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Air Changes per Hour formula and calculator
- Last Updated: January 31, 2023
The number of Air Changes per Hour (ACH) does have a big impact on the energy efficiency of the building and the well-being of the occupants.
On this page, we will 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 it easy to perform all the necessary calculations.
TABLE OF CONTENTS – QUICK NAVIGATION
1. What is Air Changes per Hour and why it its important
1.1. Recommended Air Changes per Hour
2. Air Changes per Hour formula
2.1. How to calculate the volume of a room
2.2. How to find out the airflow of a ventilation system
2.3. Air changes per hour calculation
3. Air Changes per Hour calculator
3.1. Volume of a room calculator
3.2. Cubic meter to cubic feet calculator
3.3. Cubic feet to cubic meter calculator
3.4. Air Changer per Hour (ACH) calculator – imperial units
3.5. Air Changes per Hour (ACH) calculator – metric units
4. Tips for good indoor air quality
5.1. What is mechanical ventilation?
5.2. What is natural ventilation?
5.3. Is mechanical ventilation better than natural ventilation?
5.4. Can opening a window substitute mechanical ventilation?
5.5. Do more powerful fans results in higher electricity cost?
What is Air Changes per Hour and why it is important
The Air Changes per Hour (ACH) is the number of times inside and outside air is exchanged within a room, apartment, house, or building.
A too-low ACH will lead to stale and stagnant air and an increase of toxins and viruses, … as well as mold formation and other undesired consequences.
A too-high ACH in winter will lead to losing too much heat air and increasing the heating bills.
A too-high ACH in summer will lead to losing too much cool air and increasing the cooling bills.
It is important to know the ACH of your household and the rooms, in particular, to ensure that the ACH is not too high or too low.
This page will show you how to calculate the number of Air Changes per Hour on a real household example using the Lunos e2 as a mechanical ventilation system.
Recommended number of Air Changes per Hour
To ensure that the number of Air Changes per Hour is not too high or too low, first, we need to know 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 and 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 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 Changes per Hour formula
The Air Changes per Hour (ACH) is a metric that defines how often an air exchange system can fill up the total 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 is multiplied by 60 (second) to get the ‘Airflow in cubic feet per hour.
So, if your ventilation device states 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’ 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 states 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 need to know the Length (L), Width (W), and Height (H) and use the following formula:
V = L x V x H
For example, in metric units, a room with a Length of 3.46 meters, 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, 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 a home ventilation system.
In this case, finding the air flow rate is as easy as navigating the manufacturer’s website and looking for the technical specifications.
As shown in 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.
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 in the picture below, we have looked for ‘Honeywell HT-900E air flow’, and the results have shown that this specific fan has an airflow of around 740 cubic meters per hour.
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 (ACH) Calculation
We will use the bedroom dimensions we used for the volume calculation and the Lunos e2 running at the lowest fan speed for our calculations.
ACH in metric units for a room of 31.75 cubic meters with an airflow 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 airflow 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 airflow 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 airflow of 1,341.95 cubic feet per hour
ACH = 1,341.95 / 1,114
ACH = 1.2
1.2 air changes per hour
Air Changes per Hour Calculator - ACH Calculator
Volume of a room calculator
The calculator below can 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 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 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 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 stale air and moisture accumulation.
Once the furniture and fabrics have absorbed the excess moisture, it becomes more challenging to eliminate and leads to mold growth.
The more airtight a residence is, the greater the importance of having adequate and continuous ventilation.
Also, intermittent ventilation may sometimes be necessary, for example, while cleaning the house with chemicals or at family gatherings. Still, intermittent ventilation is generally less energy efficient than continuous ventilation.
2. Keep regular maintenance of your ventilation system
If the ventilation system is poorly maintained and the filters or fans work under optimal conditions, the risk of inadequate ventilation increases.
Clean the filters regularly and ensure that the ventilation system receives adequate and regular maintenance.
3. Do not dry your clothes inside your home
If you have ever used a tumble dryer, you know that liters and liters of water are extracted from wet clothes during the drying process.
Drying clothes inside a house leads to many liters of water being released.
And in most cases, due to the lack of adequate ventilation, that moisture is absorbed by the fabrics or accumulates in cold spots with an increased risk of mold growth.
4. Keep good ventilation after showers and baths
Keep the fan operation from 10 to 60 minutes after having a shower or bath to remove the excess moisture in the air.
5. Turn on the cooking ventilation hood
While cooking, the food releases significant amounts of moisture that should be controlled to prevent that moisture from accumulating and causing undesired effects.
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 stale air and moisture accumulation.
But, to increase the energy efficiency of the houses by preventing heat or cold air losses, natural ventilation has been substituted by controlled mechanical ventilation.
Or, in the worst cases, houses have been designed with no natural or mechanical ventilation, which may be energy efficient but lead to poor indoor air quality.
Is mechanical ventilation better than natural ventilation?
Mechanical ventilation reduces 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 open 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 humidity levels are accumulated during cooking and laundry, opening a window a couple of times during the day will not fully prevent the undesired accumulation of moisture.
Do more powerful fan result in higher electricity costs?
In general, modern fans cannot be compared to older models.
Modern fans are more efficient and may use less energy than older fans with higher power ratings.
Do fans need to be loud to be powerful?
This was the case 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 reduces noise emissions.
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 may want to check the Effiworkx electricity calculators.
To save energy and lower your electricity bills, you need to know how much electricity your home appliances and devices use.
Ventilation Frequently Asked Questions (FAQ)
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