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Air Changes per Hour formula and calculator

The number of Air Changes per Hour does have a big impact the energy efficiency of the building and the well being of the occupants.

In this page we are going to go through the formulas needed to calculate the number of Air Changes per Hour so you can find out if your house or apartment is well ventilated.

Also, we have created several calculators that should make easy to perform all the necessary calculations.

What is Air Changes per Hour and why it is important

The Air Changes per Hour (ACH) is the number of times per hour that inside and outside air is exchanged withing a room, apartment, house or building.

A too low ACH will lead to stale and stagnant air and an increase of toxins, viruses, … as well as the formation of mold and other undesired consequences.

A too high ACH in winter will lead to losing too much heat air and and increase on the heating bills.

A too high ACH in summer will lead to losing too much cool air and an increase on the cooling bills.

All in all, it is important to know what is the ACH of your household in general and the rooms in particular to make sure that the ACH it is not too high or too low.

rooms layout

In this page we are going to show you how to calculate the number of Air Change per Hour on a real household example using the Lunos e2 as mechanical ventilation system.

Recommended Air Changes per Hour

To make sure that the number of Air Changes per Hour is not too high or too low, first we need to know what is the number of recommended air changes per hour. 

The ASHRAE 62.1 Standard – Ventilation for Acceptable Indoor quality recommends:

– For a living area: A minimum of 0.35 air changes per hour but not less than 15 cfm (7.5 liters per second) per person.

– For kitchens: 100 cfm (50 liters per second) intermittent, or 25 cfm (12 liters per second) continuous or openable window.

– For baths, toilets: 50 cfm (25 liters per second) intermittent or 20 cfm (10 liters per second) continuous or openable window.

– Garages: 100 cfm (50 liters per second) per car

– Common for several units: 1.5 cfm/square feet (7.5 liters per second x square meter)

But, note that the recommended value may vary depending on factors like the climatic conditions or the type of ventilation chosen.

The recommended values should be used only as an approximation.

And, do not worry if you don’t know what ‘cfm’ stands for, or if the formulas look too complicated, because in the next sections we explain all the concepts and provide all the calculators you may need.

Air Change per Hour formula

The Air Changes per Hour (ACH) is a metric that defines how many times an air exchange system can fill up the full volume of a room with air.

Depending on where you live, the ACH will be calculated using an imperial or metric formula.

The ACH formula in imperial units uses airflow in Cubic Feet per Minute (CFM) and volume in cubic feet:

ACH = (60 x Airflow in cubic feet per minute) / volume in cubic feet

Note that the ‘airflow in cubic feet per minute’ it is multiplied by 60 (second) to get the ‘airflow in cubic feet per hour’. 

So, if your ventilation device state the airflow in cubic feet per hour, you should use this other formula instead.

ACH = Airflow in cubic feet per hour / volume in cubic feet

The ACH formula in metric units uses airflow in liters per second (L/S) and volume in cubic meters:

ACH = (3.6 x Airflow in liters per second) / volume in cubic meters

Note that the ‘airflow in liters per second’ it is multiplied by 3.6 to get the ‘airflow in cubic meters per hour’.

1 liter per second = 3.6 cubic meters per hour

So, if your ventilation device state the airflow in cubic meters per hour, you should use this other formula instead.

ACH = Airflow in cubic meters per hour / volume in cubic meters

How to calculate the volume of a room

To calculate the volume (V) of a room, you just need to know the Length (L), Width (W) and Height (H) and use the following formula:

V = L x V x H

room volume

For example, in metric units, a room with a Length of 3.46 meter, a Width of 3.65 meters and a Height of 2.5 meters will have a volume of:

V =  3.46 x 3.65 x 2.5 = 31.57 cubic meters

Or, in imperial units, that same room would have a Length of 11.35 feet, a Width of 11.97 feet and a Height of 8.2 feet will have a volume of:

V = 11.35 x 11.97 x 8.2 =  1,114 cubic feet

How to find out the airflow of a ventilation system

The room in our example uses Lunos e2 as home ventilation system.

lunos e2

In this case, to find the air flow rate it is as easy as to navigate to the manufacturer website and look for the technical specifications.

As shown on the picture below, the Lunos e2 has an air volume flow between 15 and 38 cubic meters per hour.

The Lunos e2 has three fan speed options so the 15 cubic meters per hour corresponds to the lowest speed and the 38 cubic meters per hour corresponds to the highest speed.

lunos e2 specifications

If you would like to know the air flow rate of your specific device, the best would be to look for the specifications using a search engine.

As shown on the picture below, we have looked for ‘Honeywell HT-900E air flow’ and the results have straight away shown that this specific fan has an air flow of around 740 cubic meters per hour.

fan airflow rate search

But, depending if you need to know the airflow in imperial or metric units, you may need to convert the results:

1 cubic meter = 35.31467 cubic feet

1 cubic foot = 0.02831 cubic meter

So, for example, for the Lunos e2 volume flow of 15 cubic meters per hour:

15 cubic meters x 35.31467 = 529.72 cubic feet

15 cubic meters x 1 hour = 529.72 cubic feet per hour

Air changes per hour calculation

For our calculations, we will use the bedroom dimensions we used for the volume calculation and the Lunos e2 running at the lowest fan speed.

