Circulating fan vs Exhaust fan

I've been doing some "testing", or whatever you want to call it, with my fans.

I've run a week with exhaust and circulating fan, and then a week with just exhaust. Did that for 4 weeks total, 2 weeks with each on an alternating schedule.

Here's what I've noticed. With both fans running, growth seems a little slower than with just the exhaust. Just the exhaust seems to grow a little faster. I really thought it would have went the other way.

I've done some mild research to try and figure out why, or if I'm just crazy. The only thing I could find particularly of relevance was the stress by the circulating fan could be slowing growth... Maybe.

Now, I also figured that the stems/stalks would grow larger with the circulating fan, for obvious reasons. I haven't noticed a difference. I do add small amounts of silica.

Just wondering if anyone else has tried it or noticed it. I always believed a circulating fan was necessary, but now I'm not so sure. I haven't tried it on flowering plants, only veg, and different species react the same as well.

From my understanding of the fan and why they are necessary is it helps keep mold at bay and it helps with trichome production. It would be interesting to see trichome production on a plant with no fan.

It's possible to have too much air movement. My hurricane oscillator is too much for a 4x4. It'll stress young plants even pointed upwards. I just leave it off until the plants are vegged to the canopy netting.

You did this test on the same plants?

You used the exhaust only test when the plants were 2 weeks older than when you did the exhaust/circulating fan test?

The circulating fan is for canopy air flow to reduce moisture and the chance of mold on your buds . In a large enough room they also help ensure that you are actually exhausting all the old air and don't have "dead zones" with no fresh air exchange.

Obviously to much movement is too much. I only have one circulation fan, but the plants near it grow the same as the plants the farthest away.

BostonBuds said:

You did this test on the same plants?

You used the exhaust only test when the plants were 2 weeks older than when you did the exhaust/circulating fan test?

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Same plants. Did one week of both, then one week of exhaust only. Then a week of both, then exhaust only.

WingAK said:

From my understanding of the fan and why they are necessary is it helps keep mold at bay and it helps with trichome production. It would be interesting to see trichome production on a plant with no fan.

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To @Thundercat as well.

My exhaust fan pulls enough to "wiggle" all the plants, as I have the bottom flaps on both sides open. I don't think moisture is an issue, but I may be wrong. Humidity is the sane either way, both or just exhaust.

whtrabb1t said:

It's possible to have too much air movement. My hurricane oscillator is too much for a 4x4. It'll stress young plants even pointed upwards. I just leave it off until the plants are vegged to the canopy netting.

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That I agree with. I don't have it right on them, just a swirl in the tent to stimulate stem/stalk movement.

Without a fan blowing over the plants I notice the large fan leaves would touch and the undersides would be soaking wet.

BostonBuds said:

Without a fan blowing over the plants I notice the large fan leaves would touch and the undersides would be soaking wet.

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My exhaust fan seems to make them move enough that I don't have that issue. I don't have huge cannabis leaves in there yet, but I have it packed full of flowers, tomatoes, etc.

That hasn't been an issue yet, but nothing huge in there yet either.

3rd Monkey said:

To @Thundercat as well.

My exhaust fan pulls enough to "wiggle" all the plants, as I have the bottom flaps on both sides open. I don't think moisture is an issue, but I may be wrong. Humidity is the sane either way, both or just exhaust.

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I'm not saying you must have both or that it will hurt your grow not to, just explaining the specific purpose of the circulation fan in many grows. My exhaust for example is not positioned where it is creating enough draft to move the plants. In the winter I turn the circulation fan down, but in the summer its humid here, and I defnitely need it to get some canopy air flow.

Thundercat said:

I'm not saying you must have both or that it will hurt your grow not to, just explaining the specific purpose of the circulation fan in many grows. My exhaust for example is not positioned where it is creating enough draft to move the plants. In the winter I turn the circulation fan down, but in the summer its humid here, and I defnitely need it to get some canopy air flow.

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Yea, I'm not arguing with anyone here or saying anything is wrong, I'm just noticing the correlation between less wind and slightly increased growth. I haven't done anything scientific lol, just the old eyeballing, so it could just be me.

I've always believed you needed a circulating fan, always. Now, I'm not so sure though. Just wanted to know if anyone else had noticed anything similar.

I use a ceiling fan

Air circulation in a grow area is needed. It appears your question is more like if you need air blowing directly on a plant and how hard?

BostonBuds said:

Air circulation in a grow area is needed. It appears your question is more like if you need air blowing directly on a plant and how hard?

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It's not really a question, per se, but yea I guess. I know circulation is needed and fresh air is needed, but yea.

I have quite a few grows under my belt lol, so I don't want this to come off as a noob thing, just something I've noticed since not using a circulating fan. I've used one for the past 8 years indoors, never grew without it. This was... My first time unprotected lol. That last bit was a joke by the way, even if it was lame lol.

Do you think it could be less wind equals less stress which equals faster growth? Could it be that simple.

Humidity and temp don't change with both or just exhaust. Maybe at the canopy since there isn't as much circulation, but the exhaust draws from the top.

I just have a small 6" fan blowing between the light and the canopy. The 6" 400 CFM inline fan between the carbon filter at the top of the tent and blowing through the sealed hood and exhausting out the tent pulls plenty of air through three open vents at the bottom of the tent. You don't need fans blowing directly on the plants. Air exchange is more important.

xtsho said:

I just have a small 6" fan blowing between the light and the canopy. The 6" 400 CFM inline fan between the carbon filter at the top of the tent and blowing through the sealed hood and exhausting out the tent pulls plenty of air through three open vents at the bottom of the tent. You don't need fans blowing directly on the plants. Air exchange is more important.

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That's what I'm thinking too. Mine is just a 250cfm in a 4x4x8, but it pulls enough to suck the tent in. Seems like they do better than the air being circulated with another fan.

Sounds like good negative pressure . Swapped 6” for 8” exhaust ... tent has passive inlet flex tube , zipped tent up , held my jay near inlet and you can see smoke getting pulled in.

That’s how I test ...lol.

Budzbuddha said:

Sounds like good negative pressure . Swapped 6” for 8” exhaust ... tent has passive inlet flex tube , zipped tent up , held my jay near inlet and you can see smoke getting pulled in.

That’s how I test ...lol.

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I run an 8" exhaust fan for a chimney duct lol. Works fine, definitely negative pressure.