Pentair MasterTemp ERR AFS: Air Flow Switch Open

What ERR AFS Actually Means

The MasterTemp is a forced-draft heater. A combustion blower pushes air through the burner tray and out the exhaust stack before and during ignition. The air flow switch is a pressure-differential sensor that confirms the blower is moving enough air. If the switch does not close within the startup sequence, the control board displays ERR AFS and halts the ignition process.

This is one of the first checkpoints in the firing sequence. The control board needs to see the air flow switch close before it will energize the hot surface igniter (HSI). Without confirmed airflow, unburned gas could accumulate in the combustion chamber, creating a dangerous situation.

If ERR AFS does not self-clear within about 60 seconds, the heater locks out and you must press HEATER OFF on the membrane pad to acknowledge and reset the fault.

Step-by-Step Troubleshooting

Owner-Level Checks

Listen for the blower

1. When the heater receives a call for heat, you should hear the combustion blower spin up before any clicking or glowing occurs.
2. If you hear nothing from the blower, the motor may have failed, or it is not receiving power.
3. If you hear the blower running but the error persists, suspect an airflow restriction or a bad switch.

Check the exhaust and intake

1. Inspect the exhaust vent on top of the heater for bird nests, leaves, insect nests (especially mud daubers), or other blockages.
2. Check the combustion air intake openings on the sides or bottom of the unit for debris or obstructions.
3. Make sure nothing is placed on top of or against the heater that could block airflow.

Tech-Level Checks

Test the blower motor

1. With power off, disconnect the blower motor leads and check for continuity through the windings. Open windings mean a dead motor.
2. Restore power and check for voltage at the blower connector when a heat call is active. If voltage is present but the motor does not spin, replace the motor.
3. If no voltage reaches the blower during a call for heat, the control board or wiring is the problem.

Test the air flow switch

1. Locate the air flow switch mounted near the blower housing. It has a small vinyl tube connecting it to the combustion chamber.
2. With the blower running, check continuity across the switch terminals. It should be closed (near zero ohms) when the blower is at full speed.
3. Check the vinyl sensing tube for cracks, disconnections, or condensation blockage. A clogged tube will prevent the switch from sensing pressure.
4. If the tube is clear and the blower is running but the switch stays open, replace the switch.

Inspect the combustion chamber

1. Remove the front access panel and look inside the burner tray area. Soot, rust flakes, or debris can restrict airflow through the burner ports.
2. Check for signs of condensation damage (white or green corrosion on the heat exchanger fins). Operating below 68 degrees F water temp causes this.
3. Clean any debris from the burner tray and fins.

Common Parts That Fix This Problem

• Combustion blower motor assembly
• Air flow switch (pressure differential switch)
• Vinyl sensing tube for the air flow switch
• Exhaust vent cap or rain cap if original is missing or damaged
• Control board if blower is not receiving power during heat call

Model-Specific Notes

• All MasterTemp models (175K through 400K) use forced-draft combustion with the same air flow switch safety. Larger models have larger blowers but the diagnostic approach is identical.
• The MasterTemp uses HSI (hot surface ignition). The HSI will not energize until the air flow switch closes. If you see ERR AFS, the igniter has not been powered yet.
• HD (Heavy Duty) and ASME models have the same combustion airflow path as standard models. The HD designation refers to the heat exchanger construction, not the blower system.
• On installations with long exhaust vent runs or horizontal venting, verify the vent length does not exceed manufacturer specifications, which can create back-pressure that the blower cannot overcome.

How to Prevent ERR AFS From Coming Back

• Keep the exhaust vent clear of debris. Install a vent screen if insects are a recurring problem in your area.
• Ensure adequate clearance around the heater for combustion air intake per the installation manual.
• Do not operate below 68 degrees F water temperature. Condensation corrodes internal components and can restrict airflow over time.
• During annual service, inspect the blower wheel for balance and debris, and check the sensing tube for deterioration.
• On outdoor installations, verify the rain cap is in place and not restricting exhaust flow.