ACH in metric units for a room of 31.75 cubic meters with an air flow of 15 cubic meters per hour:

ACH = 15 / 31.75

ACH = 0.47

0.47 air changes per hour

ACH in imperial units for a room of 1,114 cubic feet with an air flow of 529.72 cubic feet per hour

ACH = 529.72 / 1,114

ACH = 0.47

0.47 air changes per hour

As 0.47 air changes per hour it is not sufficient for some specific rooms, like a laundry room, let’s repeat the calculations but this time let’s have the Lunos e2 running at the maximum fan speed:

 ACH in metric units for a room of 31.75 cubic meters with an air flow of 38 cubic meters per hour:

ACH = 38 / 31.75

ACH = 1.2

1.2 air changes per hour

ACH in imperial units for a room of 1,114 cubic feet with an air flow of 1,341.95 cubic feet per hour

ACH = 1,341.95 / 1,114

ACH = 1.2

1.2 air changes per hour

Air Change per Hour calculator

Volume of a room calculator

The calculator below can be used to calculate the volume of an enclosed space like a room. 

 

For example:

> For a room length of 3 (meters, feet,…)

For a room width of 3 (meters, feet,…)

For a room height of 2.5 (meters, feet,…)

The volume is 22.5 (cubic meters, cubic feet,…)

Cubic meter to cubic feet calculator

The calculator below can be used to convert cubic meters to cubic feet.

 

Cubic feet to cubic meter calculator

The calculator below can be used to convert cubic feet to cubic meters.

 

Air Changes per Hour (ACH) calculator - imperial units

The calculator below can be used to calculate the Air Changes per Hour, for airflow in cubic feet per minute.

 

For example:

> For a room with an airflow of 10 cubic feet per minute

> With a volume of 1100 cubic feet

The room will have 0.545 air changes per hour.

The calculator below can be used to calculate the Air Changes per Hour, for airflow in cubic feet per hour.

 

For example:

> For a room with an airflow of 880 cubic feet per hour

> With a volume of 1100 cubic feet

The room will have 0.745 air changes per hour.

Air Changes per Hour (ACH) calculator - metric units

The calculator below can be used to calculate the Air Changes per Hour, for airflow in liters per second.

 

For example:

> For a room with an airflow of 10 liters per second.

> With a volume of 30 cubic meters.

The room will have 1.2 air changes per hour.

The calculator below can be used to calculate the Air Changes per Hour, for airflow in cubic meters per hour.

 

For example:

> For a room with an airflow of 40 cubic meters per hour.

>With a volume of 30 cubic meters.

The room will have 1.33 air changes per hour.

Tips for good indoor air quality

1. Maintain continuous ventilation 

Our residences require continuous ventilation to prevent accumulation of stale air and moisture. 

Once the excess moisture has been absorved by the furniture and fabrics, it becomes more difficult to eliminate and leads to mold grow. 

The more air tight a residence is, the bigger the importance is of having an adequate and continuous ventilation.

Also, intermittent ventilation may be necessary at times, for example while cleaning the house with chemicals or family gatherings, but in general intermittent ventilation is less energy efficient that continuous ventilation.

2. Keep regular maintenance of your ventilation system

If the ventilation system is poorly maintained and the filters or fans are working under optimal conditions, the risk of inadequate ventilation increases. 

Clean the filters regularly and make sure that ventilation system receives adequate and regular maintenance.

3. Do not dry your clothes inside of your home

If you have ever use a tumble dryer, you know that liters and liters of water are extracted from the wet clothes during the drying process.

Drying clothes inside a house lead to many liters of water being released inside the house.

And in most of the cases, due to the lack of adequate ventilation, that moisture it is absorved by the fabrics or accumulating on cold spots with the increased risk of mold growth.

4. Keep a good ventilation during after showers and baths

Keep the fan operation from 10 to 60 minutes after having a shower or bath to removed the excess of moisture in the air.

5. Turn on the cooking ventilation hood

While cooking, the food releases significant amounts of moisture that should be kept under control to prevent that moisture from accumulating and causing undesired effects.

Ventilation FAQ

What is mechanical ventilation?

Mechanical ventilation is the intentional exchange of indoor air with fresh outdoor air using fans.

What is natural ventilation?

Traditionally, the houses were ventilated by strategically placed windows or openings that allowed the air to enter and exit the house to prevent the accumulation of stale air and moisture.

But, in order to increase the energy efficiency of the houses by preventing heat or cold air losses, natural ventilation has been substituted my controlled mechanical ventilation.

Or, in the worst cases, houses have been designed with not natural or mechanical ventilation which may be energy efficient but lead to very poor indoor air quality.

Is mechanical ventilation better than natural ventilation?

Mechanical ventilation reduce the undesired hot air losses to the outside or cold air entering the house during winter. And the other way around during summer.

Therefore, correctly designed and installed mechanical ventilation is more energy efficient than natural ventilation.

Can opening a window substitute mechanical ventilation?

The benefits of continuous and controlled mechanical ventilation can not be compensated by opening a window every so often.

If high level of humidity are being accumulated during cooking and laundry, opening a window a couple of times during the day will not fully prevent undesired accumulation of moisture.

Do more powerful fan result in higher electricity costs?

In general yes, but modern fans cannot be compared against older models.

Modern fans are more efficient and even may use less energy that older fan with higher power rating.

Do fans need to be loud in order to be powerful?

Yes, this was the case in the past when the noise emissions were not considered a problem or the fan manufacturing technology was less advanced.

But modern fans are designed with technology that reduce the noise emission.

A well designed and manufactured fan can be less noisy than a less powerful but poorly designed and manufactured fan.

How to save energy calculators

We are still working on the Effiworkx heating and cooling calculators section, but in the meantime you way want to check the Effiworkx electricity calculators.

If you want to save energy and lower your electricity bills, you need to know how much electricity your home appliances and devices are using.  

Also, we have prepared tips and advice on how to save electricity that may be of your interest.

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