QAF81.3

What does this frost monitoring device do in an air handling unit?

It monitors the air-side temperature across a heating coil and switches when frost conditions are detected, helping trigger protective actions such as stopping the fan, closing outside air dampers, and driving the heating valve open to prevent coil freeze-up.

Where is it typically installed within ductwork?

It is commonly mounted directly downstream of the heating coil on a withdrawable tray (where used), with the capillary routed evenly across the coil face.

How does it sense frost risk along the coil?

It uses a gas-filled (R134a) diaphragm and copper capillary. The switch trips when the temperature falls below the setpoint over any continuous 30–100 cm section of the capillary, rather than relying on a single point measurement.

What is the adjustable setpoint range?

The setpoint is adjustable from -5 to +15 °C, with a factory setting of 5 °C (typical for frost protection applications).

What switching differential and repeatability should I expect?

The switching differential is 2 ±1 °C, and repeatability is ±0.5 °C, supporting consistent switching behaviour around the chosen setpoint.

Does it reset automatically after a frost condition clears?

Yes. The QAF81.3 version has automatic reset once the temperature rises above the selected setpoint again.

What capillary length is supplied with this version?

This version (Siemens QAF81.3) is supplied with a 3 m capillary, intended to be laid out across the coil area for broad coverage.

Are there any mounting orientation restrictions?

No. The device can be installed in any orientation; the key requirement is correct capillary placement across the coil and protection of the capillary where it passes through metalwork.

What are the key wiring terminals and contact behaviour?

It uses a single-pole changeover (SPDT) microswitch. Terminals 1–4 indicate normal operation, and terminals 1–2 indicate frost alarm (or sensor fault condition).

What electrical ratings does the switch support?

Contact rating is up to AC 250 V, 15(1.2) A and DC 30 V, 15(1.5) A. External line protection is typically via a slow fuse up to 10 A or a circuit breaker up to 13 A (characteristic B, C, or D).

What ingress protection rating does the housing provide?

The housing is rated IP54 as standard. IP65 can be achieved when fitted with a suitable M20x1.5 cable gland (not supplied as standard).

What cable entry and terminals are used?

Cable entry is via an M20 x 1.5 threaded opening, and connections are made to screw terminals suitable for up to 1.5 mm² conductors.

What installation details matter most for reliable operation?

Route the capillary evenly across the coil on the warm side (downstream of the heating coil), maintain a minimum bending radius of 20 mm, and ensure the housing (including the test loop) is located where the ambient temperature remains at least 2 °C above the selected setpoint.

How can function be checked during commissioning?

Operation can be verified by immersing the test loop in a vessel of iced water to simulate a frost condition and confirm the contact changeover.

Does it require routine maintenance?

No. It is designed to be maintenance-free under normal HVAC operating conditions.

What is the admissible medium for the sensor?

The sensing system is intended for air temperature monitoring in ventilation and air conditioning applications.

Are any accessories included to help mount the capillary?

Yes. The unit is supplied with capillary tube clamps (six pieces, type FK-TZ1) to simplify secure routing across the coil face.

What should be considered at end of life for disposal?

It should be treated as electronic equipment for disposal and handled through appropriate recycling channels in line with local regulations, rather than being discarded as general household waste.