What is Passive Displacement Ventilation?

Passive displacement ventilation can help increase the comfort of your house while at the same time reducing your energy bills. However, a lot of homeowners are completely unaware of what passive displacement ventilation (PDV) is.

So what is Passive displacement ventilation? PDV is an innovative air distribution system that is similar to the displacement ventilation system.  It supplies air evenly at the floor level, which now gets extracted from the room at the ceiling level.  PDV, unlike DV, uses no mechanical fans to propel air into the room. Its systems rely on the process of natural convection to chill the room.

You can never exaggerate how important a working ventilation system is as well as its health benefits. Well, with passive displacement ventilation, you have one that suits your needs, so here is how it makes you live better.

How Does Passive Displacement Ventilation Work?

There is a temperature gradient set up vertically between the roof and the floor. For PDV to function properly, this stratified temperature field has to be stable. This means that the temperature of the space will have to vary linearly with spatial height in the stratified zone. The temperature is nearly constant in horizontal directions, apart from regions near diffusers.

However, you can’t easily predict the temperature gradient because some of the major parameters are difficult to account for. These parameters may include the changing heat output of material and people, the radiation between the roof and the ground, and people’s position within the space. You will notice downdrafts with cold walls and updrafts with warm walls even with the slightest temperature difference between wall and room air.

As the load in this space changes, the buoyancy forces which move airflow will also change. Computational fluid dynamics analysis can be utilized in modeling the complex air and temperature patterns to predict temperature gradients in the occupied zone. Wide temperature stratification and drafts can reduce your comfort. The air temperature at floor level and the vertical temperature gradient in the occupied zones are the most important DV evaluation parameters.  

The supply flow rate must be increased to reduce the temperature gradient, but this might lead to high draft risk and high air velocity at floor level and consume more energy. Perimeter heating terminals like fin tube or radiant panels can be used to offset heating loads during winter and augment the vertical airflow. Convective heating terminals boost the airflow driven by buoyancy within the space.

If removing heat is the objective, a temperature-based design can match the supply airflow rate and the space load temperature. If getting rid of contaminants is the major objective, a shift zone method is used. The shift zone has the highest concentration of contaminants.

The shift zone height is that height above the ground in which the total volume of air moved in convective plumes on a heat source equals the supply airflow distributed using diffusers. The shift zone approach is usually applied in the industry to free the human breathing zone from contaminants.

Relative humidity above 70% is too high and can cause discomfort and fungal growth in your home. At 74 F, relative humidity should be maintained at 50% to 60%. To accomplish this, the supply air must be cooled below the room dew point. 

In more humid regions, return air bypass channels outdoor air and some return air using the cooling coil and moisture leaves from the airstream, thereby improving dehumidification performance. This cold and dry air is now mixed with the conditioned return air to give a warmer supply air temperature with lower moisture.

How to Make PDV System Work Effectively in your Home

For your PDV system to be efficient, you need to check the following points:

  • Your house requires to be ventilated for no less than 15 minutes at least five times every week.
  • Windows and other openings on opposing sides of your home will aid draw air through.
  • Opening the windows on the east and south side are best for letting cool breeze into your interiors during the early hours of the day. Openings on the west and north sides, higher up, will make the air moving.
  • Vents and other openings in the upper floors or roof will let air escape as heat increases.
  • Built-in vents, slots, louvers, and gaps in the window or door framing can produce low-level ventilation for long periods without any draughts or security risks.
  • Different kinds of windows can be utilized to direct air into your home. For instance, side-opening windows can catch breezes and pull them into the home more than awning-opening windows.
  • If your house is on more than one level, ensure openable doors or windows on each level. As hot air comes up, high openable windows on the upper floors can be an excellent way of ventilating your home during summer. 
  • You can install flyscreens and security on windows so that you can leave them open at night or when you go out during the day. This will help keep your house cool in summer.
  • Ensure cross-ventilation under your suspended floor to control dampness.

