Proves Actual Airflow · Not Just Fan Operation · 24 or 120 Vac
PFS Positive
PFS Positive
Flow Switch
Inline Duct Airflow Confirmation · 24 or 120 Vac · 4"–9"
✓
Confirms actual airflow in the duct — not just that the fan motor is running
✓
Responds to air velocity — not static pressure, no false positives on pressurized systems
✓
Switch body mounts externally — no electrical components in the airstream
✓
Passive contact output — feeds ADP relay input, BAS point, or controller input
✓
24 Vac or 120 Vac — match to control circuit voltage
✓
Vertical (upward) or horizontal mounting only — confirm before installation
✓
Sizes 4"–9" standard · 10-year mechanical warranty
Is this the right product?
✔ Right product if
You need to confirm actual airflow is occurring in a duct run — not just that the fan is running — for safety interlock, ventilation proving, or system fault detection.
✗ Not this product if
Only need to confirm the fan motor is energized → a current sensor handles that without an inline duct section. Use the PFS when airflow itself needs to be confirmed at the duct location.
What It Does — and Why It's Different From a Current Sensor
A fan motor running at full current does not mean air is moving. A damper stuck closed, a blocked intake screen, or a collapsed duct section will all run the motor while delivering zero airflow. The PFS confirms actual flow — not just power. The switch assembly sits inside the duct and responds only to air velocity at its location — not to motor current, not to static pressure.
On a combustion air intake in a Canadian January, the intake screen ices over overnight. The fan starts in the morning, draws full current, and a current sensor reports normal operation. But airflow through the blocked screen is zero. The PFS is in the duct — it doesn't close its contact, the ADP receives no confirmation, and the appliance doesn't fire on a false signal.
PFS vs. damper end switch — these prove different things. The damper end switch confirms the blade reached open position. The PFS confirms air is actually moving. On standard residential combustion air interlock, the end switch connected to the ADP is typically sufficient. The PFS adds a second, independent layer — specify both when the AHJ requires flow proving in addition to position proving, or on exposed north-facing intakes with known screen icing history.
What Each Confirmation Method Proves
| What You Need to Confirm | Use | Limitation |
|---|---|---|
| Damper blade has reached open position | Damper end switch (HAC SPO/SPC or HOM SF1 or SF2) | Does not confirm airflow — duct could still be blocked downstream |
| Actual airflow is occurring in the duct | PFS Positive Flow Switch | Does not confirm damper blade position — only that air is moving past PFS location |
| Both blade position AND airflow | Damper end switch + PFS in sequence | Highest assurance — required on some combustion air code interpretations |
Part Numbers
| Size | 24 Vac | 120 Vac |
|---|---|---|
| 4" | PFS-0400-024 | PFS-0400-120 |
| 5" | PFS-0500-024 | PFS-0500-120 |
| 6" | PFS-0600-024 | PFS-0600-120 |
| 7" | PFS-0700-024 | PFS-0700-120 |
| 8" | PFS-0800-024 | PFS-0800-120 |
| 9" | PFS-0900-024 | PFS-0900-120 |
Mounting orientation: vertical (upward) or horizontal only. The paddle element inside the duct must be oriented correctly relative to airflow and gravity. Do not install at 45 degrees, inverted, or at other angles not specified. Also install in a straight duct section with consistent airflow — avoid locations directly downstream of elbows, transitions, or fittings. A minimum straight run of 3–5 duct diameters upstream is recommended.
Key Specs
Sizes
4"–9" standard
Voltage
24 Vac or 120 Vac — specify at order
Activation
Airflow velocity — not static pressure
Switch Type
External mount — switch body outside duct
Mounting
Vertical (upward) or horizontal only
Signal Output
Passive contact — ADP, BAS, or controller input
Construction
Galvanized steel inline duct section
Warranty
10-year mechanical
Why use a PFS instead of a current sensor on the fan motor?
A current sensor confirms the fan motor is drawing current. It does not confirm air is moving through the duct. On a combustion air application where the intake screen has iced over, the fan runs at full current against the blockage — and a current sensor reports normal operation while airflow is zero. The PFS is installed in the duct and responds only to actual air movement. For safety interlock applications where the entire point is to confirm combustion air is reaching the appliance, the PFS provides the correct signal.
Is the PFS affected by static pressure in the duct?
No. The PFS is designed to respond to airflow velocity rather than static pressure. In pressurized duct systems — zoned systems, variable air volume, pressurized make-up air runs — static pressure can be elevated even when airflow through a specific branch is low or zero. The PFS paddle deflects only when air is moving past it with sufficient velocity. A pressure-based sensor would give false positives in pressurized conditions. The PFS does not.
Can the PFS be used with a BAS or DDC controller?
Yes. The PFS provides a passive switch contact output. This can feed an ADP relay adaptor input, a thermostat or ventilation controller digital input, a BAS point, or any other control system that accepts a dry contact closure. Select the voltage — 24 Vac or 120 Vac — to match the input circuit of the receiving device.
Do I need both a damper end switch and a PFS, or just one?
It depends on the application. On most standard residential combustion air interlock installations, the damper end switch connected to the ADP is sufficient and is what the code requires. The PFS adds a second, independent confirmation of actual airflow. Specify both when the AHJ requires flow proving in addition to position proving, or when the installation has a higher-than-average risk of intake blockage — north-facing intakes, exposed locations, or cold climate installations with known screen icing history.
Ready to Order
PFS — Positive Flow Switch
Select diameter (4"–9") and voltage (24 Vac or 120 Vac) — match voltage to control circuit. Confirm mounting orientation (vertical upward or horizontal) before installation.
Configure & Order →
Diameter · Voltage
Need help specifying the PFS?
Technical support available through your Hoyme wholesaler or direct.