"You can't control what you can't measure, but you can't grow what you can't control."

Dr. Paul Stamets, mycologist and author of *Mycelium Running*

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Humidity Control With Continuous Exhaust

A Simple Strategy That Actually Holds RH

So you finally got your fruiting chamber dialed in.

Humidity is sitting pretty at 90%.

Then you flip on your exhaust fan for fresh air exchange and watch your RH drop like a rock.

Sound familiar?

This is the single most common automation bottleneck I see with tent growers.

You need continuous FAE to flush CO2 and deliver the fresh air your mushrooms are begging for.

But every cubic foot of air you push out takes moisture with it.

The good news?

You can run continuous exhaust and still hold humidity.

You just need a simple control strategy.

continuous exhaust fae

Why Continuous Exhaust Crashes Your Humidity

Most growers start with a timer.

Fan on 5 minutes, fan off 25 minutes.

That works okay until your mushrooms start pinning and transpiring hard.

Then you need more air movement, not less.

When you switch to continuous exhaust, you are replacing the entire air volume of your chamber multiple times per hour.

Incoming air is almost always drier than what you are pushing out.

Without a plan, your humidity drops fast and stays down.

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The Control Loop That Actually Works

Here is the strategy I recommend. It is not complicated but each piece matters.

Start with your exhaust fan on the lowest effective speed.

You want just enough airflow to keep CO2 below 800 ppm.

A small inline fan with a speed controller gives you fine adjustment.

More airflow than you need just means more moisture to replace.

Add fog on demand, not on a timer.

A humidistat or humidity controller triggers your ultrasonic fogger only when RH drops below your setpoint.

I run mine at 88% trigger with a 92% shutoff.

This is your actual control loop.

The fogger responds to what the exhaust fan takes away.

Place your fog inlet upstream of your exhaust outlet.

You want moist air to travel across your growing area before it exits.

If your fogger dumps moisture right next to your exhaust port, you are literally fogging the outdoors!

Buffer with a moisture reservoir. 

A wet perlite floor or damp towels inside the chamber act as a passive humidity buffer.

Just be sure to replace your perlite and damp towels very regularly (I'm not a fan of damp towels for this reason)...otherwise you are inviting in major contamination! 

They will not hold RH on their own with continuous exhaust, but they smooth out the dips between fog cycles.

Less swing means happier pins.

continuous exhaust fae

Duty Cycles To Start With

For a 4x4 tent with a 4 inch inline fan on a speed controller, try these starting points: 

  • Exhaust fan running continuously at 25 to 40 percent speed.
  • Fogger triggered by humidistat set to 88 percent on and 92 percent off. 
  • Adjust fan speed first, then fogger setpoints.

You are tuning two variables against each other.

Less exhaust means less fog needed.

Find the lowest exhaust speed that keeps CO2 in check and your fogger will barely need to run.

The Takeaway

Humidity control with continuous exhaust is not about overpowering dry air with more fog.

It is about matching your moisture input to your air exchange rate and letting a simple control loop do the balancing for you.

Start low on the fan speed.

Let the humidistat manage the fogger.

Buffer the dips with passive moisture.

That's the whole strategy! 

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