BT-3 125 — inline centrifugal fan 330 m³/h with external rotor motor

The Casals BT-3 125 is an inline centrifugal fan from the BT-3 series — a range of duct fans with external rotor motors (ERM), designed for efficient and quiet ventilation of bathrooms, small rooms and residential spaces by direct installation in circular ductwork or false ceiling voids. Manufactured in Spain by Casals Ventilación, direct importer ventilation.ro.

External Rotor Motor (ERM) — the key advantage

The single-phase external rotor motor is the technology that sets the BT-3 series apart from inline fans with conventional motors. In an ERM, the rotating element (outer winding) surrounds the fixed stator — the fan impeller is mounted directly on the rotor, completely eliminating the transmission shaft, coupling and shaft bearings of conventional motors.

Direct advantages: superior efficiency (no mechanical transmission losses), compact footprint (motor and impeller form one integrated assembly — the BT-3 125 weighs just 2.7 kg), and extended service life through permanently lubricated ball bearings rated for 40,000 hours — approximately 15–18 years at 8 hours per day operation. Automatic-reset thermal protection stops the motor on thermal overload and restarts automatically once cooled, without manual intervention.

Backward-curved impeller — high airflow, lower noise

The BT-3 125 impeller has backward-curved blades, dynamically balanced to industry standards. Backward-curved blade geometry generates the same pressure and airflow at a lower rotational speed compared to forward-curved blades — producing lower noise at equivalent airflow and a non-overloading power curve that does not stress the motor at high airflow operating points.

Galvanised steel housing

The outer housing and airway are manufactured in galvanised steel — corrosion-resistant, durable in humid environments (bathrooms, utility spaces) and suitable for installation in contact with false ceiling insulation without risk of material degradation.

Flexible installation

The BT-3 125 is designed primarily for duct installation in Ø125 mm circular ductwork — the fan is inserted inline in the duct run, with direct duct connections on both sides. Both inlet and outlet connections are standard Ø125 mm, compatible with rigid spiral, flexible and semi-rigid circular ductwork.

False ceiling installation: the compact footprint and low weight (2.7 kg) allow direct installation in the false ceiling void, suspended or fixed to the ceiling structure, with ductwork connecting to room extract grilles and the exterior discharge point.

Support feet are included in the standard package for optional surface mounting on a flat base — useful in technical applications or where false ceiling access is not possible.

External connection box on the housing provides easy access to electrical terminals without opening the main casing.

Typical applications

Extract from residential bathrooms and WCs, utility rooms, dressing rooms, small hotel bathrooms, small office spaces — anywhere requiring extraction of up to 330 m³/h through a Ø125 mm circular duct, with duct or false ceiling installation.

Note: the BT-3 125 is not suitable for transporting explosive or flammable gases. Maximum continuous operating temperature is 50°C — not suitable for hot air applications (industrial hoods, dryers).

FAQ 

What duct diameter does the BT-3 125 require? The BT-3 125 connects to Ø125 mm circular ductwork on both inlet and outlet. Compatible with rigid spiral, flexible and semi-rigid Ø125 mm standard circular ductwork.

How many air changes does it provide for my bathroom? At 330 m³/h, the BT-3 125 provides: 8 ACH for a ~14 m² bathroom (3m ceiling), or 10 ACH for ~11 m². For a standard 5–8 m² bathroom it provides 15–20 ACH — well above the normative minimum of 8 ACH. Use the airflow calculator for precise verification.

Can it be used with a speed controller? Yes — the single-phase ERM motor 230V is compatible with linear or stepped speed controllers for single-phase motors. Speed control reduces airflow and noise in normal operation and allows adaptation to the exact requirements of the installation. Contact us for a controller recommendation.

What is the difference from a wall-mounted bathroom extractor? A wall extractor (e.g. Casals Erelis, Lidero) mounts directly on the bathroom's exterior wall — simple and cost-effective but only possible when the bathroom borders an exterior wall. The BT-3 125 installs in the duct or false ceiling and can be positioned anywhere along the duct run — ideal for interior bathrooms without an exterior wall, or when multiple rooms share a single extract duct.

Does it require maintenance? Permanently lubricated bearings require no lubrication over their service life. The impeller may need periodic dust cleaning after 2–3 years of operation. Access to the impeller is through the housing after disconnecting from the mains.

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