Air Changes Per Hour – ACPH or ACH Details

Air changes per hour or air change rate is a very common term in ventilation calculation as well as ventilation design. It is abbreviated, as,

  • ACPH
  • ACH

We need to do ventilation calculations for different areas in different industries and ACPH or ACH is one of the main factors for that calculation.

What are Air Changes per hour?

Air Changes per hour basic

In ventilation design, there may be few things,

  • The room may require fresh air,
  • The room may have more CO2 gas, which needs to be replaced by fresh air,
  • The room may have chemical or toxic gases
  • The room may have fumes, which needs to be removed,
  • The room may require a temperature set like my room will be limited to a maximum of 5 or 6 deg. C temperature above the atmosphere, etc.

These all cases are associated with the ventilation system. How do we do this ventilation system?

It is simple, we add air or remove air from the area by considering air changes per hour.

Air Changes per hour definition

It is basically the ratio of the volume of air which is to be added or removed from an area to the volume of area.

  • It is assumed that air into the area is uniform.
  • Mixing of air is proper inside the area.
  • Maintain uniform velocity.
  • It can be stated as the number of times the air is entered into the area and exits from the area in one hour.

ACPH meaning

1 ACPH means,

  • 1-time air enters to a room and exits from the room in 1 hour, or
  • Air fills into room 1 time in 1 hour,

5 ACPH means,

  • 5-time air enters to a room and exits from the room in 5 hours, or
  • Air fills into the room 5 time in 1 hour,

Air Changes per Hour Formula

Let us see, how do we calculate the air changes per hour? To calculate the ACPH, we have to do the followings,

  • Need to measure the supply or exhaust air quantity, by using any device, like thermometer or velometer or vane anemometer,
  • Need to measure the room length, width & height,
  • Calculate the volume of the room,

ACPH will be air quantity divided by room volume,

That means,


  • Q = Air quantity, in
  • L = Length of room
  • W = Width of the room
  • H = Height of the room
  • V = Volume = L x W x H

Note, ACPH is defined in various standards and it is fixed for specific areas or specific applications. Suppose if it is a battery room, the ACPH for battery room is available in various standards to use it.

Air Change Rate with different units

Air change rate expressed in the below equation,

Q = V x ACPH

or ACPH = Q / V in S.I Unit

or, ACPH = Q x 60 / V in F.P.S Unit


ACPH = air changes per hour

Q = Air flow rate

V = volume of the space or room

SI Units

Q = Air quantity in m3/hr

V = volume of the in m3

F.P.S Units

Q = Air quantity in C.F.M (Cubic Feet per Minute)

V = volume of the in ft3


If 7200 m3/hr air is supplied to 120m3 volume room,

ACPH will be = Q / V = 7200/120 = 6

Hence, ACPH =6

If 800 CFM air is supplied to 4800 ft3 volume room,

ACPH will be = Q x 60 / V = 800 x 60/4800 = 10

[800 CFM = 800 CFM x 60 ft3 /hr]

Hence, ACPH =10

Standard ACPH rate

ACH or ACPH is based on industry, national standards, international standards, client-specific requirements as well as location. However, based on our experience, a reference list is given,

Name of AreaAir Changes per Hour (ACPH)
General Areas0.5 – 1
Corridors2 – 4
Meeting rooms4 – 6
Normal Stores6 – 10
Chemical Stores10 – 15
Educational Facilities
Student Classrooms6 – 12
Laboratory6 – 12
Library4 – 10
Office Buildings0.5-1.5
Office Rooms0.5-1.5
Municipality Buildings0.5-1.5
Public Assembly Spaces0.5-1.5
Halls for public gathering4 – 6
Auditorium4 – 12
Electrical Room0.5
Pump Room10-15
Battery Room (Lead-acid)12-13
Battery Room (Ni-Cd)4- 6

Calculate of Air Changes per Hour

We have already learned the formulae of calculating air changes per hour, here, look at all the simple steps for the calculation in S.I units

Step 1

Measure the air quantity, Q in m

Step 2

Measure the length, L in m

Step 3

Measure the width, W in m

Step 4

Measure the height, H in m

Step 5

Calculate the volume,

V = length x width x height = L x W x H in m3/hr

Step 6

Air changes per hour (ACH or ACPH) = Q / V = Q / (L x W x H)

Example of ACPH Calculation

Let’s take a simple example. You are in a room and the room needs to be ventilated. The size of the room is 10m x 5m x 5m.

