Lennox furnace Alert code 227: Low pressure switch open in run mode
Alert 227 means the low pressure switch opened while the furnace was already running. The draft was proved at ignition and then lost partway through the cycle.
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
This alert is distinct from the switch failing to close at all, and the distinction is diagnostically valuable. The furnace lit successfully, which means draft was adequate at that moment. Something then changed during the run. That pattern points at conditions that develop over a cycle rather than being present from the start: a condensate trap filling as the furnace produces water, ice building at a cold vent termination, or an inducer weakening as it heats. Lennox's checklist is the same as for the switch failing to close, but the timing tells you which of those causes is plausible.
Most common causes
- A condensate drain or cold end heater box that backs up during a run and clears between cycles.
- Ice forming progressively at the exhaust termination during a long run.
- An inducer weakening as it warms up.
- A partly restricted vent where draft is marginal rather than absent.
- Cracked hoses in the pressure switch circuit.
- The unit not level, so condensate accumulates during operation.
- A pressure switch operating at the edge of its setpoint.
How to diagnose it
- Note that the furnace lit successfully before faulting. That is the key distinction from an alert where it never proved draft at all.
- Note how long the furnace runs before the alert appears, since a consistent interval points at something accumulating.
- Check the condensate drain for slow drainage rather than complete blockage.
- Note the weather and whether the fault clusters on cold days, which fits ice at the termination.
- Check whether the furnace is level.
How to fix it
- Clear a partly blocked condensate drain, which is the most common cause of a run-mode drainage fault.
- Clear the vent termination and keep it free of snow and ice.
- The pressure switch, hoses, inducer, and cold end heater box are technician work.
- Tell the technician how long the furnace runs before faulting. That single observation distinguishes several of the causes and is something only the householder can supply.
Notes
Intermittent faults that only appear after the furnace has been running are among the hardest to catch, because the furnace usually works when a technician arrives. Noting the elapsed time before the fault is genuinely useful diagnostic information.
Related
- Alert code 223 — Low pressure switch open
- Alert code 272 — Low pressure switch open in run mode — Watchguard
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.