How to calculate the extraction and intraction in a closet without falling short
The extraction and intraction in a cabinet is calculated based on the volume of the culture, but the number that appears on the extractor box is not always the one that reaches the cabinet when you add filter, tube, bends and reductions. That is why it is advisable to work with real m³/h: the useful flow rate that remains after assembling the entire ventilation circuit.
In indoor cultivation, well-sized ventilation helps to renew air, evacuate heat, control humidity and keep the carbon filter working at its correct range. When the extractor goes too tight, the cabinet accumulates temperature, the odor can escape through joints or zippers and the intraction stops supplying the system with sufficient fresh air. When it is oversized without control, the crop can suffer sudden drops in humidity, unnecessary noise and excessive negative pressure.
The idea is not to choose the largest extractor, but to calculate the necessary flow rate and add a realistic margin. In this guide you will see the formula, the correction factors for losses, examples by cabinet measurements and how to combine extractor, filter and tube so that everything works as a system.
What does it mean to work with real m³/h
The m³/h indicate how many cubic meters of air a piece of equipment can move in one hour. In indoor ventilation, this data is used to know if the extractor can renew the air in the closet quickly enough. The problem is that many flows are measured under favorable conditions, without a carbon filter, without meters of tube, without closed curves and without diameter reductions.
When air passes through a carbon filter, it passes through filter material. When it runs through a long tube, it rubs against the walls. When there are elbows, reductions or narrow outlets, the extractor needs more pressure to move the same air. All of this reduces the final performance. Therefore, when calculating extraction and retraction in a cabinet, it is not enough to multiply volume by renewals: the result must be corrected.
A practical calculation must always differentiate between theoretical flow and useful flow. The theoretical flow rate is what you need before mounting accessories. The useful flow rate is what you really want to achieve after installing the filter, duct and outlet. To avoid falling short, the chosen extractor must have a nominal flow rate higher than the target useful flow rate.
Base formula to calculate flow
Start by measuring the inside of the closet in meters. Multiply length by width by height and you will have the volume. Then decide how many times you want to renew that volume per hour. In small and medium-sized closets, frequent renovations are usually carried out because heat and humidity concentrate quickly, especially with strong lighting or poorly ventilated rooms.
The basic formula is simple: closet volume in m³ multiplied by renovations per hour. A margin for losses is added to this result. If the assembly includes a carbon filter, flexible tube, several meters of route and curves, the margin is not optional: it is what makes the calculation a useful choice for everyday life.
Quick example: a 1.20 x 1.20 x 2.00 m closet has 2.88 m³. If you want to renew the air 60 times per hour, the base flow rate would be 172.8 m³/h. If you add 40% per filter, tube and curves, the target reaches about 242 m³/h. In that case, a 250 m³/h unit can be used for a very clean assembly, but an extractor with more margin will be more comfortable if there is heat, a long tube or a restrictive filter.
| Calculation | Formula | Example |
|---|---|---|
| Volume | Length x width x height | 1.20 x 1.20 x 2.00 = 2.88 m³ |
| Base flow | Volume x renewals/h | 2.88 x 60 = 172.8 m³/h |
| Corrected flow | Base flow x 1.40 | 172.8 x 1.40 = 242 m³/h |
Common losses: filter, tube, bends and outlet
The losses are not the same in all assemblies. A cabinet with the extractor on top, a well-sized filter and a short pipe leading to a straight outlet does not work the same as one with ten meters of duct, several sharp bends and a final reduction. That is why it is advisable to apply cumulative correction percentages.
The carbon filter is usually the main leak. The more compact the filter is or the closer it is to its flow limit, the more effort the extractor will have to make. That is why it is recommended that the filter have a flow rate compatible with or slightly higher than that of the extractor in use. Pure Filter filters, for example, are offered in different capacities, from 300 m³/h models to higher options, making it easy to choose the appropriate diameter and flow rate for each installation.
The tube matters too. The flexible aluminum tube is convenient because it adapts to the route and is easily installed, but each added meter and each curve reduces efficiency. The ideal is to keep the route as short and straight as possible, stretch the tube to avoid folds and not strangle the diameter at the outlet.
| Element | Guidance loss | How to reduce it |
|---|---|---|
| carbon filter | 20-30% | Choose filter with sufficient flow and clean pre-filter |
| flexible tube | 5-10% for long haul | Use as little travel as possible and stretch it well |
| closed curves | 10-20% for demanding curve | Make smooth curves and avoid unnecessary elbows |
| Reductions | 10-25% | Maintain constant diameter whenever possible |
| Narrow exterior exit | variable | Do not block grates or crush the duct |
How to choose the extractor according to the result
Once you have the corrected flow rate, it is time to choose the extractor. For small or medium-sized installations, a compact equipment may be sufficient if the distance is short. For installations where noise matters, a soundproof exhaust fan such as the ISO-MAX Professional Air Extractor It is interesting because it is designed to move air with less acoustic disturbance and has different flow models.
