Extraction and the Irish Problem

What You Need to Know

Two lessons ago you learned the air does half the work. Now we deal with how the air actually gets in and out of the tent — and why that’s harder in Ireland than almost anywhere the big grow channels are filming.

Extraction is the system that pulls stale, warm, damp air out of the top of your tent and lets fresh air fall in through vents at the bottom. Done right, it does three jobs at once: it removes the heat your light dumps in, it carries off the moisture your plant exhales, and it keeps the CO2 around the leaves topped up. Done wrong, it’s the single most common reason a tent ends up with the wrong temperature, the wrong humidity, and therefore the wrong VPD. The fan you bought is upstream of every number from the last two lessons.

Negative Pressure — Why the Tent Should Suck In Slightly

A properly running tent sits under slight negative pressure. The extraction fan pulls air out faster than it leaks in, so the canvas walls draw inward a touch and air is forced in through your intake vents in a controlled path: in low, across the canopy, up and out through the fan. That controlled path is the whole game — it means you decide where fresh air comes from and where stale air goes.

Negative pressure does a second favour: it stops smell escaping through gaps, because air is always being pulled in through any opening rather than pushed out. Lose negative pressure — fan too weak, or too strong and the intake can’t keep up — and you lose control of the airflow path and the smell at the same time.

Sizing the Fan — The Maths Nobody Does

Most growers buy whatever the forum recommended or whatever was on sale, then spend three months wondering why the humidity won’t behave. Here’s the bit that saves you that.

The rule of thumb: exchange all the air in your tent about once a minute — sixty times an hour. So the fan’s rating in cubic metres per hour (m³/h) should be roughly sixty times the tent’s volume in cubic metres. But the headline number on the box is a fantasy, because the filter and the ducting eat a big chunk of it.

Worked example for a 1.2m × 1.2m × 2m tent:

1. Volume: 1.2 × 1.2 × 2 = 2.88 m³.
2. One change a minute: 2.88 × 60 ≈ 173 m³/h — the bare minimum.
3. Add for the carbon filter: pulling air through carbon costs 20–30%. Now you need ~220–250 m³/h.
4. Then subtract ducting losses: a straight run costs 5–10%, and every 90-degree bend costs another 10–15%. Six metres and two bends can take 20–30% off the top.

So for that tent you spec a fan rated around 250 m³/h or a bit more, knowing it’ll deliver maybe 175 once the filter and bends have taken their cut. A 150mm (6-inch) inline fan handles it comfortably. A 100mm (4-inch) fan suits a 60cm × 60cm tent.

Seb’s Corner — why bends cost so much. Air moving through a duct has momentum. A straight run lets it keep that momentum, so the fan only fights friction along the walls. A 90-degree bend forces the whole airstream to change direction at once — it crashes into the outer wall of the bend, turbulence forms, and the fan has to spend energy re-accelerating air that’s just been stopped. That’s why one sharp elbow can cost more airflow than several metres of straight pipe. The practical rule writes itself: keep the duct short, keep it straight, and where you must turn, use the gentlest sweep you can. A fan rated 250 m³/h on paper is delivering what physics leaves it after the route home.

The Irish Problem — You’re Fighting Damp Air to Begin With

Here’s where Ireland makes this lesson its own. A grower in a dry climate starts the day at maybe 40% ambient humidity and uses extraction mainly for heat. You don’t. Irish ambient humidity sits at 65–75% just from the weather outside the window. The air your fan pulls in is already nearly as damp as the air it’s pulling out.

That changes the spec. Your extraction has to work harder to drop humidity, because it can’t simply swap wet tent air for dry outside air — the outside air is wet too. In flower, when you need humidity under 55%, raw extraction often can’t get you there on its own, and you’ll lean on a dehumidifier to finish the job. This is why “just open a window” is worse than useless here: you’d be inviting damp Irish air straight into the tent, with no control over temperature, humidity, or smell. A window isn’t ventilation. It’s a humidity leak with a view.

