Reznor Duct Furnace 2-Flash Issue
Reznor Duct Furnace Pressure Switch Lockout in NYC
This page targets a narrower Reznor duct-furnace failure than the parent furnace-repair page's broad low-heat and cycling coverage: the combustion venter is running, the burners never light, and the ignition board is flashing 2 flashes because draft never proved within the 30-second safety window. On Reznor duct furnaces and makeup-air cabinets, that usually means moisture in the sensing tube, a clogged venter pressure tap, a slowing combustion blower, or a heat exchanger problem neutralizing draft.
What We Check First
A Reznor 2-flash pressure-switch fault is not a generic no-heat call. The first question is whether the combustion venter starts and stays running while the ignition board blocks gas because the pressure switch never closes within the documented 30-second proving window.
Because Reznor duct furnaces and makeup-air units often handle cold outside air, we check the pneumatic side early: water trapped in the clear silicone sensing tube, rust scale clogging the tiny aluminum pressure-tap orifice at the venter housing, and whether the venter is actually pulling the draft the switch is calibrated to see.
Quick Answer
A Reznor duct furnace pressure-switch lockout is the documented 2-flash fault: the pressure switch did not close within 30 seconds of venter-motor start, so the board locked out ignition. On Reznor commercial duct furnaces, the real causes are usually condensate trapped in the sensing tube, a clogged venter pressure-tap orifice, a combustion blower that still spins but has lost RPM, blocked exhaust termination, or a cracked heat exchanger that lets supply-air pressure cancel the draft signal.
Common Causes
Condensate trapped in the sensing tube
Reznor duct furnaces and cold-air makeup units can condense flue moisture in winter operation. When that water settles in the clear silicone tube, it forms a plug that blocks the vacuum signal from reaching the pressure-switch diaphragm, so the switch never closes even though the venter motor is running.
Clogged venter-housing pressure tap
The aluminum pressure barb at the combustion blower housing uses a very small drilled orifice. Rust scale, soot, or corrosion can close that opening down enough that the switch sees little or no vacuum, producing the same 2-flash lockout as a bad switch.
Venter motor bearings worn enough to drop RPM
A Reznor combustion blower can still make noise and appear to run while worn bearings slow it below the speed needed to create the required negative pressure. That is why manometer testing matters: the motor may be energized but still not generating enough draft to prove the switch.
Cracked heat exchanger neutralizing draft
On older duct furnaces, a cracked or separated heat exchanger can let the main supply blower push positive air pressure into the combustion side. That counter-pressure offsets the venter's vacuum and keeps the pressure switch from proving even when the vent path itself is open.
Reznor Error Codes For This Issue
Codes below are informational — a code alone doesn't confirm the fix, and resetting power without addressing the underlying fault often just delays the problem.
2 Flashes
What it means: Verified: pressure switch did not close within 30 seconds of venter motor start.
When service is needed: Service is needed when this code repeats because the real fault may be trapped condensate, a clogged pressure-tap orifice, weak venter draft, blocked venting, a failed switch, or a heat exchanger integrity problem rather than the switch alone.
DIY-Safe Checks vs. Call for Service
DIY-Safe
- Inspect the outdoor exhaust termination for visible blockage from leaves, nests, soot, or ice before resetting the unit.
- With power off at the service switch, look at the clear pressure-switch tube for obvious water droplets, sagging loops that can trap moisture, or a tube that has slipped off either fitting.
- Cycle power off for 30 seconds and back on once to clear the lockout, then watch whether the same 2-flash code returns on the next call for heat.
Professional Required
- Connecting a digital manometer to the pressure-switch tubing to compare actual venter draft against the exact Reznor switch rating before condemning the switch.
- Clearing the venter-housing pressure-tap orifice and draining or rerouting the sensing tube so condensate cannot keep plugging the vacuum signal.
- Replacing the combustion venter assembly or the exact OEM Reznor pressure switch when testing proves the draft or switch has failed.
- Inspecting the heat exchanger for cracks or separation and confirming combustion integrity before returning the duct furnace to service.
FAQ
What do 2 flashes mean on a Reznor duct furnace?
It is the documented pressure-switch failure code: the switch did not close within 30 seconds after the venter motor started, so the ignition board locked the furnace out before opening the gas valve.
Why does my Reznor venter run but the burners never light?
That is the classic 2-flash pattern. The venter is energized, but the pressure switch never proves draft because the sensing tube is waterlogged, the pressure tap is clogged, the motor is not pulling enough vacuum, the vent terminal is blocked, or the heat exchanger is leaking air into the combustion path.
Can condensation really stop a Reznor pressure switch from closing?
Yes. On Reznor duct furnaces and makeup-air units handling cold outdoor air, condensate can collect in the clear sensing tube and physically block the pneumatic signal to the pressure switch.
Schedule Reznor Service
Need Reznor Repair in NYC?
A Reznor duct furnace pressure-switch lockout is the documented 2-flash fault: the pressure switch did not close within 30 seconds of venter-motor start, so the board locked out ignition. On Reznor commercial duct furnaces, the real causes are usually condensate trapped in the sensing tube, a clogged venter pressure-tap orifice, a combustion blower that still spins but has lost RPM, blocked exhaust termination, or a cracked heat exchanger that lets supply-air pressure cancel the draft signal.