If you need more margin or a system with a professional approach, the Duxfan Foam Box Extractor offers a wide range of flow rates, from 250 m³/h models to high-capacity options. This variety allows sizing from contained cabinets to rooms or assemblies with greater resistance per filter, long ducts or more demanding environmental control needs.
The right choice does not depend only on the size of the closet. The power of the lighting, the temperature of the room, the type of crop, the distance to the exit and the acceptable noise level also play a role. If the calculation is between two models, it is usually better to choose the superior one and regulate it, instead of buying a fair one that always has to work at its maximum.
Intraction: passive, active and pressure balance
Extraction and intraction in a closet must work together. Extraction draws out hot, humid or odor-laden air; intraction allows new air to enter. If not enough air enters, the extractor forces the cabinet, the walls sink too much and the actual flow rate drops. If too much air actively enters and exceeds the extraction, you can lose negative pressure and facilitate odor leaks.
In small cabinets, passive intraction is usually sufficient if the air inlets have a large free surface and are well placed. As a practical reference, the total inlet surface area should be several times larger than the exhaust outlet r, because the air enters without mechanical assistance. If the inlets are small, covered or have a very dense mesh, the extractor will not be able to perform well even if the nominal flow rate is correct.
Active intraction is used when the closet is large, the room has little fresh air, or the system needs to feed multiple points. In this case, the inlet fan must move less flow than the exhaust fan to maintain negative pressure. A common setting is for the active intake to work below the extraction, leaving the carbon filter to remain the preferred path for the outgoing air.
Guidance table by closet size
The following table serves as a starting point for calculating the flow rate. It is designed for cabinets with normal heights, a carbon filter and a reasonably short assembly. If you use high-power LEDs in a hot room, if you have many meters of tubing, or if the air comes out through a very restrictive grille, go up the flow rate one step.
Do not take the table as a closed rule. It is used to understand ranges and avoid common errors, but the final calculation must be made with the actual measurements of the cabinet and the assembly losses. In ventilation, a small regulation margin usually gives better results than going to the limit from the first day.
| Wardrobe | Volume approx. | Base flow | Recommended corrected flow rate |
|---|---|---|---|
| 60x60x160cm | 0.58 m³ | 35-60 m³/h | 100-150 m³/h |
| 80x80x180cm | 1.1 5 m³ | 70-115 m³/h | 150-250 m³/h |
| 100x100x200cm | 2.00 m³ | 120-200 m³/h | 250-350 m³/h |
| 120x120x200cm | 2.88 m³ | 175-290 m³/h | 350-500 m³/h |
| 150x150x200cm | 4.50 m³ | 270-450 m³/h | 550-750 m³/h |
| 240x120x200cm | 5.76 m³ | 345-575 m³/h | 750-1000 m³/h |
Carbon filter: do not choose it to the limit
The carbon filter must accompany the extractor. If the filter admits less flow than the extractor, it can become a bottleneck: performance drops, noise increases and effectiveness against odors is reduced. The Pure Filter Carbon Filters They include capacities such as 300, 350, 500, 680, 900, 1125, 1420, 1875 and 2500 m³/h depending on the variant, so it is advisable to choose the model that fits the diameter and flow rate.
The practical rule is simple: the maximum flow rate of the filter must be equal to or greater than the flow rate at which the extractor will work. If you use a regulator and the extractor rarely works at 100%, you can adjust the assembly better. If the extractor always works at maximum and the filter is tight, any additional tube loss is much more noticeable.
You must also keep the pre-filter clean. The accumulated dust increases the resistance to the passage of air and reduces the actual m³/h. Checking it during cultivation and changing it when necessary is a simple way to recover performance without changing the entire system.
Tube and diameter: the route rules
The diameter of the tube should match the mouth of the extractor and filter whenever possible. The Flexible Aluminum Tube It is available in common diameters such as 102, 127, 152, 203, 254 and 315 mm, which allows the system to be adapted to different extractors and filters. The most common mistake is to use reductions because you already have a smaller tube: it works, but it increases the loss and can generate noise.
To improve performance, leave the tube as straight as possible. Avoid 90 degree bends close to the extractor outlet, do not crush the duct against walls or zippers and check that the exterior outlet is not blocked. In long assemblies, a larger diameter or an extractor with more pressure available can make a difference.
It is also important to seal joints. Leaks before the filter can allow odor to escape without passing through activated carbon. Leaks after the filter reduce efficiency and can create turbulence. Clamps and aluminum tape help keep the circuit airtight.
Complete example: wardrobe 120 x 120 x 200 cm
Imagine a closet measuring 1.20 x 1.20 x 2.00 m. The volume is 2.88 m³. For a crop with powerful lighting, we propose 70 renewals per hour as a base objective: 2.88 x 70 = 201.6 m³/h. Now we add losses: carbon filter, flexible tube, a bend and exit to the outside. If we apply a 45% margin, the target flow rate is about 292 m³/h.
