Quick answer: A temperature and humidity fan controller reads a sensor and switches or speeds up a fan when temperature or RH passes a setpoint, then slows or stops it when conditions return, with a gap (hysteresis) so it does not cycle constantly. Choose one that matches your fan (on/off, AC speed control, or 0–10 V for EC fans), handles the motor's current, and has a sensor you can place where the plants, goods or equipment are. Remember the limit: a fan only lowers humidity if the air it brings in is drier than the air it removes. In humid weather, you need a dehumidifier.
Quick answer
- Works by thresholds plus hysteresis: e.g. fan on at 70 % RH, off at 65 %.
- Match output to fan: relay (on/off), triac speed control (AC), 0–10 V (EC).
- Fans can't dry the air when the replacement air is humid.
What these controllers do
| Function | Typical behaviour |
|---|---|
| Temperature control | Fan speeds up or switches on above a temperature setpoint |
| Humidity control | Fan speeds up or switches on above an RH setpoint |
| Combined | Fan responds to whichever is out of range first |
| Minimum speed | Fan runs slowly all the time for air exchange |
| Day/night or lights-on/off modes | Different setpoints for each period |
| Alarms and logging | High/low alarms, data export, app or Wi-Fi access on "smart" models |
| Extra outputs | Switch a heater, humidifier or dehumidifier as well |
Settings that work
- Hysteresis (deadband): set a gap of about 3–5 % RH or 1–2 °C between on and off, so the fan does not switch every few seconds.
- Minimum speed: a low base speed keeps air moving and avoids stagnant pockets.
- Separate night setpoints: at night or lights-off, RH rises as temperature drops; a lower RH setpoint at night prevents condensation.
- Ramp, don't jump: on speed-controlled fans, a proportional band avoids sudden pressure changes and noise.
Matching the controller to the fan
| Fan type | Controller output | Check |
|---|---|---|
| AC fan, single speed | Relay (on/off) | Relay current rating ≥ fan running and start-up current |
| AC fan, speed-controllable | Triac or transformer speed control | Fan maker says it can be speed-controlled; minimum speed setting |
| EC fan | 0–10 V or PWM signal | Signal type and range match; fan has its own power supply |
| Large or three-phase fans | Controller drives a contactor or VFD | Use a contactor or variable-frequency drive, not the controller directly |
Undersized relays and incompatible speed controls are the most common causes of controller failure and humming fans.
Sensor placement
- At plant, product or equipment level, not next to the fan or a heater.
- Shaded from lights and sun.
- Away from the intake, where incoming air would give a false reading.
- One sensor per zone; long cables or wireless sensors for remote points.
For greenhouses, see greenhouse humidity sensor placement.
When a fan cannot fix humidity
A fan replaces indoor air with replacement air. It lowers humidity only when the replacement air holds less water:
| Indoor | Replacement air | Fan effect on humidity |
|---|---|---|
| 24 °C, 70 % RH | 18 °C, 60 % RH (cooler, drier) | Lowers it |
| 24 °C, 70 % RH | 28 °C, 70 % RH (humid summer) | Raises it |
| 20 °C, 85 % RH (night) | 22 °C, 75 % RH (warm, humid night) | Almost none: both hold about 12.4 g/kg |
Compare water content (g/kg), not RH, to judge it. When the replacement air is humid, the answer is dehumidification: a dehumidifier removes water whatever the outside weather. Many controllers can switch a dehumidifier on a second output, with the fan held at minimum speed while it runs. See ventilation vs dehumidification and our industrial dehumidifiers.
Commissioning
- Check the sensor against a reference hygrometer.
- Run the fan through its whole speed range and check it starts reliably at minimum speed.
- Test each setpoint by warming or humidifying the sensor gently.
- Simulate a sensor failure and check what the controller does (fan to full speed is the usual safe default).
- Record the settings.
FAQ
Frequently asked questions
How does a humidity fan controller work?
It reads RH and switches or speeds up the fan above a setpoint, then slows or stops it below a lower setpoint.
What hysteresis should I use?
About 3–5 % RH or 1–2 °C between on and off points.
Can a fan controller lower humidity?
Only when the replacement air is drier than the indoor air. In humid weather, it can raise humidity.
Can I use a fan controller with an EC fan?
Yes, if it has a 0–10 V or PWM output that matches the fan.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- University of Alaska Fairbanks Cooperative Extension, Controlling the greenhouse environment (sensor placement).