Furthermore, it is a great idea always to clean your indoor air. You can do this in many ways. They include:

  • By using a single-room or whole-home dehumidifier to reduce mold potential. 
  • By changing your air filter for about four times every year – you should do this more frequently if your house has air quality problems.
  • By cleaning your carpets using a HEPA-filtered vacuum regularly

Another effective method of improving the air quality in your indoor space is by exhausting thoroughly. You can use exhaust fans to draw air from your house and vent it to the outside. This practice is known as spot ventilation. Make sure your kitchens and bathrooms have exhaust fans. It would be best if you always turned on the bathroom fan after bathing for at least 45 minutes.

Whenever you notice that the air outside is drier and cooler than the air inside, you should open the windows. You should allow the fresh air to flow through and if it is not windy outside, turn on a window or whole-house fan. This will encourage better air movement, both into and out of your home.

Factors to Consider while Designing a Home for Passive Cooling

In warm and humid regions, it is important to include passive ventilation for cooling in house design. However, while designing a home for passive cooling, you need to consider the following:

  • Locate, orient, and design a form that maximizes exposure to cooling breezes.
  • Where possible, specify low (near the floor) horizontal openings as they are more effective than vertical openings for good ventilation.
  • Make sure there are good airflow paths through the building.
  • Specify windows that maximize airflow but minimize unwanted heat gain

What are the Benefits of Passive Displacement Ventilation?

Using a passive displacement ventilation system over traditional AHUs/FCUs system has several benefits. I have mentioned a couple of them below. Let us take a look.

  • It is virtually maintenance-free
  • It requires no plant room or under-floor or over-head ducting
  • It reduces building dead load 
  • It integrates smoothly into interior design
  • It can be tested and pre-fabricated off-site
  • It eliminates undesirable draft
  • It is Vibration and noise-free
  • It saves at least 20 percent of needed energy as compared to the traditional cold water system

Energy efficiency

Passive Displacement Ventilation systems are known for their energy efficiency. However, if you want to get the most out of your PDV system, there are a few things to do. They include:

  • Ensuring that the fans are controlled properly
  • Make sure you use maximum free cooling
  • Operate the system on a variable volume basis and not constant volume

Running costs for PDV

Passive displacement ventilation systems do not cost as much as other traditional cooling systems. And this is because the system is supported by local cooling systems especially for places with very high demands for cooling. However, for optimal performance, your PDV system needs a building management system (BMS). 

PDV Improvement opportunities

A few areas require improvement in displacement ventilation systems. They include:

  1. Dehumidification controls optimization
  2. Controls for chillers, boilers, and associated pumps need to be upgraded
  3. Control improvements can be implemented with the tenants in-situ
  4. The chiller plant needs to be upgraded.  Chillers have a lifespan of 15 to 20 years.
  5. It is difficult to retrofit passive displacement ventilation into an existing house because you will need duct riser spaces and enough height between the floor and the ceiling.
  6. Airflows in occupied spaces need to be controlled and commissioned to make sure that they are not above the permissible level.

Applicable buildings

Passive displacement ventilation is most-suited for buildings and spaces with high ceilings. Such buildings may include auditoria, atria, and low-density spaces with a floor to ceiling height of above 3m.

Floor plate implications

Passive displacement Ventilation (PDV) systems require the following to function:

  • High ceiling (above 3m)
  • A floor void or plenum (about 0.3m to 0.45m deep) or column/wall diffusers (you should know that they do take up your floor space)

Why Do You Need Passive Displacement Ventilation?

Passive displacement ventilation is important because of several reasons, and I have listed them below.

1. Passive Displacement Ventilation helps you reduce temperatures

When there are many people in a confined space, whether for a conference, work, or a public event, the environment can soon become hot and stuffy. PDV makes the room well-ventilated, thereby making it comfortable. And this will result in a more productive workplace.

2. PDV stops the condensation that causes damage to your home

We all know what condensation is, but what causes it, and what do you do when you encounter it? You would have noticed that droplets of water form on the outside of a canned drink when you bring it out of your fridge. That phenomenon condensation and the reason it happens is due to factors like temperature, air, and water vapor.  

Once you turn off the heating, the inside temperature will rapidly drop until it reaches the point of complete saturation. As the air becomes colder, some of the water will no longer be held in an invisible water vapor form and form liquid droplets.

