Lennox furnace Alert code 272: Low pressure switch open in run mode — Watchguard

Alert 272 means the low pressure switch opened during a run and the system entered Watchguard mode. Lennox rates it Service Urgent.

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 combines two things worth separating: the fault occurred while the furnace was already running, and it was serious enough to trigger Watchguard. The run-mode timing means draft was adequate at ignition and was lost during the cycle, which points at conditions that develop as the furnace operates. Lennox's specific question here is whether the switch is stuck open during a heat call, and its checklist covers vent blockages, inducer operation and restriction, the cold end heater box and drain, cracked hoses, and levelness.

Most common causes

• A condensate drain or cold end heater box backing up during a run.
• Ice forming progressively at the exhaust termination during a long cycle.
• A pressure switch stuck open during the heat call.
• An inducer weakening as it warms.
• Cracked hoses in the pressure switch circuit.
• A partly restricted vent where draft is marginal.
• The unit not level, so condensate accumulates during operation.

How to diagnose it

1. Note that the furnace lit successfully before faulting — that distinguishes this from a failure to prove draft at all.
2. Note how long the furnace runs before faulting, since a consistent interval points at something accumulating.
3. Check the condensate drain for slow drainage.
4. Check the outside vent termination, and note the weather.
5. Check whether the furnace is level.

How to fix it

1. Clear a partly blocked condensate drain and the vent termination — both homeowner work.
2. The pressure switch, hoses, inducer, and cold end heater box are technician tasks.
3. Tell the technician how long the furnace runs before faulting. That observation distinguishes several causes and only the householder can provide it.
4. Never bypass a pressure switch.

Notes

Run-mode faults are harder to reproduce on a service visit than start-up faults, because the technician has to wait out a full cycle. Knowing the elapsed time in advance saves that wait and makes the visit more likely to be productive.

Related

• Alert code 227 — Low pressure switch open in run mode
• Alert code 271 — Low pressure switch open — Watchguard

Sources

• Lennox Residential Communicating Systems Alert Code Guide, document 100017, March 2023 — Table 2, Alert Codes and Troubleshooting