Okay, let's break down the best way to handle the booster fan situation you've described. Combining bath fan exhausts and using a booster fan is fairly common, but the control strategy is important for effectiveness and efficiency.

Understanding the Goal

The purpose of the booster fan is to ensure proper airflow when either or both bath fans are running, helping to exhaust moisture and odors effectively.

Why Running the Booster Fan Constantly is Not Ideal Wasted Energy

Running the booster fan 24/7 consumes unnecessary electricity.

Noise

Constant fan noise can be annoying.

Potential for Backdrafting

If the booster fan is powerful and only one bath fan is on, it could potentially pull air back in through the off-state bath fan (though this is less likely with a well-designed system and backdraft dampers).

Premature Wear and Tear

Constant operation shortens the lifespan of the booster fan.

Option 1: Simplest - Booster Fan Runs Whenever Either Bathroom Fan Runs How it works

Wire the booster fan to a relay (or a contactor if it's a higher-amp booster fan). Connect the existing switch circuits for both bathroom fans to the coil of the relay. When either switch is flipped on, it energizes the relay coil, which closes the relay contacts and turns on the booster fan.

Pros

Simple wiring. Ensures the booster fan runs whenever either bath fan is operating. Avoids constant operation of the booster fan.

Cons

Requires installing a relay/contactor near the booster fan or in a junction box along the wire run.

Wiring Details for Option 1 (Relay/Contactor Control)


Disconnect Power

Always disconnect the power at the breaker to both bathroom fan circuits and the booster fan circuit before working on any electrical wiring.2.

Locate the Relay/Contactor

Choose a suitable location for the relay. It's often near the booster fan itself, inside a junction box.3.

Connect the Relay Coil

Run a wire from the "hot" (switched) wire of

Bathroom Fan 1 to one side of the relay coil (A1 terminal on most relays). Run a wire from the "hot" (switched) wire of Bathroom Fan 2 to the other side of the relay coil (A2 terminal on most relays). Connect the neutral wire from both bathroom circuits to the neutral connection of the relay.4. Connect the Relay Contacts

Run the "hot" wire from the booster fan's power source through the normally open (NO) contacts of the relay. When the relay is energized, it closes these contacts, providing power to the booster fan. Connect the neutral wire from the booster fan's power source directly to the booster fan's neutral connection.5.

Important Considerations



Relay/Contactor Sizing

Choose a relay with a coil voltage that matches your household voltage (120V in North America). The relay contacts must be rated to handle the amperage draw of the booster fan motor. A contactor is typically used for higher amperage fans.

Safety

All wiring must be done in accordance with local electrical codes. Use appropriate wire gauges and connectors. Ground everything properly.

Option 2: Your "Two Separate Contacts" Idea - GENERALLY NOT RECOMMENDED

You described having two separate contacts for the booster fan, each controlled by a separate bathroom fan switch. While technically possible, I strongly advise against this for several reasons: Complication

This adds unnecessary complexity to the wiring.

No Benefit

It doesn't provide any advantage over the simpler relay/contactor approach.

Potential for Problems

It introduces the possibility of mismatched voltage or current if the "coils" are not perfectly identical.

Important Safety Considerations (Applicable to Both Options)



Electrical Code

All wiring must comply with your local electrical codes. If you are not experienced with electrical work, hire a qualified electrician.

Grounding

Properly ground all components (fans, booster fan, relay/contactor).

Wire Gauge

Use appropriate wire gauge for the amperage load of the fans and booster fan.

Disconnect Power

Always disconnect power at the breaker before working on any electrical wiring.

Enclosures

Use proper junction boxes and enclosures for all wiring connections.

Recommendation

I strongly recommend

Option 1 (relay/contactor control) as the best approach. It's the simplest, most reliable, and most energy-efficient way to ensure the booster fan runs only when needed. Make sure to choose a relay/contactor that is appropriately sized for the booster fan's amperage draw.

Before You Start


Determine Booster Fan Amperage

Find the amperage rating of the booster fan motor. This information should be on a label on the fan.2.

Choose Relay/Contactor

Select a relay/contactor that meets the voltage and amperage requirements.3.

Consult an Electrician: If you are not comfortable with electrical work, consult a licensed electrician to ensure the wiring is done safely and correctly. They can also help you select the appropriate relay/contactor.Good luck! Flag for review