Condensation is the most common type of dampness and will eventually result in mold growth.  If you allow it to develop over time, damp patches may start to appear on your walls. This means that wallpaper may peel, and ultimately black mold will grow. 

This will result in musty smells, damage to the fabric of your house, and may even cause health issues. However, you can solve this problem with PDV.

Passive Displacement Ventilation provides adequate ventilation, with a condensation control unit that gently ventilates your home from a central position. PDV transforms a stagnant and stale atmosphere into a fresh, healthy, and condensation free environment.

3. PDV has many health benefits.

Passive displacement ventilation has the following health benefits:

It reduces asthma

If your house lacks a good ventilation system, it becomes a good breeding space for dust mites, especially when humidity is high. These dust mites thrive in homes that do not have adequate ventilation. 

When you inhale their detritus or if it touches your skin, it can cause various allergic reactions. These reactions could lead to asthma attacks, watering eyes, eczema, itching, sneezing, and a runny nose.

However, you can reduce these problems significantly with an effective ventilation system like PDV. Some people who have asthma have seen immediate benefits after fitting such a system in their homes.

It helps alleviate hay fever symptoms

In the summer, many people tend to suffer from hay fever. A passive displacement ventilation system can filter out larger particles, like pollen, and prevent them from entering the home.

If you have such a ventilation system in place, you do not need to open the window. If you open the windows, it may cause problems for those with pollen allergies. A PDV system could be the solution to a more comfortable summer.

Some people who suffer seasonal health conditions, like pollen allergies and hay fever, have noticed improvements in their condition when the PDV system is fitted.

PDV system enables a constant supply of filtered fresh air into your home. And this is an excellent way to control pollutants in your indoor atmosphere, which leads to a positive impact on health.

4. Passive Displacement Ventilation controls impurities

The air quality you live in is probably not too good, especially if you live in a busy city. And in most cases, the air indoors is more polluted than the air outside. PDV system will help expel a buildup of pollutants, moisture, bacteria, and unpleasant odor.

5. PDV regulates airflow cost-efficiently.

PDV enables you to regulate the airflow in your home. Too much fresh air means more energy bills, but with PDV you have regular access to fresh air in a cost-efficient way.  

6. PDV reduces the effects of naturally-occurring Radon gas.

Radon gas is a colorless, odorless radioactive gas formed by decayed small amounts of uranium that occur naturally in soils and rocks. When this happens, it gives another radioactive element known as Radon’s Daughters. These substances can stick to dust particles in the air, and, when inhaled, they get stuck to the lung’s airways.    

Some parts of the country are usually affected by this naturally-occurring gas. According to the World Health Organization (WHO), Radon exposure causes between three and fourteen percent of all lung cancer cases. Radon is mostly found in places with high levels of granite and other igneous rocks.  It is particularly prevalent in areas such as the East and the South West Midlands.

If your area is Radon-affected, you need to have some tests carried out in your home. The solution may be a Positive Input Ventilation system like PDV to help lower Radon to safe levels. When you do this, you will reduce the risks to your health.

7. Passive Displacement Ventilation reduces the impact of Volatile Organic Compounds.

Volatile Organic Compounds (VOCs) are invisible gases that can cause many health issues in your home.  According to the Scientific Committee on Health and Environmental Risks, there are about 900 chemicals in indoor air.

These chemicals possess negative effects and are very more common in buildings without adequate ventilation. The numbers from the Environmental Protection Agency indicates that the air inside homes could contain 70 percent more pollutants than it does outdoors.

VOCs usually come from a wide range of sources, such as air fresheners, cosmetics, or permanent markers. When you perform certain activities like stripping paint and cleaning, VOC levels could increase to 1,000 times higher than outdoor background levels. You can take measures to protect you and your family by installing an effective ventilation system. PDV will dilute and control VOCs in your home by constantly introducing clean, fresh air from the outside.


It is undeniable that fresh air is essential to humans as we all need it to survive. Since we can’t constantly stay outdoors and breathe clean air, we need good ventilation systems. Passive Displacement Ventilation (PDV) is one effective way to keep the air fresh, whether in your home or workplace.

While choosing a ventilation system, keep in mind that the PDV system effectively filters and improves indoor air, thereby improving your health.

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