Air Change in A Room: The What, Why and How Answered

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Imagine living in a space, which is deprived of fresh air. It is sure that you will feel smothered and choked within no time. There is no doubt that natural ventilation is the most important factor to make a space liveable and here we are going to shed some light on the ‘air change in a room’!

Natural Ventilation

All buildings need to be naturally ventilated, keeping the focus on making the set up energy efficient.

Two factors ensure natural ventilation or movement of air inside the building and they are:

  1. Wind pressure
  2. Stack pressure
Wind Pressure & Stack Pressure

However, mechanical ventilation may be required when the area is too huge or there are too many people living inside the set up or if complex activities are undertaken inside the building.

When wind hits the building, it induces a positive pressure on windward side (the direction from which the wind is blowing) and negative pressure on leeward side (side that is protected from the wind). This allows the air to enter the building and flow from the high pressure windward openings to low pressure leeward direction. Similarly, the temperature difference within the building, i.e. hot air rising upward, will also make the air to flow.

Thus, the air continuously keeps flowing in and around the building, providing ventilation to the interior rooms and spaces.

Now, to start with, let us understand it in detail and get introduced to the term ‘air change’.

What Is Air Change In A Room?

Air Exchange

Air change in a room is nothing but the number of times the air enters and exits a room in one hour. An air change is defined as the air equivalent to the volume of room/space replaced with the set of same air volume.

In sixty minutes how many times the room will fill up the air from the supply is known as an air change. It will be called Air Changes per Hour.

Air Change Formula

Air change in a room is a measure of the air volume added to or removed from a space (normally a room or house) divided by the volume of the space.  

In other words, it is the amount of air escaping into or out of the room in connection to the room volume.

According to IS: 3362 – 1977 Indian Standard CODE OF PRACTICE FOR NATURAL VENTILATION OF RESIDENTIAL BUILDINGS, in rooms occupied by only a small number of persons, normal air change will automatically be attained in cool weather by normal leakage from the windows and other openings. This may easily be secured in warm weather by keeping the openings open.

Here, it is important to get familiar with this term because the air change rate is a vital factor in knowing how well the Heating, Ventilating and Air Conditioning (HVAC) system or the natural ventilation of your house is working, and whether the conditions inside your house is healthy, favorable, comfortable, warm and worth living for the members of your house or not.

In brief, the air change rate will depend on following factors for naturally ventilated spaces:

  1. Wind speed
  2. The size of openings on both faces
  3. The temperature inside and outside the building

It depends on local climatic factors and vary from place to place depending on the environmental parameters and also on the density of buildings in a given location.  

The air change per hour varies in different parts of the world depending upon different climatic factors. Thus, it is advisable to refer to the national standards of your area to know the perfect and precise air change per hour so as to achieve conducive living conditions inside your house. National Building code of India (NBC)[343] suggests air changes per hour for different rooms as follows:-

Sr. No.Application for HousingAir Change per Hour
1Living Rooms3 – 6
2Bedrooms2 – 4
3Bathrooms6 – 10
4Toilets6 – 10
5Shower baths15 – 20
6Kitchen8 – 12
7Utility Rooms15 – 20

Now let’s have a look at air change rate formula for your room.

Air Change Rate Formula

Air Change Rate

 For comfortable living, first compare the number of room air changes to the required air changes. If it’s in the required range, one can proceed to design or can balance the airflow and get assurance that you are doing a right thing. If it’s out of range, you need to take another look. Thus, for knowing the number of room air changes, it is mandatory to know the air change rate formula and steps involved in its calculation.

For calculating air changes in room, follow these steps:

  1. Measure the supply of airflow in room
  2. Multiply the CFM (Cubic Feet per Minute) times 60 minute per hour.
  3. Then divide the volume of room in cubic feet (volume of room = Width x Length x Height). In simple words, we are changing CFM into CFH (Cubic Feet per Hour).

Air Change Rate Imperial Units

Air change rate can be expressed in imperial-units as:

N = 60Q / V

Where,

N = number of air changes per hour

Q = Volumetric flow rate of air in cubic feet per minute (CFM)

V = Space volume L × W × H, in cubic feet

Air Change Rate – SI Units

Air change rate can be expressed in SI-units as:

n = 3600 q / V        

Where,

n = air changes per hour

q = fresh air flow through the room (m3/s).

V = volume of the room (m3)

Example of Calculating Air Change Rate in SI Units

For example, for a room with a volume of 200 (m3) with an airflow of 4 (m3/s),

n = 3600 q / V        

n = (3600 x 4) / 200

n = 72 per hour

Air change rate is generally measured per hour. Different rooms have different air change rate requirements depending upon their use and ventilation.

Here are some FAQs to clear doubts that you have about air change:

Please note that quick ventilation can be achieved by opening the windows completely about 5 times a day, which gives an air exchange of 0.3 to 4.

