AFS
Pentair Pool Heater
Severity: ModerateWhat Does This Error Mean?
Pentair MasterTemp AFS means Air Flow Switch.
During start-up the heater runs the blower, then expects the air flow switch to close once airflow is established.
If the switch never closes, the heater will not light the burner — that protects against firing with no exhaust.
The cause is almost always the blower itself, the switch, or something blocking airflow through the unit.
Affected Models
- Pentair MasterTemp 200
- Pentair MasterTemp 250
- Pentair MasterTemp 300
- Pentair MasterTemp 400
- Pentair MasterTemp HD
Common Causes
- Blower motor not spinning or spinning too slowly
- Mouse nest, leaves, or debris blocking the blower intake
- Air flow switch tubing pinched or disconnected
- Switch itself stuck open
- Vent pipe blocked or terminated incorrectly
- Loose wiring at the switch terminals
How to Fix It
-
Power off at the breaker.
Pool heaters run 240V — switch off the dedicated breaker before opening the unit.
Wait 5 minutes for capacitors in the blower circuit to discharge. -
Open the front panel and look at the blower.
Remove the screws holding the front access panel.
The blower is the squirrel-cage fan unit inside.
Spin the wheel by hand — it should turn freely.
Stuck or scraping = bearing failure or debris. -
Clear nests and leaves.
Mice love idle pool heaters.
Look in the blower housing, near the burner, and in any open ducting.
Vacuum out everything you find.
Check the vent terminal outside for spider webs or screens choked with lint. -
Inspect the air flow switch.
The AFS is a small disc with two wires and a length of clear tubing running to the blower housing.
Disconnect the tubing.
Blow into it gently — you should hear or feel the switch click.
No click means the switch is dead. -
Check the tubing.
The signal tubing between switch and blower must be open and free of moisture.
Replace any pinched, kinked, or water-filled section.
Replacement silicone tubing is a few dollars. -
Test the blower under power.
Restore power and start the heater.
The blower should spin up within a few seconds and run smoothly.
If it grunts and stops, the capacitor or the motor itself has failed.
Capacitors are 15-40 USD; replacement blower motors are 150-300 USD. -
Replace the AFS.
If the blower runs strong and the tubing is clear but AFS still posts, replace the switch itself.
Genuine Pentair AFS parts are 40-90 USD.
Two screws and two spade terminals — straightforward swap.
Frequently Asked Questions
Why does AFS only happen on the first start of the day?
A weakening blower or a borderline switch can be just strong enough once warmed up but not on a cold start.
If AFS clears once the heater has run for a minute, the parts are aging — replace before they fail completely mid-season.