Ventilation – The Open Loop

• By Hydro Harvest
• 07 January 2025

So, like all right-thinking indoor growers you want a decent ventilation system. What exactly do you need to create it?

Before getting down to brass tacks it’s worth saying that there are various types of ventilation system and what you need will depend on what you’re looking to do.

Any number of arrangements are possible but broadly speaking the setups for indoor gardens fall into two categories – ‘open loop’ or ‘closed loop.’

For the purposes of this we’ll look at the simple, commonly used, yet highly effective ‘open-loop’ variety.

An inlet

To start at the start, you need a suitably sized hole in your wall from which you can access outside air.

If you’re running an Active ventilation system (where you pull air in using an intake fan) you’ll really want a solid fixing point on your interior wall. A GAS Metal Wall Plate / Flange will do the job nicely. To this fixture you’ll attach ductwork and an intake fan (see below) so any air is only really able to travel in one direction – inwards.

If you’re running a Passive ventilation system (where you rely on the natural inward flow of air through vents etc) then you’ll just need to know you’ve enough air coming in to satisfy your plants. Of course, you don’t really want unfiltered air flowing out of those same vents, so you’ll need to take steps to ensure your extraction is suitably strong to overcome backwash.

Ductwork

Is a form of wide, flexible pipework that can carry air into and out of growing spaces. It connects your inlet, and outlet points to whatever fans and filters you choose to use. Typically, indoor growers suspend their ductwork to maximise floorspace, simplify connections, and ease access.

Ductwork is available in a variety of types with varying levels of acoustic damping to keep noise to a minimum. It does get used and abused but unless you’re incredibly confident that your space is discreet then it is well worth spending a few quid on something decent. Sure, look for something that suits your budget, but be aware that you get what you pay for. The hierarchy of ducting runs roughly as follows:

Silver, which is decent enough from a reputable brand but won’t really reduce noise and is vulnerable to tears. Combi, which is essentially Silver but with an added layer of tear protection. Acoustic, which takes Sliver or Combi and adds a soundproofing, heat-retaining insulation layer. And finally, Phonic, at the top of the tree. Phonic is triple layered, highly resistant to tears, reduces the possibility of noise, and minimises the emission of any noise that does occur.

As the insulation on Phonic ducting is ultra-smooth smooth and light in weight it also helps ensure that your ventilation throughput is always maximised.

Whichever ductwork you choose, remember to match the diameter of your ductwork to that of your fans and filters, more of which below.

Fans

Are the beating heart of your ventilation system, drawing fresh air into your growing space and pulling dirty air out. If you’re running an Active ventilation system, you’ll need an intake fan and an extraction fan. If your setup is Passive, then extraction is all you need.

Types of fans vary but can typically be separated into In-line and Box.

In-line fans boast some of the most advanced performance and silencing technologies out there, even on budget models. These need to be suspended from the ceiling of your room or grow tent.

Box fans offer more versatility in terms of positioning. They’re relatively inexpensive to buy whilst still having the capacity to shift industrial quantities of air. Acoustic damping on these fan types keeps noise to a minimum but they are a little louder than In-line equivalents.

Either way, look for something with the capacity to properly serve your space. Required capacity is easy to figure out.

Simply multiply together the length, width, and height of your space in metres – that’s your cubic meterage.

Ideally, you want to change all the air in that space every minute. So, multiply your cubic meterage by 60 (as in minutes of the hour).

Now you know how many cubic metres of air your fan needs to move per hour.

All the fans listed with Hydro Harvest include cubic metre per-hour ratings. But crucially you don’t want something that just-about matches your calculations.

That would mean running the fan flat out, with no margin for extreme conditions, and would ignore the inevitable drag on performance caused by ducts and filters. A rule of thumb is to get something that gives you at least 33-50% more performance than your numbers say you need.

If running an Active system, you should then choose an intake fan that is about 25% less powerful than the extraction fan that you actually buy. This is vital to maintain negative pressure – an effect that maintains proper circulation and locks in odours.

You can also create negative pressure using a dual fan speed controller and running intake at 75% of extraction.

Filters

Are essential if you want safe, discreet, nuisance-free emissions. These cylinder-shaped units scrub off the VOCs and smells you want to remove before you expel your extracted air. If you don’t use a filter, then your growing space and the area outside it will become very obvious and unpleasant very quickly.

Filters fit into your system before your extraction fan and are typically the first stop for spent air in the extraction process. Experience has taught manufacturers precisely the right composition and arrangement of carbon beds required by indoor gardeners.

Look for a filter that matches to your fan, usually dictated by the diameter shown on the listing. Once you know the size you seek you can start looking at features. Units with easily replaceable Pre-Filters are a must.

It’s important to note that Filters are designed to remove tiny airborne compounds but not larger debris, dust, or dirt. For that you’ll want a Pre-Filter.

Pre-Filters

Remove larger bits of dirt, natural occurring organic matter, and debris before they can get into your Filter. With a clean Pre-Filter in place extracted air moves more quickly through your ventilation system, improving fan performance, efficiency, temperature control, humidity balance, and induction of CO2. Put that all together and you can get meaningful gains in plant development and yield.

Spare Pre-Filters are a grow room essential. Unless you’ve got a replacement to hand, you’ll have to shut down your ventilation system until you’ve washed a dirty one and dried it. Clearly this is not a desirable option so always keep a spare Pre Filter or two at the ready.

Silencers

Help you chase out annoying chatters, rattles, and hums in your ventilation system. You simply add them in after an appliance or near a bend to dampen unwanted sounds. A handy addition for discreet growers.

Metalwork

Really can ease the strain on your fans and help minimise noise. There are bends that are far better achieved using a smooth metal elbow than by simply contorting your ductwork. Metal Elbows and Y’s also allow you to create configurations that just aren’t possible with ductwork alone.

Couplers

Hold it all together, so choose something secure and reliable. Always ensure that your ductwork and appliance connections are snug and secure by choosing quality couplings. Doing this will help minimise noise, maximise performance, and prevent accidents.

Making the magic happen

The checklist above might look a little daunting but there really is nothing to fear from it. Because the kit described is produced by dedicated indoor horticulturalists you can be sure that it is designed to suit the needs of growers. They know that indoor gardeners are busy, busy people who want easily compatible equipment. They want you to succeed.

With these essentials you can create an incredibly productive, healthy, discreet growing space. But as with everything green, there’s always scope for more. If you want to take this equipment on further adventures check out our guide to ‘closed loop’ ventilation systems here.