  • What would happen if there is/are:
  • No windows

According to Time Saver Standards for Architectural Design Data[382], “air changes imply that the air in a space is being changed so many times in an hour. It is not necessarily “new air” or outside air. It may be completely re-circulated.” Hence if there are no windows in a room, still air might be re-circulated, but with the least air change rate of 0.37hr–1.

  • Windows only on one wall

A room with one side window, however big, gets least air since there is no provision for air to exit, but, the ventilation will be still present due to diffusion. The air change rate gets a bit higher at about 0.8hr–1 than the one with no windows as the air change rate becomes roughly proportional to the width of window opening.

  • Windows on two opposite walls

As per the study conducted by Cynthia Howard-Reed, Lance A. Wallace & Wayne R. Ott, “Opening windows and doors produced the greatest increase in air change rates. Opening multiple windows increased the air change rate by as much as 2.8 hr–1. These increases are far greater than could be produced in a closed house even by extreme winds and temperature differences.”

However, it should also be remembered that, just because a window is open, required air change per hour (ACH) may not happen.

  • What would happen if the doors are kept open?

As mentioned above, air change rates are the greatest when both the doors and windows are kept open. With open doors and windows, the rate is about 4 changes per hour, meaning that it takes only 15 minutes to exchange air.

  • Does it have any relation with velocity of wind and size of openings?

Yes, the windowmaser has provided calculators to calculate air change, which is based on the following factors:

  1. Building location (i.e. open terrain, scattered vegetation, suburb, city).
  2. Height of opening from ground
  3. Orientation i.e. North, South, East, West
  4. Window height and its area
  5. Room volume
  6. Window location – Façade, Gable, Pitched etc.

According to calculators in windowmaster, in cities depending upon the temperature the air change for a 15.00 sqm room with 3 m height, with façade windows, will vary from 8 to 16 for a wind velocity of 3.2 km / h (mph) to 45 km/h (28 mph) respectively.

According to Helenice Sacht & Marieli AzoiaLukiantchuki (Published in Windows Size and the Performance of Natural Ventilation) “regarding the distribution of the airflow in the internal environment, it was observed that it was dependent on the size of the window, because a larger opening allows the passage of air resulting in a better utilization of the winds in the internal environment and in greater air change rates per hour.”

  • Will air change happen if windows are open but wind velocity is zero?

When the windows are open, air flows from high pressure to low pressure. But, when the wind velocity is zero, the air change cannot happen. As the formula says

Q = a * v, where v is the velocity of air.

When v =0, Q also becomes 0 resulting in no air change. However, some air may flow if the temperature difference between the outside and inside is substantial. In such case recourse should be taken for mechanical ventilation through fans.

  • Does air change happen in a/c rooms with closed windows?

With normal window air conditioner, the fresh air is not drawn upon. The same air is re-circulated. However, fresh air may enter, while you open the door for going in and out of the room. Some window a/c’s do allow entry of fresh air in the room.

  • What will happen in central a/c plant?

In mechanical ventilation, fans, ducts and control system are used to drive the ventilation. Apart from providing air outside, they are also used to distribute cool/hot air. The proportion of this air is also re-circulated to improve the efficiency of the system.

According to L A Wallace, S J Emmerich & C Howard-Reed, “the home HVAC system uses 100% re circulated air, resulting in minimal exchange with outdoor air. It was considered not to affect air change rates because all ductworks were contained in the house. Very little effect of either wind speed or wind direction was noted in this study. ”

  • What is the minimum area of windows/openings required in terms of percentage?

According to David H. Mudarri (Published in Building Codes and Indoor Air Quality), the minimum openable area to the outdoors shall be 4 percent of the floor area being ventilated. When openable area is provided through adjoining rooms, the openable area must be more than or equal to 8% of floor area of the interior room. For residences, a minimum ventilation rate of 0.35 air change per hour must be maintained.

Summing up, air change rate is a vital and an inevitable factor to be kept in mind for creating a perfect healthy and user-friendly atmosphere in your interiors. Air change rate becomes roughly proportional to the width of window opening. According to the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Fundamentals Handbook, “flow through an open window depends on several variables, including area of the window opening, number and location of window openings, wind speed, indoor/outdoor temperature difference, and height of the neutral pressure level (the point where the indoor pressure is equal tothe outdoor pressure).”

Air change rate is primarily dependent on window-opening area (width). As width of window opening increases, the airflow also increases resulting in greater air change rates per hour. Thus, refer the standards of your area and calculate the air change rate as we saw earlier.

Do refer our articles shown below which talk more about fresh air ventilation for your house and air quality –

What is Make Up Air?
Importance of Fresh Air in Your Home & Life!
Tips to Ensure the Good Ventilation in Home!

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