switch testing with a manometer, and verification of the terminations against the installation manual.
Do not jumper or bypass the pressure switch, do not repeatedly reset and retry the furnace, do not dismantle the vent, and do not tell the client the furnace 'works but needs service.' A furnace that locks out on a safety control is a CO and no-heat hazard until a qualified technician clears it.