Yes, it's definitely possible to interlock 120V exhaust fans with motorized dampers controlled by a reverse-acting thermostat. Here's a breakdown of how it can be done, along with considerations and potential solutions

Understanding the Requirements
Reverse Acting Thermostat

A reverse-acting thermostat means the control signal increases when the temperature decreases. So, colder temperature -> higher signal to the control circuit -> dampers open and fans turn on.

Interlocking

This ensures the exhaust fans only operate when the dampers are open. This is crucial for safety and efficiency:

Preventing Damage

Running the fans with the dampers closed could overwork the fan motor, leading to overheating and potential failure.

Preventing Backdraft

If dampers are closed, fans could create negative pressure, potentially drawing in air from undesirable sources.

120V System

The whole system being 120V simplifies the wiring a bit compared to mixed voltage systems.

Methods for Implementation

Here are a few ways to achieve the interlocking, from simpler to more complex (but potentially more robust)

1. Relay-Based System (Most Common & Recommended)

This is the most common and generally reliable approach. Components Needed

Reverse Acting Thermostat (120V Output): Look for a thermostat designed for HVAC applications, specifically reverse-acting. Relay(s): 120V coil relay(s) with sufficient contact rating to handle the damper and fan motor loads. (Check the amp ratings on the equipment.) Wiring & Junction Boxes: Appropriate gauge wiring, junction boxes, and conduit for 120V applications.

How it Works


Thermostat Control

The reverse-acting thermostat's 120V output is wired to the coil of a main control relay (Relay 1).2.

Dampers Activated

When the thermostat "calls" for exhaust (temperature is below setpoint, so thermostat provides 120V), the relay (Relay 1) is energized, and its normally-open contacts close. These contacts power the motorized damper.3.

Interlock Relay

A second relay (Relay 2) is wired so its coil is powered by the same power source as the motorized damper's motor. This means Relay 2 only activates when the damper motor is actively opening (or fully open, depending on how you wire it - see notes below).4.

Fan Activation

The normally-open contacts of Relay 2 are wired to the exhaust fan's power supply. Only when Relay 2 is energized (meaning the dampers are at least attempting to open) will power be supplied to the fan.

Wiring Diagram (Conceptual)

120V AC Supply
----------------------------------------------------|
                                                  |Thermostat (Reverse Acting)       Damper Motor (120V)    Fan Motor (120V)|                                   |                    
120V Output                       |
-------------------> Relay 1 Coil   |                    
|Relay 1 Contacts (NO) ----------|-------> Damper Motor
-- (Parallel with Damper Motor) --> Relay 2 Coil||Relay 2 Contacts (NO) ---------> Fan Motor

Important Notes for Relay Approach



Damper Position Feedback (Highly Recommended)

Ideally, the motorized dampers will have end-position switches (auxiliary contacts) that indicate when the damper is fully open. This allows you to use these contacts (wired to the Relay 2 coil) to only energize the fan after the damper is confirmed to be fully open. This prevents the fan from running even if the damper motor is struggling or has failed.

Relay Contact Ratings

Make absolutely sure the relay contacts are rated to handle the inrush and running current of both the damper motor and the fan motor. Over-sizing is always a good idea.

Enclosure

Mount all relays and wiring inside a suitable electrical enclosure, following local codes.

Safety

Use proper wiring techniques, grounding, and overcurrent protection (fuses or circuit breakers).

2. Using a Programmable Relay (Smart Relay/PLC Lite) Components Needed

Reverse Acting Thermostat (120V Output) Programmable Relay (e.g., Crouzet Millenium 3, Eaton Easy Relay, Schneider Zelio Logic, Allen-Bradley Micro800 series) - Must accept 120V inputs. Wiring & Junction Boxes

How it Works


Thermostat Input

The thermostat's output is wired to a digital input on the programmable relay.2.

Logic Programming

You program the relay with logic to: Detect the thermostat signal (low temperature). Activate an output that powers the damper motor. Wait a specified time (sufficient for the damper to open). You can also use a damper open/closed signal, as in the relay-based system. Activate a second output that powers the fan.3.

Outputs

The programmable relay's outputs are wired to control the damper and fan motors.

Advantages

More flexible control logic. Timers and delays are easily implemented. May be more compact than multiple relays.

Disadvantages

Requires programming knowledge. Potentially higher initial cost.

3. Solid State Relays (SSRs) Components Needed

Reverse Acting Thermostat (120V Output) Solid State Relays (SSRs) - Rated for the voltage and current of the damper and fan. Heat Sinks (for SSRs, especially if switching large loads) Wiring & Junction Boxes

How it Works

Similar to the relay-based system, but using SSRs instead of electromechanical relays. SSRs switch the AC power electronically.

Advantages

No moving parts (potentially longer lifespan). Faster switching. Quieter operation.

Disadvantages

SSRs generate heat and require heat sinks, especially with larger loads. Can be more sensitive to voltage spikes and transients. Generally more expensive than electromechanical relays. Leakage Current: SSRs have a small amount of leakage current when "off". This might be a problem for some sensitive equipment but is usually negligible for motors.

Important Considerations and Best Practices Local Codes and Regulations

Always comply with local electrical codes and regulations. This might require permits and inspections.

Qualified Electrician

Have a qualified electrician perform the wiring and installation. 120V AC is dangerous.

Safety First

Always disconnect power before working on any electrical circuits.

Overcurrent Protection

Use fuses or circuit breakers to protect the circuits from overloads and short circuits.

Wiring Diagram

Create a detailed wiring diagram and keep it for future reference.

Testing

Thoroughly test the system after installation to ensure it is working correctly and safely. Simulate different temperature conditions to verify proper operation.

Documentation

Provide clear operating instructions to the client.

Maintenance

Recommend a regular maintenance schedule to inspect the system and ensure proper operation.

Damper Type

What kind of motorized damper are you working with? Spring-return (fails closed) or power-to-close/power-to-open? This affects the wiring. Spring-return dampers are usually easier to work with because they provide a fail-safe in case of power loss.

Which Method to Choose? For most applications, the relay-based system is the best combination of cost-effectiveness, reliability, and ease of understanding. Especially if you incorporate the damper position feedback (end switches). The programmable relay offers more flexibility if you need advanced control logic.* SSRs might be suitable if you need very high switching speeds or long lifespan, but be mindful of heat dissipation.In Summary

Yes, interlocking 120V exhaust fans with motorized dampers controlled by a reverse-acting thermostat is a common requirement. The relay-based system is the most common and generally reliable solution. Ensure proper wiring, safety precautions, and compliance with local codes. Don't forget to test and document the system thoroughly. Flag for review