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How To Apply This

1. Measure your tent volume — length × width × height in metres, multiplied together.
2. Do the sizing maths: volume × 60, then add 30% for the filter. That’s your target rated airflow before ducting.
3. Buy to your tent size, not bigger. A 150mm fan for a 1.2m tent; a 100mm for a 60cm tent. Oversizing just makes noise and heat the tent can’t use.
4. Keep ducting short and straight. Filter inside the tent at the top, shortest run to the fan, gentle sweeps instead of sharp elbows.
5. Set the speed for slight negative pressure. The canvas should draw inward a little. If the walls are sucking in hard and folding, you’re over-extracting — back off. If they’re slack, increase speed.
6. Plan for a dehumidifier in flower. In an Irish winter especially, extraction alone often won’t pull humidity under 55%. Run a small dehumidifier during lights-on to finish the job, and recheck your VPD afterward.

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Watch Out For

Extraction is where money gets spent in the wrong direction.

The jet engine. Someone buys a 250mm fan for a 60cm tent because the spec sheet looked impressive. The tent sucks in so hard the canvas folds and eats growing space, the intake can’t keep up, and the fan screams. They’d have been far better with a properly sized 100mm fan running at 70%. Bigger is not better — matched is better.

The fantasy box number. The cheap fan with the huge headline figure and six metres of thin plastic ducting won’t deliver half its label. Restrictive duct, sharp bends, weedy motor. Spec for real-world delivery, not the number on the cardboard.

Cranking the fan to fix humidity, then crashing everything. Turn extraction to maximum to fight damp and you’ll also drop the temperature several degrees and maybe overshoot humidity down to 35%. Now the leaves crisp and the plant’s cold-stressed. Extraction is a system — you can’t move one number without the others following. One change, then read the response.

Thinking the back room is far enough. Cannabis in flower smells, and that smell travels through walls, under doors, into lofts. In Irish semi-detached houses and apartments, your wall is someone else’s wall. A maintained carbon filter and negative pressure aren’t optional extras — they’re the baseline that keeps the whole operation quiet.

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Quiz

1. What is negative pressure in a grow tent, and name one benefit of it beyond airflow control.
2. A tent measures 1m × 1m × 2m. What is the bare-minimum extraction rating (one air change a minute) before you account for the filter?
3. Roughly how much airflow does a carbon filter cost you, and why must you add for it when sizing the fan?
4. Why does a single 90-degree bend in your ducting cost so much airflow?
5. Why is “just open a window” a particularly poor humidity strategy for an Irish grow?

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Sources

Chapter 7, The Grower’s Guide (book draft) — extraction sizing maths, filter and ducting losses, negative pressure, fan-to-tent matching, and the Irish ambient-humidity context. Airflow physics (momentum loss at bends) is general engineering knowledge; no paywalled sources used. Ties into the GGB VPD Calculator.

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Answer Key

1. Negative pressure is when the extraction fan pulls air out slightly faster than it leaks in, so the tent walls draw inward and intake air follows a controlled path. Beyond airflow control, it stops smell escaping (air is pulled in through gaps rather than pushed out).
2. 1 × 1 × 2 = 2 m³; 2 × 60 = 120 m³/h before the filter.
3. Around 20–30%. The carbon resists the airflow, so the fan delivers less than its rating once the filter is in line — you size up to compensate.
4. Because the bend forces the whole airstream to change direction at once, creating turbulence as air crashes into the outer wall; the fan then spends energy re-accelerating air that was effectively stopped.
5. Because Irish ambient humidity is already 65–75%, so an open window invites damp air straight in — and gives you no control over temperature, humidity, or smell.

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Next lesson: Soil, Coco, and What Roots Want — we’ve sorted the air above the canopy; now we go below it, into the stuff in the pot and the choice that decides how forgiving your whole grow will be.