With that number, a 250 m³/h system may be just enough if the pipe is long or the room is heated. An extractor with a higher regulated flow rate can work more comfortably. In filters, a 300 m³/h model could fit in a short assembly, but if the chosen extractor exceeds that range, it is advisable to move to a higher capacity. This is where looking at the extractor, filter and tube together avoids unbalanced purchases.
If you want a closed solution for small spaces, the Grower Extraction Kit 100mm It already combines a 100 mm extractor, 100 x 200 mm Pure Filter of 300 m³/h, 5 m Combiconnect tube and aluminum tape. For larger cabinets or longer runs, it will be best to size each component separately.
Errors that reduce flow without being seen
The first mistake is placing the filter, the extractor and the tube with different diameters without need. Each reduction forces the air to accelerate, generates turbulence and makes the extractor work worse. The second mistake is rolling the excess tube on top of the cabinet. Although it may seem comfortable, that extra journey becomes a permanent loss.
Another common problem is removing the air from the closet but leaving it inside the same closed room. In this case, the crop breathes hot, humid air again. Whenever possible, it is advisable to evacuate to another room, a window, a prepared exit or an area with sufficient renovation.
The air intake is also often forgotten. Opening just a small grate is not always enough. If the cabinet deforms too much inwards, there is a whistling noise at the entrances or the extractor fan changes its sound when you close the door, there is probably a lack of intraction. Before purchasing another extractor, check the inlets, filter, pre-filter and tube route.
How to adjust the system after installing it
The calculation helps to choose well, but the final adjustment is made by observing the closet. With the system working, a slight negative pressure should be felt: the walls sink inwards a little, but without collapsing. The air should come out through the filter and not through zippers, seams or holes. If there is a smell outside the cabinet, check the seal and filter before simply turning up the power.
Control temperature and humidity with the light on and off. If the temperature rises too much with the extractor at maximum, there is a lack of flow, too much travel or the entrance room is already hot. If the humidity drops excessively, the extractor may be renewing too much for that phase or you may need to regulate it by time periods.
In growth, a stable and moderate renewal may be of interest. During flowering it is usually more important to maintain negative pressure and control humidity. In drying, the objective changes: to renew without creating an excessive direct current on the flowers. For this reason, an adjustable or margin extractor allows you to adapt the system without disassembling it.
Recommended products depending on the type of installation
For silent mounting, the ISO-MAX Professional Air Extractor It is an option designed to reduce noise thanks to its soundproof design. Their models cover different flow rates and some formats incorporate several speeds, useful when you want to adjust the flow without always depending on an external regulator.
For installations with greater demand, the Duxfan Foam Box Extractor offers a very wide range, with models from 250 m³/h to professional flow rates. It is an interesting option when you are looking for power, acoustic insulation and margin for filter, tube and accumulated losses.
To complete the circuit, combine the extractor with Carbon filters, Ducts and accessories and, if you prefer a ready-made solution, check the Ventilation kits. The key is that all elements share diameter and flow range.
Checklist before buying
Before choosing, write down the actual measurements of the closet, the diameter you want to use, the distance to the exit and the number of curves. Then calculate the volume and apply renewals per hour. Finally, add losses and compare the result with the flow rate of the extractor and filter.
If the closet is in a hot room, add margin. If the route is long, add margin. If the filter is tight, change to a higher filter. If noise is a problem, prioritize soundproof extractors, well-stretched tubes and fewer bends instead of forcing a small equipment at 100%.
The ideal system is one that moves the necessary air without working to the limit. With margin, you can regulate. Without margin, you can only accept the problem or change parts.
- Measure the length, width and height of the closet in meters.
- Calculate volume and renewals per hour.
- Apply margin per filter, tube, curves and outlet.
- Choose extractor by corrected flow rate, not by base volume.
- Choose a filter with a flow rate equal to or greater than actual use.
- Maintain the constant diameter between extractor, filter and tube.
- Check negative pressure after installing.
Frequently asked questions
How many times per hour do you have to renew the air?
It depends on the heat generated, the humidity and the size of the closet. As a practical basis, many setups work between 30 and 60 renewals per hour, increasing the margin when there is strong lighting, high temperatures or long extraction paths.
Should the intraction have the same flow rate as the extraction?
Not necessarily. If you use active intraction, it usually moves less air than extraction to maintain negative pressure. In small closets, a wide passive intraction is usually sufficient.
Can I use a filter with a lower flow rate than the extractor?
It is not recommended. The filter can limit air flow, increase flow loss and reduce system efficiency. It is best that the filter equals or exceeds the working flow rate of the extractor.
What happens if I use a smaller tube?
The system will work worse: there will be more resistance, more noise and less useful flow. Whenever you can, keep the same diameter of the extractor, filter and tube.
Is it better to oversize the extractor?
It is better to have an adjustable margin than to go just. An uncontrolled fan that is too large can lower the humidity too much or generate noise, but a regulated one offers more flexibility in the face of heat, restrictive filters or long tubes.
