Sizing grow tent ventilation: fans, filters and ducting

Ventilation is the part of a setup people size by guesswork and then spend a whole season fighting. It is one multiplication and one adjustment, so it is worth doing properly.

What the fan is for

Four jobs, and they matter in this order in a Brisbane summer:

• Removing heat. Every watt going into a light comes back out as heat.
• Controlling humidity. Plants transpire constantly. Without exchange, humidity climbs until you have a fungal problem.
• Replenishing CO2. A sealed room runs itself out of carbon dioxide surprisingly fast, and growth simply stops.
• Negative pressure. Slightly lower pressure inside the tent means air is drawn in through the intakes rather than pushed out through the seams, so odour and dust go through the filter rather than around it.

The basic sum

Work out tent volume in cubic metres, then decide how often you want to replace all of it.

Volume = length x width x height, all in metres.

A 1.2 x 1.2 x 2.0 metre tent is 2.88 cubic metres.

The usual target is a complete air change every one to three minutes. One minute is the safe assumption in Queensland. So:

2.88 cubic metres per minute x 60 = 173 cubic metres per hour.

Most fans sold here are rated in cubic metres per hour. If you see a fan rated in CFM, multiply by 1.7 to get cubic metres per hour.

Now add the losses

That 173 figure is the fan running free with nothing attached to it, which is not how you will use it. Every restriction reduces real-world flow.

• Carbon filter: add 25 to 40 percent. A filter is a wall of packed carbon and the fan has to pull through it.
• Ducting: add roughly 5 percent per bend, and more for long runs. Flexible ducting is worse than rigid because the corrugations create turbulence.
• Silencer, if you run one: add another 10 percent.

For our 1.2 metre tent with a filter and a couple of bends, 173 becomes around 240 cubic metres per hour as a realistic requirement.

Then buy bigger and turn it down

This is the part that saves people money in the long run. Buy a fan comfortably above your calculated figure and run it on a speed controller.

A fan running at 60 percent is quieter, uses less power, lasts longer and gives you headroom for the worst week of summer. A fan running flat out just to keep up has nothing left when it hits 38 degrees in February, and it is loud.

For a 1.2 x 1.2 tent, a 150mm fan on a controller is the sensible buy. For a 1 x 1, a 125mm. For anything approaching a small room, 200mm.

Matching the filter

The filter must be rated at or above the fan output, not below it. An undersized filter is a restriction and the carbon does not get the contact time it needs to work.

Match the collar size to your fan and ducting -- a 150mm fan wants a 150mm filter. Filter life depends on humidity more than anything else, and running one in consistently high humidity shortens it considerably.

Intake

Passive intake means opening the tent vents and letting the exhaust fan pull air in. Simple, quiet, and fine for most tents. Rule of thumb: total passive intake area should be around three to four times the area of your exhaust duct.

Active intake means a second fan pushing air in, and you want it slightly weaker than the exhaust so you keep negative pressure. Worth it for bigger rooms and for pulling cooler air from somewhere specific.

Ducting, done properly

• Keep runs as short as you physically can.
• Every bend costs you flow. A gentle sweep is much better than a right angle.
• Pull ducting taut. Slack corrugated duct is a flow killer and a noise generator.
• Insulated ducting is worth it purely for noise if the tent is anywhere near where people sleep.
• Seal joins with duct tape properly -- leaks upstream of the filter defeat the whole point.

Brisbane-specific

Summer here is the design case, not winter. If the room your tent sits in hits 34 degrees, no amount of air exchange gets the tent below that -- you are exhausting hot air and drawing in hot air. Ventilation moves heat, it does not create cold.

If that is your situation, the answers are drawing intake air from a cooler part of the house, running lights at night when ambient is lower, or accepting that you need cooling rather than just extraction.

What we stock

We keep HVC Dura inline fans from $175, HVC carbon filters in 150mm and 200mm, and boxed five metre silver ducting from $11.

If you would rather not do the sum, bring your tent dimensions into the shop on Ipswich Road and we will size it with you. It takes about two minutes and it is a lot cheaper than buying the wrong fan twice.