Ventilation methods can be broadly classified into two; active ventilation and passive ventilation. Both these categories are extensive, they have many subcategories under them. In this article let’s see what these ventilation systems are and how they are different from each other.
Passive ventilation
A passive ventilation system is one that doesn’t use any forced air movements. In this system, the air movements happen due to wind velocity and heat buoyancy. Heat buoyancy is a term used to express the movement of hot air from below to the top. This movement is also called convection.
Passive ventilation systems make use of these air movements due to natural forces. They ventilate houses without expending any energy. This ventilation method is definitely the most recommended system for cooling whether it is for your home or industry.
Pros of passive ventilation
- Uses natural forces to achieve cooling
- No expense of fossil fuels, and hence it is a ‘green’ system
- Needs no or less maintenance
Cons of passive ventilation
- The system may not be powerful enough to achieve ventilation in complex structures
- Passive ventilation systems require early planning to make maximum use of natural forces
Active ventilation
While passive ventilation is great it is not possible in every case. For example, if the design of the attic of a house is complex then passive ventilation alone might not be able to suffice the ventilation requirements. In this case, a forced mechanical system, a fan, is used to circulate air. This type of mechanical ventilation system is called active ventilation systems.
As you know a fan uses electric energy. So active ventilation systems, unlike those passive ventilation systems, work at the expense of energy, and so they aren’t exactly ‘green’ systems. However, there are active ventilation systems that use solar energy and wind energy so there are eco-friendly versions as well under this category.
Pros of attic ventilation
- Effective throughout the day irrespective of weather conditions
- Effective venting complex structures
Cons
- Uses electric energy
- Has moving parts that causes wear and tear
- Requires maintenance and costs more to run
A back to back comparison of active and passive ventilation systems
Now that you have a general idea of what active and passive ventilation systems are, let’s do a back to back comparison of these;
| Active Ventilation | Passive Ventilation |
|---|---|
| Used to forcefully ventilate spaces | Ventilates spaces by using natural forces |
| Works at the expense of energy | No energy expenses |
| Has moving parts and motors | No moving parts but there are exceptions (roof turbines) |
| Higher running costs | No or less running costs |
| Can cause noises | No noise pollution |
| Malfunctions can cause damages and fire hazards | Fewer chances of malfunction and no chances of causing damages |
| Improper implementation can cause negative pressure build-up and back-drafting of gases in some cases. | No hazardous negative pressure buildups |
| Not eco-friendly | Nature-friendly ventilation method |
| Effectiveness remains the same throughout the operating time. | Effectiveness varies with weather conditions |
| Requires less planning in advance | Requires early planning |
| Tends to be costly to install | Tends to be cheaper to install |
Let us now discuss these differences in detail;
Working
Active ventilation systems forcefully move the air in a space to achieve ventilation. The best example of such a system is an attic fan. It forces the hot air out of the attic space which creates a negative pressure which in turn draws in the cooler air from the outside.
Passive ventilators on the other hand use natural air movements to achieve ventilation. A good example of such a system is a combination of soffit and ridge vents in the attic. When the wind flows over the ridge vents it creates a negative pressure which draws out the hot air inside the attic. This also lets the cooler air into the attic through the soffit vents.
Energy consumption
Active ventilators use electric energy for their working. There are some solar-powered systems as well. The point here is that an active ventilation system consumes energy which is not great from the perspective of an environmentalist.
Passive systems, in this regard, are super cool. They don’t consume any energy, they work entirely based on the natural flow of air. And surprisingly these natural currents can be more powerful than an active ventilator.
Moving parts
All active ventilation systems have moving parts while passive ventilators are generally motionless. An exception for a passive ventilator with moving parts is a roof turbine which has a rotating turbine but it doesn’t consume any energy for its working and hence it is a passive ventilation method.
All active ventilation systems have a motor while passive ventilators don’t have this. For this reason, passive ventilation systems are quiet even though they can create slight noises sometimes. On the other hand, active ventilation systems are noisy.
Having moving parts also means more wear and tear, and that means more maintenance. So active ventilation systems need more maintenance than passive systems, and this makes them more costly to run. Since there are no moving parts, a passive ventilation system requires minimal maintenance. However, it is advised to clean this system yearly to make sure it is properly working.
Safety
Passive ventilators are safer compared to active ventilation systems mainly because they don’t forcefully vent air. If the intake vents of the attic are covered, an active ventilator like an attic fan can create a negative pressure inside the attic because it forcefully exhausts all the air inside and there is no path for cool air to come in. This negative pressure build-up can draw conditioned air from inside the house which increases the load on the air conditioning system. At its worst, this can even cause structural damages to the roof.
The negative pressure in the attic can also cause flue gases to backdraft filling the living areas with dangerous gases like carbon monoxide.
In the case of a passive ventilation system, like a ridge vent, even if the intake vents are closed there won’t be any build up of negative pressure in the attic. There will be a slight depressurization as the wind blows over the vents however it won’t be strong enough to cause any damages or back-drafting.
On a side note: whether the ventilation is active or passive there should be unobstructed intake ventilation for them to work properly. Otherwise, the attic won’t be vented adequately.
Noise
Constant noise from the attic, however small it is, can be really annoying if you are exposed to it throughout the day. This is a common problem with active ventilation systems like attic fans and whole-house fans. You will have to bear with the noise their motors make, as well as the swooshing sound of the wind against their blades. There are many whisper-quiet fans available which considerably reduce this noise problem, but even they are not truly silent.
On the contrary, a passive roof ventilation system has no motor or moving parts. This makes them totally silent. You may occasionally hear a whistling sound if the vents are narrow but other than that a passive ventilation system is completely silent.
Effectiveness
The effectiveness of active ventilation is generally way more than that of passive ventilation. This is the reason why there are active ventilators in the first place.
Passive ventilation doesn’t work all the time, their performance depends heavily on the weather conditions. For example, the effectiveness of soffit-ridge ventilation depends on the temperature difference between the attic and the outside, and also the speed of the wind. If the temperature difference between the two spaces is minimal then there won’t be much convection which means there won’t be much ventilation.
You may think that there is no need for forced ventilation if the temperatures are not so different between the attic and the outside, but the problem here is the moisture. With less air movement the moist air will be trapped inside the attic which over time causes the growth of molds.
Active ventilators, because powered by electric energy, present consistent performance all the time. These systems are effective in venting humid air from the attic unsusceptible to temperature differences.
Passive ventilation is also not very effective if the structure of the space to be vented is complex. For example, if an attic has many separations and height differences, a passive ventilation system may not be the best choice as these complex structures often prevent the natural flow of air. In such cases, an active ventilator is a better choice.
Advance Planning
Implementation of passive ventilation systems generally requires beforehand planning to take into account their effectiveness by quantifying the natural forces available. For example, the direction of wind flow, or the side of the roof that gets the most sunshine, needs to be taken into account for the best implementation of a passive ventilation system.
Active ventilators, especially when it comes to attics, requires less advance planning. In many cases, an attic fan is installed not because it is absolutely necessary but to correct the faulty implementation of a passive ventilation system. This may not be true in all cases but I know some people who did that.
Cost
Even though passive ventilation requires some advance planning they are cheaper to install and run. But active ventilation systems commonly cost more to install as well as to run.
Examples of active ventilation systems
- Attic fans
- Whole house fans
- Exhaust fans
Examples of passive ventilation systems
- Roof vents
- ridge vents
- louvre vents
- Roof turbines
