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Ask Erik: RAW! - Can I Blow It All Through Two Holes? Grand Master Ventilation Tactics

Ventilation is important for your grow room, and there’s more than one way to skin this cat. Ventilation is important for your grow room, and there’s more than one way to skin this cat.


I want to construct a grow operation that uses IN/OUT ventilation, but only have access to one eight inch diameter duct for IN and one eight inch diameter duct for OUT. I want to run about 12 lights (not all in the same room or timer), and don't have a problem with supplying some supplemental air conditioning for the three or four weeks of the year where fans just won't cut it around here.

BUT, I don't want to be relying on an AC and running a totally sealed environment, not yet anyways.

Can it be done like this: one eight inch duct each for IN and OUT for 12+ 1KW HID (translation: a dozen or more one thousand watt high intensity discharge lights – ed.) lights?


Yes, it can. However, how well it's going to work is going to depend on a few factors in the set-up, and how you manage your fan system.

What I have revealed here is a very useful and often overlooked principle in ventilation that has loads of application for indoor growing.

You are going to need a lot of 8” diameter inline fans. You could use fewer, but more powerful box-blowers with 8” duct ports, but they aren't going to be nearly as efficient and will create more logistical issues for the set-up and operation.

For the number of lights you describe, maybe six to eight individual 8” duct diameter centrifugal in-line fans will be required. On top of that, you will also need a few boxes of 8” diameter flex duct and probably half a dozen of each “Y” and adjustable Elbow (45 to 90 deg) connections. For hardware, pipe strap, aluminum foil tape, small self drilling, self tapping screws (TEK), eye-hooks, S-hooks, anchors and EPDM bungee straps (for suspending fans vibration free) are going to be required to hold it up overhead neatly and securely.

All of your lights should be housed in air-cooled reflectors. Lamps not isolated and cooled directly may produce too much heat to keep up with; a/c shades direct heat away before it even enters the growing area.

You may also need back draft dampers, and possibly a couple of motorized dampers, depending on how extensive the duct-work gets relative to the number of individual growing zones that need ventilation for healthy growth.

The principle you are going to be relying on for success here is that while an eight inch diameter duct doesn't seem like much for all of those lights, it can actually be plenty if you have it as powerful every foot along the way through the duct work as it is blasting straight out of the fan from a few feet away. What hurts ventilation output in most grow setups is that the length and bends in duct work overload what is typically a single fan connected to one or either end of all that tubing and fittings. Static pressure, and friction loss greatly reduce the air flow, meaning your fans are forced to work very hard to move less air.

Now, if you have the same eight inch diameter centrifugal fan reoccurring every ten feet or so, or after every major bend in the duct work, you have very little loss in air movement, making as much air come out the end of 100 feet of duct as it does 8 feet of duct, the only difference being the number of fans it took to do it.

What I have revealed here is a very useful and often overlooked principle in ventilation that has loads of application for indoor growing. Couple all that air movement with precision control in each of your growing zones with a TV-1 or TV-2 temperature/fan speed control module, and you have a very effective and highly adaptable IN/OUT ventilation system that uses considerably less energy, and has fewer issues and start-up costs versus CEA (sealed) growing environments.

Your eight inch IN can branch-out with “Y”s and Elbows for fresh air intake for as many rooms as you need to. The 8” OUT duct can be set-up in a similar fashions with A/C reflectors and carbon filters sending out old air 24/7 through ducting with Ys and Elbows. Motorized dampers will allow you to automate and actuate various zones - a gate opens and closes electronically when either you or a thermostat tell it to. These are available in normal On or normal Off, depending if you want it activated when power is interrupted or supplied by a controller device.

I highly advise you map out your project on some graph paper and sketch a few different ways of setting it up, trying to keep the number of bends and fans required to a minimum in order to reduce your start-up and operating costs. Play around with it a little before anchoring it all securely and moving your plants into the grow; this way you can fine tune things without hurting the crop or creating more work for yourself.

Cheers, Erik Biksa

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Last modified on Friday, 29 June 2012 17:17

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