Air changes and heat load are two different questions
The air change calculation is the one everybody does. Take the volume in cubic feet, multiply by the air changes per hour you want, divide by sixty, and you have the airflow in cubic feet per minute. A four by four by seven foot tent is 112 cubic feet, so a full exchange every minute is 112 CFM. It is an easy number and it is almost never the number that governs.
The heat calculation is the one that governs. Every watt going into the space comes back out as heat, and moving that heat out on an air stream requires airflow proportional to the heat and inversely proportional to the temperature rise you will tolerate. The standard relationship for air at ordinary conditions is:
CFM = BTU per hour ÷ (1.08 × temperature rise in degrees F)
The 1.08 bundles up the density and specific heat of air. Watts convert to BTU per hour by multiplying by 3.412. So 530 watts of lighting and pumps is about 1,808 BTU per hour, and holding the exhaust within 5 degrees F of the intake needs 1,808 divided by 5.4, which is 335 CFM. That is three times the air change figure for the same tent, and it is why fans sized from air changes alone run hot in summer and adequately in a cold garage.
Look at what happens as the allowed rise shrinks. At 5 degrees you need 335 CFM. At 3 degrees you need 558. At 1 degree you need 1,674, and no sensible fan will do it. The airflow required rises without limit as the target rise approaches zero, which is the mathematical version of a simple truth: ventilation cannot make a space cooler than the air you are feeding it. Below a few degrees of rise, the answer stops being a bigger fan and starts being cooler incoming air.
The rated figure on the box is not what you get
Fan ratings are measured in free air, with nothing attached. Attach a carbon filter and you lose a large fraction immediately. Attach flexible ducting, and every foot of it and every bend costs more. Add a silencer and it costs more again. The losses compound rather than add, so a 25 percent filter loss followed by a 15 percent duct loss leaves 0.75 times 0.85, which is 64 percent of the rated figure.
| Restriction | Typical cost | What makes it worse |
|---|---|---|
| Carbon filter, new | 15 to 25 percent | A filter sized for the duct rather than the fan |
| Carbon filter, loaded | 30 percent and up | Dust, age, high humidity packing the carbon |
| Flexible ducting | A few percent per foot | Ribbed interior, sags, compression at the ends |
| Each 90 degree bend | Several feet of equivalent duct | Tight radius bends, bends near the fan inlet |
| Silencer | 10 percent or so | Undersized bore, placement right at the outlet |
| Undersized intake | Anything up to crippling | One small passive opening doing all the work |
The intake is the restriction people forget, and it is often the biggest. A fan can only push out what it can pull in. If the passive openings total less area than the duct itself, the fan spends its capability fighting a vacuum, the tent walls suck inward, and the delivered airflow falls far below anything on the box. Two to three times the duct free area, spread across more than one opening, is the usual guidance, and more is better.
Where carbon dioxide fits, and where it does not
Plants consume carbon dioxide during photosynthesis, and in a well-sealed space with a dense canopy they can pull the level down below what is outside. Ventilation solves that by continuously replacing the air, which is one of the reasons an exchange target exists at all alongside the heat calculation. At 60 air changes per hour the entire volume is renewed every minute, and the concentration inside stays essentially at whatever it is outdoors.
Enrichment above outdoor levels is the opposite strategy and the two do not combine. Adding gas to a space that is exchanging its air every minute simply sends it out of the duct. Enrichment is done in sealed spaces where the heat is removed by cooling rather than by exhausting, which is a different and more expensive set of equipment entirely, and it brings its own hazards since a gas that displaces oxygen in an enclosed room is a safety matter rather than a horticultural one. This calculator sizes exhaust ventilation, which is the exchange approach.
Noise, velocity and the practical limits
Air velocity through the duct is the airflow divided by the duct free area, in feet per minute. A six inch duct has about 0.196 square feet of free area, so 400 CFM through it moves at roughly 2,040 feet per minute. Above about 2,000 feet per minute duct noise becomes the dominant sound in a quiet room, and it is generated by the air itself rather than by the fan, so a quieter fan does not fix it. The remedies are a larger duct, smooth rigid duct instead of flexible, and gentler bends.
Reducing the fan speed reduces both noise and airflow, and the relationship is not proportional: sound power falls much faster than flow does, which is why a larger fan run slowly is quieter than a small fan run flat out for the same airflow. That is a genuine reason to size up beyond the calculated requirement rather than exactly to it.
Related pages
For air changes in a general room rather than a plant space, with no filter or heat load in the picture, the CFM and air changes calculator is the simpler tool. The heat you are moving comes from the fixtures sized in the grow light DLI calculator, and the humidity that ventilation carries away is the subject of the VPD calculator. If the duct run is long enough that pressure loss needs proper treatment rather than a percentage, the duct static pressure calculator handles it, and for a room where the problem is moisture rather than heat the dehumidifier sizing calculator is the right page.
Questions people ask
How many air changes per hour does a grow space need?
The exchange rate is a floor rather than a target, and in most setups it is not what sizes the fan. Something in the range of one full exchange every one to five minutes is what people commonly run, which is 12 to 60 air changes per hour, and the purpose is keeping carbon dioxide near outdoor levels and stopping stagnant pockets forming. Then you calculate the heat load, find it needs several times that airflow, and the heat number wins. Work out both and take the larger. If the heat number comes out below the exchange number, the space is either very lightly loaded or the room around it is doing the cooling.
How much airflow does a carbon filter cost?
Commonly 15 to 25 percent when new and correctly matched, and considerably more when it is undersized, loaded with dust or has been sitting in high humidity. The most frequent mistake is buying a filter to suit the duct diameter rather than the fan output, which puts a restriction in the run that no fan can overcome. Match the filter rating to the fan rating with margin, and treat a noticeable drop in airflow as the signal the carbon is spent. Manufacturer loss figures are measured clean, on a bench, with straight duct, so treat them as the best case rather than the expected case.
Do I need an intake fan as well?
Usually not, if the passive openings are large enough. An exhaust fan pulling from a space with adequate intake area creates a slight negative pressure, which is generally what people want because it means odours and air leave through the filter rather than through gaps. Adding an intake fan of similar size cancels that and pushes air out through every seam. Where an intake fan does earn its place is when the intake path itself is long or restricted, such as ducting fresh air in from another room. The more common fix is simply more passive opening area, two to three times the duct free area spread across several openings.
Why is my tent hotter than the calculator says it should be?
Check the four usual causes in order. First, the incoming air: ventilation cannot deliver air cooler than what it is pulling in, so a tent inside a warm closet is recirculating its own exhaust and the calculation never applied. Second, the intake area, which if too small collapses the delivered airflow far below the rated figure. Third, the filter, which if loaded is costing far more than the derate you entered. Fourth, the heat total, which needs to include everything plugged in inside the space and not just the light. If all four check out and it is still hot, the answer is less heat in the box rather than more air through it.
Should I run the fan at full speed all the time?
That is a temperature and humidity question rather than a fixed rule, and it is worth noting that speed controllers change more than the noise. Airflow falls roughly in proportion to speed, so a fan at half speed delivers roughly half the airflow and holds roughly twice the temperature rise at the same heat load. Sound power falls much faster than flow, which is why a larger fan running slowly is meaningfully quieter than a small fan running flat out for the same delivered airflow, and that is a good reason to size up rather than exactly to the calculated figure. What does not change with speed is the filter and duct restriction, which take their percentage regardless.