Pressure Switch Proving Failure on a Condensing Furnace
A 90% AFUE, two-stage, 60,000 BTU/h condensing furnace locks out with a three-flash code that the label defines as 'pressure switch stuck open.' The inducer starts and draws 0.9 A against a 1.1 A nameplate FLA. A manometer teed at the pressure switch reads -0.62 in. w.c.; the rating plate requires -1.10 in. w.c. for low stage. The hose from the inducer housing to the switch contains water droplets, and the condensate trap is full of rust-colored water. The drain line leaves the trap, loops upward 6 in., then runs to a floor drain. The technician jumps the pressure switch, the burners light, and the furnace runs. He proposes leaving the jumper in place until the weather warms because the customer has no heat.
1. Explain the sequence of operation and why the pressure switch is not closing.
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On a call for heat, the control starts the inducer, then checks the pressure switch. The switch must close before ignition is allowed; after proving, the igniter energizes, the gas valve opens, and flame is proven. Here the switch remains open because the measured draft is only -0.62 in. w.c. against a required -1.10 in. w.c. The switch is doing its job. The water droplets in the hose and the rust-colored water in the trap point to a blocked or restricted condensate path. The upward loop in the drain line can trap water and raise pressure in the inducer housing, reducing the negative pressure seen at the switch. Other possibilities include a blocked inducer wheel, restricted flue, or heat exchanger passage, but the condensate system is the first suspect.
2. What checks and measurements would you perform before replacing the switch? Include the drain/trap and heat exchanger.
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Remove and clean the condensate trap, flush the drain, and correct the upward loop so the line slopes continuously to the drain. Inspect the pressure-switch hose for water, kinks, and cracks; blow it clear and reconnect it securely. Measure pressure at the inducer housing and at the switch with a manometer and compare with the rating plate. Check inducer amp draw and wheel condition, inspect the flue and termination for blockage, and examine the heat exchanger for restriction or cracks. If the switch receives the required -1.10 in. w.c. but does not close, replace the switch. If pressure remains low after the trap and flue are clear, continue troubleshooting the inducer and heat exchanger rather than replacing parts blindly.
3. Explain the hazard of the jumper and the correct repair/verification.
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Jumpering the pressure switch bypasses a safety that proves combustion air and flue flow. If the flue or heat exchanger is blocked, the furnace can produce carbon monoxide and distribute it into the occupied space. The correct repair is to remove the jumper, clear the condensate trap and drain, correct the drain slope, clear the pressure-switch hose, and verify the required pressure at the switch. Then check ignition, flame signal, combustion analysis, CO in the supply air, and limit operation. Confirm the inducer and flue are clear and the heat exchanger is not compromised. If parts are unavailable, arrange safe temporary heat rather than leaving a safety bypassed.