Measure air quantity = 2000 CMH,

So, what is the air changes per hour?


  • Air quantity, Q = 2500 CMH
  • Length, L = 10m
  • Width, W = 5m
  • Height, H = 5m

Hence, Volume V = 10 x 5 x 5 m3

= 250 m3

As per formulae,

ACPH = Q / V = 2500 / (10 x 5 x 5) = 10

Is ACPH is the only method for ventilation calculation?

In the case of ventilation calculation, ACPH is not the only method by which the calculation or the equipment selection is done.

For example,

Let us consider, an electrical room which has heat-generating equipment. To understand, consider,

  • Electrical room length = 10m
  • Electrical room width = 6m
  • Electrical room height = 5m
  • Equipment heat generation = 12.0 kW

So, volume = 10 x 6 x 5 = 300 m3

We know from the standard ACPH list,

ACPH for electrical room = 6

Or, Air quantity, Q1 =ACPH x 300 = 6 x 300 = 1800 m3/hr

Now, if 1800 m3/hr is provided in the electrical room, the room will be ventilated, but what about the equipment heat, generated by electrical items?

Will it be restricted to be heated?

No, unless, a separate calculation method to be adopted. This second method is called the heat load method.

A standard formula for this method,

Heat, q

  • q = m x Cp x Δt
  • q = ρ x Q x Cp x Δt
  • Q = q / (ρ x Cp x Δt)


  • q = Heat Generated in kW = 12.0 kW
  • Cp = Specific Heat = 1.005 kJ/kg-K or kJ/kg-Deg. C (Same)
  • ρ = Density of air = 1.2 kg/m3
  • ∆T = temperature difference between outside temperature and inside temperature= 5ºC [∆T depends on the project requirements, here it is 5ºC]

As per the equation,

Q2= q / (Cp x ρ x ∆T) in m3/s

or, Q2 = 12.0 / (1.2 x 1.005 x 5) in m3/s

or, Q2 = 1.99 m3/s = 7177 m3/s = that is > Q1

Hence, Q2 is more than Q1.

Air quantity in the ACPH method is less than the heat load method, in this example.

Hence, during ventilation calculation, both methods are to be considered, as in many cases, it varies.

Project Example

Let us take a project example,

Reference drawing

Input Data

From the drawing, the following data, are as follows,

  • Length of pump hosue, L = 60m
  • Width of pump house, W = 12m
  • Height of pump house, H = 10m
  • ACPH = 10
  • Heat Load data, q = 14.4 kW
  • Temperature difference = 5ºC

Design Calculation

ACPH Method

Volume of pump house, V = L x W x H = 60 x 12 x 10 m3

Air quantity, Q  = V x ACPH = 60 x 12 x 10 x 10 = 7200 m3/hr

Heat Load Method

Q2 = q / (Cp x ρ x ∆T)

or, Q2 = 14.4 /(1.2 x 1.005 x 5) m3/s

or, Q2 = 2.39 m3/s = 8604 m3/hr

Hence, 8604 m3/hr air quantity is selected.

Equipment Selection

The pump house needs to be positively ventilated, which means supply and exhaust to be provided and supply shall be more than exhaust.

As per drawing, 3 nos of supply fans, each of 2868 m3/hr (8604/3) nos.

Exhaust fans, 3 nos. 2868 x 0.8 = 2295 m3/hr.

The numbers and capacity of supply fans and exhaust fans shall be based on the building layout, space availability.

Why ACPH is an important factor?

ACPH is very important to design a ventilation system, and proper ACPH selection incurs a proper ventilation design. A proper ventilation system helps, as follows,

  • It removes pollutants from the ventilated space.
  • It supplies fresh air,
  • Improves indoor air quality (IAQ)
  • It helps to maintain inside temperature, humidity up to a certain level.
  • It can help pressurize the space by supplying more air and exhausting less air.
  • It gives a certain level of comfort to human beings.
  • It restricts to increase the temperature up to a certain level which helps all equipment to maintain it’s limited temperature.


Refer to ASHRAE 62.2-2016, “Ventilation and Acceptable Indoor Air Quality in Residential Buildings” for ventilation details.


So, finally, we have got the basic idea about air changes per hour or ACH or ACPH along with various examples, diagrams.


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