Why You Should Always Have Negative Air Pressure in a Hydroponics System

This post explains why you should always have negative air pressure in a hydroponics system for optimal plant growth and development.

One of the key aspects of hydroponics is the ability to create an enclosed system that contains exactly the optimum environment for our plants to grow and flourish so that we can more or less guarantee a fast crop and a healthy yield both in terms of quality and quantity. If we ensure that we have a slight negative air pressure within that environment we’d be giving our enclosed environment a little bit of an edge.

The slight negative pressure within our grow tent – and grow tents are highly recommended for enabling a growing environment that is super-friendly to our growing crop – will ensure that all the conditions inside the tent will always stay inside the tent. That includes odours (although they can be dealt with by proper ventilation and filters as well) and a controlled temperature and humidity which we know should not be influenced by anything outside as long as the air pressure inside is slightly lower than the air pressure outside.

What Is Negative Air Pressure in Hydroponics?

In the context of hydroponics, negative air pressure simply means that the air pressure inside a given system or environment is lower than the pressure surrounding it.

Examples of negative air pressure are all around us. It’s negative air pressure that causes large weather systems to blow wind into the low-pressure areas, often heralding a showery spell for those living beneath. At the other extreme, negative air pressure surrounds an inflated balloon, so that when we stop pinching the opening end of the balloon the air will come rushing out.

In both cases, the air always seeks equilibrium, it always wants to even itself out, so high air pressure flows in the direction of low air pressure and will join it if allowed to. The difference here is that, in a sealed grow tent, we just don’t allow it to.

This is because negative air pressure will ensure everything we want within the closed system will remain stable, in this case, the sealed system of our grow tent. We want to preserve all the environmental aspects which we know are best for growing our plants, and we also want to keep the smell from offending anyone nearby!

Negative Air Pressure in Your Grow Tent

A grow tent is actually a good example of a closed system or sealed environment. The air inside the grow tent is maintained at a slightly lower pressure than outside by regulating the ventilation so that, at any given time, more air leaves the grow tent than enters it from outside. Simply ensuring that this is constantly happening will guarantee negative air pressure which will be beneficial to your crop.

This negative air pressure should be kept up, and sustained throughout the entire growth cycle of your crop from seedling until it’s time to harvest.

As well as controlling temperature and humidity, you also want carbon dioxide levels to be controlled. The aim here is not so much low air pressure for its own sake, but for the good of a sustained and stable environment.

You may find that making slight variations to the environment through your plants’ growth cycle will be beneficial for a good crop. These are things that you may discover through your own testing or maybe the advice given for specific types of plants at varying stages of that plant’s growing cycle. But such subtle differences will likely only be possible if you keep control of your environment in the grow area, and having a slightly lower air pressure will make this much easier to sustain.

The odour of both your plants (depending on what you’re growing) and some fertilizer may be something you won’t want to share with the outside world either. While the use of a carbon filter as the air passes out of the growing area will remove the odour issue, having a slightly lower air pressure within the grow tent will solve all the others.

Negative Air Pressure and the Ventilation System

In general terms, the air pressure inside a grow tent will be controlled by your ventilation system. Basically, the venting of air from your grow tent should be at a faster rate than the system is pumping fresh air back in, so as to sustain a slightly negative pressure inside.

Of course, the opposite will also be true: if the air is coming in at a faster rate than it’s going out then the air pressure inside the grow tent will be higher than the outside air and will be positive rather than negative.

The differences between the two are easily spotted just by looking at the walls of your grow tent. If the tent walls are a bit bowed out this means that the air pressure inside is positive, and if the walls are slightly bowed in then the air pressure inside is negative, which is ideally what you want. You want tent walls to be slightly bowed in all the time.

Odours coming from your plants or fertilizer will be prone to escaping from the grow tent if this contains positive air pressure and if the sides bow outward; such smells easily escape through the smallest of cracks or holes under these circumstances and leak into the air outside.

How to Get Negative Pressure in Your Grow tent

A good way to ensure that negative air pressure is sustained within your grow tent is to use an extractor fan that has a greater power rating than any other fan which brings the fresh air from the outside into the grow tent.

In another article, we looked at an accurate way of working out the power rating of your hydroponic filter system, based on the size of your grow tent and other factors, and how this was calculated in CFM (cubic feet per minute). Applying what we discovered then, we can work out the power needed to contribute to the negative air pressure inside the tent. In this case, we need to have an extractor fan that is about 100 CFM greater than the power rating of any intake fan.

The example that we used then was of a 350 CFM output fan when used with a carbon filter. If this is the case then we should ensure that the air coming in does so at the rate of about 250 CFM. You could also achieve negative air pressure inside the tent by having a powered exhaust fan combined with some form of passive intake.

In such a setup the non-powered intake will always provide less internal air pressure as long as your exhaust fan and filter system is working.

How to Optimise Air Pressure

So far we’ve established the reasons why slightly negative air pressure is better for your grow tent, what to look for and how to go about getting it.

But there is a danger that the air pressure may be too negative. If this happens then there are ways to correct this, ensuring that you end up with the slight negative pressure which you want, but not too much of it.

There are three possible ways of achieving this, so let’s look at these briefly in turn.

Use an Active Intake and Not a Passive Intake

If you’re using a passive (unpowered) method such as just having a hole or vent in your grow tent, this may be overwhelmed by the exhaust rate of the extractor fan-filter system.

Smaller grow tents will not need to be fitted with a powered intake fan. But as the size of your tent increases, you may find that a powered intake fan is a good solution. There are useful ready-reckoners published by manufacturers of such equipment so there’s no point in going into great detail here, except to say that you can get intake fans that are powered from less than 300 CFM right up to powerful models of well over 2500 CFM for large grow tents.

Control Your Exhaust Fan Speed

It may be stating the obvious, but if your exhaust fan or exhaust fan-filter system comes with a speed controller you’ll be able to set this to the optimum speed so that the air pressure inside the tent is slightly negative and no more. You may have to experiment a bit to get this just right.

Extra Air Vents in the Grow Tent

A simpler solution would be to add extra vents (i.e. holes or ducts) in the fabric of the grow tent itself. You should make these holes carefully, preferably punching in a small reinforcing ring which will ensure that the hole will not be prone to tearing; once you’ve established the best size for your extra ducting you’ll want to keep it that way. You’ll probably find that you’ll need several of these.

You can buy larger ducts ready-made. Good quality ducting should be able to keep out daylight completely, as you don’t want anything from the outside to disturb the optimum growing environment on the inside; you also don’t want to encourage the growth of algae which is very fond of sunlight. Ducting with an s-bend will be good at this.

In conclusion, you want your grow tent to have a slightly negative air pressure relative to the surrounding air pressure, for all the reasons given above.