Pool Error Codes

Pentair ERR AFS: Air Flow Switch Open

ERR AFS means the combustion blower on a Pentair MasterTemp or MiniMax NT heater hasn't proven it's moving enough air before the burner is allowed to light.

medium severity

Applies to: Pentair MasterTemp and MiniMax NT residential gas pool/spa heaters

What it means

Every gas heater has to prove two things before it's allowed to light a burner: that water is flowing through the heat exchanger, and that the exhaust side is actually venting. ERR AFS is about the second one. Inside the cabinet, a pressure-differential switch is plumbed to sense the vacuum the blower creates once it spins up; if that switch never registers enough negative pressure, the control board assumes air isn't moving properly through the flue and refuses to proceed with ignition. It's a carbon-monoxide safeguard rather than a nuisance fault — a heater that fired without confirmed venting is exactly the scenario this switch is there to prevent. Outdoor installations pick this fault up more than indoor ones, mostly because the vent termination and intake sit exposed to whatever the yard blows into them.

Likely causes

The switch itself is rarely the actual problem — something is usually keeping air from moving the way the blower expects it to.

What you can check yourself

These checks are safe to do with the heater powered off and the cabinet closed back up before testing a restart.

  1. Check the outside vent termination and intake for blockages (leaves, debris, insect nests) and clear them
  2. Confirm the blower runs and sounds normal when the heater tries to start
  3. Clear the fault and retry once obstructions are removed
Call a professional if…

If the vent is clear and the blower spins normally but ERR AFS persists, the switch or its tubing needs professional testing.

Sources

Related reading

This site is an independent reference, not a substitute for a licensed pool professional. Gas pool heaters can leak carbon monoxide and cause fire; pump and heater electrical circuits carry mains voltage. Anything involving the gas line, burner assembly, or house wiring should be handled by a licensed technician or electrician.