Lennox furnace Alert code 226: High pressure switch stuck closed
Alert 226 means the high pressure switch will not open. Lennox rates it Service Urgent and specifically directs checking for moisture in the pressure switch.
Applies to: Lennox communicating gas furnaces on systems using the S30, S40 or comparable communicating thermostats, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
The high pressure switch must open when the inducer is not producing its rated draft, and this alert means it will not. A switch stuck closed proves a draft that may not exist, which is why Lennox rates it Service Urgent rather than Service Soon. The guidance names a cause that is easy to overlook: moisture in the pressure switch. On a condensing furnace, water is produced by design, and water reaching a pressure switch will hold its contacts closed as effectively as a mechanical fault. Lennox again points to Service and Application Note H-13-07 for detail.
Most common causes
- Moisture in the pressure switch, which Lennox lists explicitly.
- Pressure switch contacts mechanically stuck closed.
- A vent blockage affecting the pressure the switch sees.
- A combustion air inducer not operating correctly, or restricted.
- Water in the sensing hoses from a condensate drainage problem.
- A switch whose operating pressure has drifted out of specification.
How to diagnose it
- Check the condensate drain for backing up, since moisture in the switch is Lennox's named cause and drainage is where it comes from.
- Check the outside vent termination for obstruction.
- Note whether the furnace is level, since drainage depends on it.
- Note whether the fault appears after long runs or in particular weather, which fits a moisture-related cause.
How to fix it
- Clear a blocked condensate drain, which addresses the source of the moisture Lennox names.
- Clear the vent termination if obstructed.
- The pressure switch, its hoses, and the inducer are technician work.
- Never jumper a pressure switch. It proves safe venting, and defeating it can allow the furnace to fire into a blocked flue.
Notes
Moisture as a cause of a stuck pressure switch is specific to condensing furnaces and is genuinely counterintuitive — the water is a normal product of the furnace working correctly, and it only becomes a fault when drainage fails.