In short
- The CO2 comes from the substrate and the mushrooms, not people, so room-size and occupancy calculators give the wrong answer.
- Measure the load: close the room at peak flush, fan off, and time how fast CO2 rises.
- Fresh air needed (m³/h) = room volume × rate of rise (ppm per hour) ÷ (target ppm − outdoor ppm).
- Then add headroom: installed fans move 20–40% less than their rating, and a fan sized to run continuously has no reserve. Aim for a fan that runs about half the time.
- Every exhaust fan needs an inlet. Without one it moves far less than its rating.
Ask how big an exhaust fan a mushroom room needs and you will be told a number of air changes per hour, or a fan “one size bigger than the last one”. Neither is based on the thing that matters: how fast this room, with this crop, produces CO2.
The good news is that you do not need a table from a textbook. You can measure your own room's CO2 load directly, and the fan size follows from one line of arithmetic.
Why fan calculators for homes and offices don't apply
A standard fan size calculator works from the number of people in a room, because in a home or office people are the CO2 source. In a mushroom room the source is the substrate and the fruit bodies respiring, and that source changes: it rises as the flush develops, differs with the number of bags and their age, and increases with temperature. The only reliable way to know it is to measure it.
Step 1: measure how fast CO2 rises
- Pick the right day: the peak of a flush, when the room is fullest and the load is highest. A fan sized for that day handles every other day.
- Start with the room ventilated, with CO2 near your fruiting level (for example, under 800 ppm).
- Switch the fan off, close the door, and note the CO2 reading and the time.
- Wait 20–30 minutes and note the reading again. Keep the test short: you are measuring, not letting the crop sit in high CO2.
- Work out the rate of rise in ppm per hour. A rise of 600 ppm in 30 minutes is 1,200 ppm per hour.
You need an NDIR meter placed among the bags for this (see how to choose a CO2 meter).
Step 2: the formula
Fresh air needed (m³/h) = V × R ÷ (Ctarget − Coutdoor)
V = room volume in m³ · R = rate of rise in ppm per hour · Ctarget = the level you want to hold · Coutdoor = outdoor CO2, about 420–450 ppm
It comes from a simple balance: with the fan off, all the CO2 the crop produces stays in the room, so the rate of rise tells you how much it produces. With the fan on, the room settles where fresh air removes CO2 as fast as the crop makes it.
A worked example
| Step | Value | Note |
|---|---|---|
| Room | 6 × 4 × 3 m = 72 m³ | An example room, not a typical one |
| Measured rise | 600 ppm in 30 min = 1,200 ppm/h | Peak flush, fan off |
| Target | 900 ppm | Middle of a Fan ON 1,000 / Fan OFF 700 setting |
| Outdoor | 450 ppm | Difference: 450 ppm |
| Fresh air needed | 72 × 1,200 ÷ 450 = 192 m³/h | The minimum, running all the time |
| Allow for installation losses | 192 ÷ 0.7 ≈ 275 m³/h | Ducts, mesh and grilles cost 20–40% |
| Headroom to run in bursts | 275 × 2 ≈ 550 CMH (≈ 325 CFM) | Fan runs about half the time at peak |
Your numbers will be different. That is the point: the example shows the method, and the answer comes from your own measurement. To convert, 1 CFM is about 1.7 CMH.
Why aim for a fan that runs half the time
A fan sized exactly to the minimum has to run continuously at peak flush just to hold the level, and it cannot recover after the door has been open for harvest. It also means the room is being ventilated all the time, which is the fastest way to dry the crop.
A fan with roughly twice the required capacity clears the room in short bursts and then stops. Total fresh air is the same, since the crop decides that, but the room spends more time sealed and humid between bursts, and there is reserve for the heavy days.
Do not go far beyond that. A much larger fan drops CO2 and humidity in a rush, and in a cooled or heated room it swings the temperature with every burst.
Mistakes that undersize the fan
- Measuring on a quiet day. Between flushes the load is a fraction of the peak. Size on the peak.
- No inlet. An exhaust fan can only remove air that can get in. Give the room an inlet with a filter or insect mesh, about as large as the fan opening. A sealed room with an exhaust fan and no inlet mostly just spins the blades.
- Trusting the box rating. The CMH on the box is measured with no duct and no mesh. Installed, expect 20–40% less.
- Leaky test room. If the room leaks during the test, the measured rise is lower than the real load. Another reason for headroom.
How to tell if the fan you have is the right size
Put the fan on a CO2 controller and watch it through a peak flush. If it runs in short bursts and the room stays inside the band, it is right. If it runs almost continuously and CO2 still drifts above the ON threshold, it is too small: you need more air-moving capacity, not a different threshold.
Let the room tell you
VentPlus logs CO2, temperature and humidity through the whole crop, so you can see the rate of rise and the fan's duty cycle for yourself, and switches the fan at the thresholds you set. Fans up to 3 A / 700 W plug straight in; larger fans run through a contactor. ₹8,499, made in India.
See VentPlus for mushroom farmsThe short version
Measure how fast CO2 rises in your room at peak flush with the fan off. Multiply by the room volume, divide by how far above outdoor air you want to hold, and you have the minimum fresh air. Allow for installation losses, then size so the fan runs about half the time. Give it an inlet. Then let the CO2 reading, not a timer, decide when it runs.
Sources
- Directorate of Mushroom Research (ICAR), Solan. dmr.icar.gov.in
- Stamets P., Growing Gourmet and Medicinal Mushrooms.
- Fan airflow conversion: 1 CFM = 1.699 m³/h.