In a way, our homes are just like our lungs. They need to breathe and expel the stale air from inside them. Whether in your home or the workplace, a supply of fresh, pure air is essential for staying healthy.
One of the easiest and cheapest ways of ventilating a space is by harnessing the cooling power of the ambient air around us. A zero-energy passive ventilation system for your home will reduce your power consumption significantly.
Question: what is passive ventilation and how can you use it in your home?
This article will discuss all you need to know in order to give your air conditioner a break!
It’s Been There Since Forever: A Short History of Passive Ventilation
Let us not assume that passive ventilation systems are a new-age concept. The word ”ventilation” is derived from the Latin term “ventilatio,” which means wind. In the 17th century, the word was used to describe a process to ‘replace poor air in an enclosed space with new and clean air’.
Even before humans started using such terms, other life forms on the planet were using ingenious ventilation strategies. One of the best examples of passive ventilation is the tall termite mounds that use wind flow to control the temperature inside the mound.
In fact, architect Mick Pearce used this concept to design a tall office and retail building in Zimbabwe. As a result, the building consumed 35 per cent less energy than similar structures. This is a fantastic example of using biomimicry or bio-inspired design to develop a natural cooling system.
Archaeologists have uncovered ancient Persian temples dating back to 3000 BC, which acted as windcatchers or wind towers. This chimney-like structure is often used with canals or water reservoirs to cool the interior space.
Around 2,000 years ago Hypocausts were used in Roman baths and private villas. The system was designed to distribute heat generated by an underneath furnace throughout a building. The walls of the building were kept hollow, to allow the heat and smoke to pass through them and ensure sufficient air circulation.
In 1556, a hand-driven wooden fan was set up in the House of Commons in London. In the year 1600, King Charles I of England issued a decree about building design. It mentioned that the ceiling of all buildings should have a minimum height of 10 feet to prevent the air inside the room from turning foul.
In 1836, a Cornish mining engineer, T.Tredgold, became the first person to calculate the quantity of ventilating air needed for miners. However, his calculations did not consider the carbon dioxide and moisture exhaled by the occupants. So, the numbers were not accurate to ensure human comfort.
For several centuries there were conflicting priorities among the scientific community about ventilation systems. Architects and engineers were concerned about human comfort and the removal of odours and carbon dioxide from homes. On the other hand, physicians thought of using ventilation to prevent the spread of diseases.
The first half of the 20th century saw the confirmation of the hypothesis that excess temperature and humidity in a room are the two main causes of human discomfort. It was also established that an excess amount of carbon dioxide in a room was dangerous for the occupants.
What is Passive Ventilation?
The phrase says it all: passive ventilation does the job without actually doing anything.
In other words, it’s a process of circulating air in a room by using natural methods only. This process of natural ventilation uses wind, temperature differences, and buoyancy to promote air movement. The basic mechanism uses the incoming fresh air to push out the old stale air from an enclosed space. Passive heating and cooling techniques work best in dry and moderate weather conditions.
This is contrary to the process of active ventilation which uses mechanical devices like fans or air conditioners to circulate the air in a room. While these systems consume more energy, active ventilation has its own advantages. It delivers consistent performance and is not affected by weather changes. However, installation difficulties, noise from the fans or even small annoyances (like figuring out how to hide your outdoor AC unit) are the cons of an active system, aside from the fact that it feeds on electricity.
In some cases, where the rate of natural ventilation is too low, hybrid (mixed-mode) ventilation is used. Such a system uses exhaust fans to increase the ventilation rate.
Here are the main ways in which passive ventilation systems for houses work.
Wind-Driven Ventilation
The mechanism of wind-driven passive home ventilation is simple. As the wind comes in contact with a building, the wall facing the wind, or the windward wall develops a positive pressure. At the same time, the opposite wall, the leeward wall, develops a negative pressure.
As a result, fresh air will enter through the windows on the windward side and move out through the windows on the opposite side. The amount of airflow will depend mainly on the wind velocity, wind direction, and the size of the windows.
Buoyancy or Stack Ventilation
This process of passive stack ventilation works due to the temperature differences in a house, resulting in convection. As the air inside a room heats up, it rises just like a hot air balloon. This air will escape through an opening located near the roof and cool air will be drawn in through the windows in the lower portions.
This will be a continuous process that does not depend on the wind. However, the pressure difference created in this process is less than that created by wind flow.
A classic example of passive stack ventilation is the use of solar chimneys that have been around for centuries. They use solar heating to enhance the overall effect of natural stack ventilation by creating a suction effect. Additionally, a solar chimney can also save lives during an accidental fire in a building.
Night Cooling
This is as simple as opening up all windows on a summer night to close them back and draw the curtains in the morning.
The thermal mass of a building includes the concrete and masonry elements. These elements absorb heat during the day and release the same to the indoor air during the night. Then, the cool night air cools the thermal mass and it can absorb the heat during the next day. This helps in keeping the interior cool during the day.
In locations where solar radiation is excessive, night cooling may not be sufficient to maintain the minimum comfort level. However, it can help in reducing the daytime cooling requirements.
Pros and Cons of Passive Ventilation Systems
When properly designed and installed, passive ventilation solutions have several advantages. Take a look.
- An effective passive ventilation system can ensure a high air exchange rate while keeping the costs low. Since the system is simple, maintenance requirements are minimal.
- The reduction of heating by using passive ventilation systems can save energy and reduce the carbon footprint of a home. These systems can be an effective component of a green building design.
- Large openings or windows that are part of a passive home ventilation system allow higher amounts of daylight to reach the rooms.
At the same time, passive ventilation systems have a few drawbacks too.
