How to control inline HVAC DUCT Fan

Very new to SmartThings, so hoping someone can guide me on a solution to controlling my in-line Duct Fan.

I have a home that has an installed inline Duct Fan (Fantech FG6 -, that is controlled manually via a Variable speed switch ( The problem is I have to manually turn on the fan when I want to have it come on, and then leave it running (or run downstairs every time the HVAC comes on and off). It’s purpose was to provide extra airflow to the downstairs storage area, which works fine if/when it’s on. I just want to automate the turning on/off of it.

I was hoping to control this fan via SmartThings, by changing the existing variable speed knob switch with a controllable relay (I’m thinking the FS20Z-1 ??), and then monitoring my HVAC system for when the fan motor goes out of Idle (either heating or cooling). I have a Honewell THX9421R5021WW thermostat (Redlink connection), so am using the Honeywell 9000 Thermostat device driver to talk to it (I’ve tried also the Honeywell VisionPro Thermostat). SmartThings controls it just fine, but when I try to use CoRE to monitor the ThermostatOperatingMode to determine when it changes from Idle, I ran into issues. The problem is, if I manually reset the cooling (or heating) setpoint, SmartThings receives a ThermostatOperatingMode event such as “Cooling to 72 F”, and my piston is triggered. Great. BUT, if I simply leave the thermostat at a given setpoint, and it comes on and off to maintain that setpoint, NO event is ever posted to Smartthings when the air comes off or on. SmartThings always reports the status as Idle. I only get an event posted IF I reset the setpoint to make the air (or heating) come on, not when it comes on/off by itself to maintain the setpoint. So, monitoring for change of Idle won’t work.

So, my current thought is to put some sort of relay(?) on the furnace system (or Thermostat, which apparently talks to an Equipment Interface Module sitting next to the furnace), to monitor when the blower motor is on or off. This is where I am totally lost, as I know nothing about relays, etc. Can anyone suggest to me a way of simply monitoring when the blower comes on and off, so I could control the duct fan? What sort of “relay” might I need to wire into the furnace? Also, is the above FS20Z-1 the correct device/relay to replace my Variable Speed relay with? I was thinking of some sort of “dry contact” monitor on the furnace, that I would monitor for, then send a signal to the FS20Z-1 to turn on/off the fan?? But again I’m lost with relays, contacts, etc, as to what I would need.

Sorry for the long question, but wanted to provide all the relevant details to hopefully help someone help me.


So with a quick read of this i would think you could solve this problem by using a relay that monitors power consumption. That way when it kicks out of idle the power consumtion goes up. I do this with my washer and dryer. If the fan plugs into a socket then it couldl be super simple by just adding a smart plug. But im assuming its a wired fan so a relay would be the way to go. How do you have the setup wired?

On top of that you could then create a virtual switch that turns on and off based on power consumption so you can easily see in app its its on or off or ask alexa or google.

The variable control switch is hard wired in, so I’d have to unwire it. Can you give info on the power monitor you used? Where would I wire it in (at the furnace, or at the breaker)?

old thread but i am working on a similar issue with an inline duct fan. I want it to come on based on the hvac. I have an aeon HEM connected in my fuse panel reading power consumption and a smart plug connected to the duct fan. were you able to find an app to turn on switch based on power consumption?

Hi Dave,

I did mine a slightly bit different, but it works great. Instead of
monitoring power consumption, I tapped off the available Humidifier lead in
the furnace (I don’t have a humidifier, so these were available). The
Humidifier lead is a simple contact, that was unused. So I purchased this

Wired 2 small wires, one from the positive (+) screwdown block for the
Humidifier lead, and one from the negative (+) screwdown block for the
Humidifier lead, to the two ports on the left of this relay (there is an
instruction manual with it for wiring). In a furnace, the Humidifier
contact is energized when the furnace comes on, and de-energized when it
goes off. On your furnace board, you’ll look for a screwdown block with 7
or 8 wires going to it, and one of them will be labeled HUM+ and one HUM-

I then ordered this smart dry contact sensor:

On the relay, you wire from the Command to the sensor, and then either from
the NO (normally open) or NC (normally closed) plug to the sensor,
depending upon what state you want to indicate (I used NC, but it really
does not matter, your just watching for a change in state, so you’ll write
your little smartapp based on whatever is changing NO–>NC or NC–>NO).

Lastly, I then ordered this smart switch:

This replaced a rotary knob switch that had been installed, to control the
duct fan.

I have a Samsung Smartthings hub, that I use for home automation. I add
both the smart Sensor and the Smart fan switch (I named them "HVAC Sensor"
and “HVAC fan”). If you have the Smartthings device, you have to install
the CoRe Smartapp. Once installed, I just went into it and created a new
Basic Core routine (called a Basic Piston, it’s all menu driven), that says
"If “HVAC Sensor” -changes- to CLOSED, Then -Turn Off- “HVAC fan”. I named
this routine "Heating Off. I made a corresponding routing called “Heating
On”, that says "If “HVAC Sensor” -changes- to OPEN, Then -Turn On- “HVAC

Once I did the above, it’s been working perfectly. Summary:

  1. Find the HUMidifer leads on your furnace board
  2. Wire + and - to Relay
  3. Wire Sendor to Relay
  4. Install Smart Switch
  5. Assuming you have Smartthings - Install Core, create 2 Basic Pistons
  6. Have some hot cocoa and celebrate.

Hope this helps!


Ok thanks. My duct fan can plug into a smart plug so i had done that and I had the extra energy monitor so i put that in the fuse panel. I will have to work something out but this looks like a way i could possibly run another accessory off that. we have 2 carrier infinity systems so integrating with smartthings isn’t really possible

I actually als just was able to set up WebCore and I think I have this working now. Time to test it

I want to do something similar, I want to control a 600 CFM cooktop ventilation inline blower via a home automation system.

I can’t get information from manufacturers on how to vary speed (I’m ok with off/low/medium/high) - Best told me they don’t have multiple taps and I should vary the voltage but my reading suggests that can easily damage the motor.

Anyone do this before?