Garden & Greenhouse


Choosing Between Passive or Active Greenhouse Ventilation

Maintaining stable greenhouse temperatures during seasonal changes keeps growers on their toes year-round. When correctly designed and installed, passive (natural) and active (mechanical) ventilation systems help to ensure optimal plant health and crop production. Less obvious to growers however, is whether a passive or active ventilation system is ideal for their individual greenhouse operation. When considering which types of ventilation tools to integrate, growers must first determine how aggressive their grow needs to be, their proximity to an on-site power supply and their budget.

Passive Ventilation

Passive ventilation relies on two factors: the wind effect and thermal buoyancy. Thermal buoyancy occurs when denser, cold air lifts warm air up and out of a building. When wind passes over the backside of the roof vents, a sort of chimney effect is created. As hot air is drawn out of the roof, cooler outside air is drawn in through the wall vents. When done properly, this can be a very efficient and cost-effective way to ventilate a greenhouse.

The wind effect simply refers to the fact that wind is almost always blowing around structures. It is hardly ever completely still. These air movements create small pressure differences on the windward and leeward side of a structure. By placing ventilation windows strategically, pressure differences will move air through the greenhouse. The wind effect is the dominating force as long as it is blowing at around 2.3 mph. Therefore, only under very wind still conditions will the wind effect have a major impact on ventilation.

Passive ventilation typically uses a combination of roof and wall vents that work with the forces of wind and temperature to create a natural air exchange. Other ventilation passages include doors, roll up sides, louvers and shutters. Rolling up a portion of the greenhouse wall will suffice in some cases. However, the best performing systems make use of a combination of accurately-sized roof and wall vents. It’s important to remember that insect screening adds additional air resistance; the tighter the screen, the less room there is for air to flow.

One of the main advantages of passive ventilation is affordability. The initial purchase price is inexpensive,

especially when buying a manual system.  Operating costs are also very reasonable since very little electricity used. Another fringe benefit is having access to crops from the end walls and side walls. For crops that can handle wider temperature and humidity fluctuations or for growers who just grow for themselves and have flexible timelines, a passive system makes more sense. Passive ventilation is an efficient option for growers who live on the site of their greenhouse and don’t mind keeping a constant eye on it.

Important to note is that what users of passive ventilation gain in savings, they lose in control. Passive systems are less precise than their active counterparts, because they rely on wind and temperature. They are also more labor-intensive, as they require someone to open and close the vents regularly. The vents have to be sealed well, or air is lost through infiltration. Improperly installed vents can cause heating costs to go up. Labor costs associated with running such equipment add up also, so owners should assess the option to automate upon initial investment.

Active Ventilation

Active ventilation relies on mechanical devices, like inlet vents, exhaust fans, horizontal airflow fans and evaporative coolers, to produce air circulation and temperature control. Similar to passive ventilation, fans exhaust hot air and bring in cooler outside air to lower the greenhouse temperature.

Fans have a set of guidelines developed by AMCA (Air Movement and Control Association) to confirm capacity, static pressure, ventilation efficiency ratio and power requirement. Growers looking to buy fans for greenhouse ventilation should keep an eye out for a sticker indicating it has passed AMCA inspection.

With fan ventilation alone, growers may not be able to lower temperatures to the ideal level. If greenhouse temperatures must be brought down even further than those outdoors, evaporative cooling is the most effective method. Evaporative cooling works by taking warm outside air through a damp pad to cool it. Fans at the other end of the structure then pull the cool, moisture-laden air from the pad and across the greenhouse to cool the environment. This action not only cools the air, but also provides some ventilation. This type of cooling can lower greenhouse temperatures by as much as 10 to 20 degrees below the temperature outside. Evaporative coolers require a tightly sealed greenhouse; gaps must be sealed to ensure it works with the highest efficiency. The pads have to be cleaned and treated for algae, water quality has to be maintained and filters have to be cleaned and replaced. How long an evaporative cooling system lasts is determined by how well it is maintained.

The advantages of using an active system are having precise control, as well as avoiding drafts and potential plant damage along the outside edges of the vents. Syncing ventilation equipment up to a greenhouse controller allows growers to create detailed equipment schedules with almost no labor required. With a controller, a grower can manage humidifying and dehumidifying equipment, like exhaust fans, with the flip of a switch or by automating the system to respond to environmental changes. Controllers can even be fully integrated with other atmospheric controllers and timers. Having more precise control allows growers to extend the growing season or create a year-round growing operation. It is wise for growers on strict delivery schedules to opt for active systems, because the added element of control allows them to more easily achieve optimal growth.

As an added bonus, active ventilation also limits the entrance of pests, since they are relatively sealed environments. Growers who need an especially high level of control should explore the option to automate. Automating active ventilation systems ensures consistency, no matter the conditions outdoors.

While active ventilation systems are excellent for ensuring tight control of a grow environment, they do come with high monthly operating costs due to their heavy reliance on electricity. Choosing the appropriate system all comes down to a grower’s specific needs.

Passive and active ventilation systems come with their own unique set of advantages and disadvantages. Passive ventilation works great in certain parts of country and during certain times of the year. Community gardens are a great example of where passive ventilation makes sense. Growers won’t hit the exact temperature they want every day but they will save a substantial amount of money on utilities. Growers with hard deadlines to hit absolutely need active ventilation.

Passive and active ventilation equipment is not too far off in initial purchase price. However, once growers with passive systems begin to automate motors and other such equipment, the price differential starts to even out. Eventually, an automated passive system can become as costly as or even more costly than an active system.

Essentially, if delivery schedules are lax, on-site power supply is far and capital cost is limited, passive ventilation is likely the best method. If delivery schedules are stringent, on-site power supply is nearby and an operation can accommodate it in their budget, active ventilation makes more sense. With these considerations in mind, growers will be more readily equipped to choose a ventilation system that works best with their budget, location and application.

Jessica Batchelor is a content writer for GrowSpan Greenhouse Structures She has an interest in horticulture and how it is affected by new technology.

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