Lennox furnace Alert code 225: High pressure switch will not close
Alert 225 means the high pressure switch did not close during a heat call. On a two-stage furnace that usually shows up as a unit which runs but never reaches full output.
Applies to: Lennox communicating gas furnaces on an S30, S40 or comparable communicating thermostat, as documented in the Lennox Residential Communicating Systems Alert Code Guide.
What it means
A two-stage or modulating furnace has a second pressure switch set for the greater airflow that high fire produces. Alert 225 says that switch never closed. The practical effect differs from a low pressure switch fault: the furnace is not dead. It lights, runs on low fire, and simply never steps up. In cold weather that presents as a house that will not reach temperature and a furnace that runs continuously — which most people do not recognise as a fault at all. Lennox's troubleshooting is specific about where to look, naming the cold end heater box, the condensate drain and the drain strainers at its outlet alongside the usual vent and inducer checks.
Most common causes
- A partially restricted vent or intake — enough draft for low fire, not enough for high.
- A blocked cold end heater box, condensate drain, or the strainers at the outlet of that box.
- The inducer not reaching the speed high fire requires.
- A cracked or disconnected hose on the high pressure switch specifically.
- A failed high pressure switch.
How to diagnose it
- Note whether the house struggles to reach temperature in cold weather while the furnace runs constantly. That is this fault's signature.
- Check the vent and intake terminations outside for partial obstruction as well as complete blockage.
- Note whether there is standing water near the furnace, which points at the condensate path.
- The fault can sit unnoticed for a whole season, because the furnace still produces heat.
How to fix it
- Clear any obstruction at the terminations, which is the one homeowner-checkable item.
- Lennox specifies measuring the pressure at which the switch closes during a heat call, and the operating pressure, both in inches of water column. That is technician work.
- Worth resolving before deep winter rather than during it — a furnace stuck on low fire will not fail dangerously, but it will fail to keep up when it matters.
Notes
Alert 226 is the same switch in the opposite condition, stuck closed. Together they are the high-fire equivalent of the 223 and 224 pair.
Related
- Alert code 226 — High pressure switch stuck closed
- Alert code 276 — Furnace calibration failure, or high pressure switch fault
All codes for this manufacturer
Lennox furnace alert codes — every code, with what differs between models.