- Passive ventilation depends on external climatic conditions which are variable. Since wind and temperature differences are difficult to control, the ventilation rate can come down during extreme climatic conditions. This can result in condensation when humidity levels are high.
- The negative pressure developed may not be sufficient to control the airflow. Remember that room in the back of your house that always feels somewhat stuffy, no matter how much air you let in? It’s because of the formation of stagnant zones.
- Some environments require the use of particulate filters which are not a part of passive ventilation systems. If climate and security conditions demand windows and vents remain closed, the ventilation rates drop.
- The cellars and basements in a home cannot be ventilated by using passive systems due to lack of airflow. So you’ll have to resort to an active ventilation system.
- In extreme cold conditions relying on natural ventilation techniques can cause condensation, leading to rot and moulds.
Passive Ventilation with Heat Recovery
To reduce energy consumption and greenhouse gas emissions, modern ventilation systems need to be as efficient as possible. That’s why engineers have developed the idea of integrating passive ventilation systems with heat recovery technology. This is an emerging technology that can reduce energy demand in power-guzzling buildings.
So how does it work?
As the process of passive ventilation pushes out the warm air from a room, a lot of heat is lost with it. Recovery of this heat is possible by using a mechanical device like a heat exchanger placed in the path of the airflow. There are various types of heat exchangers including rotary and fixed plate exchangers. In some scenarios, two exchangers are placed in a closed loop to create a run-around system.
Another common method is using a heat-pipe recovery unit with natural ventilation systems. A heat pipe has sealed pipes divided into the evaporator and the condenser sections that absorb and release heat. A fluid inside the pipe undergoes a phase change and transfers heat between the incoming and outgoing airstreams.
But there is a small catch.
For passive ventilation systems, a big challenge is to reduce the pressure drop across the heat exchanger. The velocity of the airflows in a passive system is on the lower side. Excess pressure loss in the exchanger can significantly reduce the ventilation rate.
In short, further fine-tuning of the technology is required to make it more efficient. Additionally, the complexity of the process makes it challenging to determine the best possible energy-recovery solution for your home.
How to Implement Passive Ventilation Solutions for Your Home?
Passive ventilation strategies can be implemented in a home in multiple ways. For an existing or future homeowner, choosing the best strategy depends on quite a few factors. These are the size of the house, orientation of the sun, location of openings, your home’s general thermal performance and more.
Here are a few effective tips you can use for effective passive heating and cooling in your home.
1. Effective Door and Window Orientation
Future homeowners should plan to orientate their houses in a manner that can make the best use of the wind patterns in the area. Matching the building orientation with the wind direction is crucial. In coastal regions, the wind direction is onshore, whereas in mountainous belts, the wind travels downslope.
Make sure to place windows on at least two ends of a room for better airflow. Quite simply, the more openings you provide, the more wind-flow paths you get.
In existing homes, windows on the windward side can be opened a bit less, while those on the leeward side should be opened fully. This causes a vacuum effect and increases the speed of the airflow.
2. Select the Best Window Design
Casement windows are the most common options and come with hinged frames that can be opened inwards or outwards. They offer maximum airflow when fully opened. Besides, casement windows can also help in redirecting some of the wind flowing from the opposite direction into the building.
Other common options are sliding windows and awning windows. However, these are not the best options when you are looking for maximum airflow.
Many homes use the so-called louvre windows in combination with casement windows to control the airflow. A louvre arrangement can also be used in the interior of a home for better internal cross ventilation and also for aesthetic emphasis. The development in the design and styling of these windows in recent years makes them a good choice for modern homes as well.
To encourage passive stack ventilation, you can use clerestory windows and operable skylights. The ridges on the top floor can be vented to let the hot air flow out. Horizontal openings near the floor are more effective in allowing the inflow of cool air than vertical openings.
3. Use Vents and Bottlenecks
Sometimes, the air needs YOU to show it the way in or out.
You can use roof vents or windows with vents at the top to create a pathway for the warm air to flow out. The other option is to place vents on the walls separating the rooms and also place passive vents above doors. This will allow the movement of hot air as well as fresh wind. Remember to clean the vents and keep them free from dust, debris, and bird nests.
When wind flows into a large area like a patio, it can be forced to move out through a smaller opening at the other end. This method of creating a bottleneck increases the wind velocity and allows it to move further into the house.
4. Use the Courtyard Effectively
Yes, you read that right. It’s not just what’s in the house that affects the airflow.
When designed in the right way, the courtyard acts as an air funnel that effectively discharges indoor air into the sky. Likewise, it can also act as a suction zone and help in drawing air through the windows. Using a fence or a row of trees in the courtyard can help in directing the wind.
The amount of impact the courtyard will have on natural ventilation depends on its shape, size, and the number of windows around it. The building type and the climate of the area are other factors that determine the overall impact of the courtyard. It is best to consult an architect who can consider all the parameters and suggest the best ideas during the design phase of a building.
Frequently Asked Questions
What is natural ventilation cooling?
Natural or passive ventilation uses the wind and convection currents to keep the insides of a house cool. Both wind pressure and convection use the cool and heavier air coming in to drive out the hot and light air from the top of the building.
How effective is passive ventilation?
Passive ventilation depends on external factors and is difficult to control. So, it is not the best solution to generate the desired cooling effect inside every type of building. That said, it is an inexpensive and energy-efficient way to lower the internal temperatures of a house.
What is the most effective ventilation system for a home?
The most effective ventilation system for your home depends on the prevailing climate conditions and your preferences. For moderate climates and breezy locations, combining passive room ventilation solutions with mechanical exhaust fans is a good way to keep the interior fresh and cool. However, make sure to understand the limitations of the system and compensate accordingly.
