The exhaust rate in m3/Hr is a measure of airflow. In simple terms, it is a measure of the rate at which a certain volume of air is moved. The higher the exhaust rate, the greater is the volume of air that is moved.
This page serves as a residential kitchen chimney exhaust rate requirement calculator. When there are standards availalbe, the calculations are based on the recommendations of the Home Ventilation Institute (HVI) of the USA as similar guidlines do not seem to exist for India.
There is a lot of false information out there regarding the calculations of kitchen ventilation exhaust rates for homes. Most of the so-called guides base it on commercial establishment requirements which results in people needing 1000 - 2000 m3/Hr when in reality just 1000 m3/Hr is more than what most homeowners will ever need.
Moreover, there are plenty "rules-of-thumb" with exhaust rate numbers being thrown around without solving the real simple mathematics. I have explained the process and taken care of it for you through this calculator.
Since this page serves as a calculator, I’m not going to get into the theory right now, but you can find details and formulae used just below the calculator. To get there, you can click here.
NOTE: These are mainly recommendations based on the HVI and you are free to pick based on your personal judgment. In the USA if you have an exhaust rate of more than 675 m3/Hr you are required to have a makeup air system in many municipalities because some municipalities tend to have closed windows during the winter. If you do not have open windows this is something you will have to consider.
How much Ventilation Do I need?
The amount of ventilation that one needs in their home, or for the purpose of this discussion, the kitchen is that value which is adequate to remove any excess moisture, cooking odors, indoor air pollutants, etc. depending on what is cooking and how well the kitchen chimney captures the fumes.
According to the HVI, the amount of ventilation needed varies a lot depending on both the type of cooking and the position of the cooking range. kitchen chimneys which are placed above the cooktop capture contaminants with their canopy and effectively exhaust them with a lower volume of air when compared to downdraft exhaust systems, which require a higher volume of air for the capture and exhaust of the contaminants. The HVI does not directly specify recommendations for downdraft kitchen chimneys and they ask you to consult the kitchen chimney manufacturers to determine what you need.
When it comes to kitchen chimneys, it’s ability must be compatible with the exhaust generation rate of your cooking range and the type of food you cook. If you cook on high settings and if you’re cooking food with a lot of emissions, you will need a relatively powerful kitchen chimney, above the minimum requirements. Ducting of the right dimensions is crucial for the effective performance. With bad ducting, it will not perform at its rated capacity. The efficiency is further lowered based on the length of the duct and the bends.
Suggestions Based on the Home Ventilation Institute (Length and Location of Your Cooktop)
The Home Ventilation Institute, on their website has a list of minimum ventilation requirements for kitchen exhaust systems as well as their suggested CFM values per foot of your range.
|Positioning of the Cooking Range||Recommended Ventilation Rate per Linear Foot of Range||Minimum Ventilation Rate per Linear Foot of Range|
|Against a Wall||100 CFM||40 CFM|
|Island||150 CFM||50 CFM|
Using the specifications above, the tables below give you the requirements for standard kitchen chimney dimensions for wall backed cooking ranges (under cabinet and wall-mount kitchen chimneys) and island ranges (island kitchen chimneys) respectively.
|Wall Backed kitchen chimneys|
|Width of the hood||30 Inches (2.5 feet)||36 Inches (3 feet)||48 Inches (4 feet)|
|HVI Recommended||250 CFM||300 CFM||400 CFM|
|Minimum||100 CFM||120 CFM||160 CFM|
|Island kitchen chimneys|
|Width of the hood||30 Inches (2.5 feet)||36 Inches (3 feet)||48 Inches (4 feet)|
|HVI Recommended||375 CFM||450 CFM||600 CFM|
|Minimum||125 CFM||150 CFM||200 CFM|
It is important to realize that these requirements only take into account the dimensions of your cooking range and not the ducting, etc. Moreover, the recommended values are based on the average cooking requirements. So if you think you cook more (or you cook food with more emissions of vapor, smoke, and grease) than the average homeowner, you're going to have to go a little higher on the CFM value for your kitchen chimney to do a great job and Indian cooking emits a lot more fumes and odor and therefore in the introduction I already increased the suggested minimum exhaust rates to something suitable for Indian homes.
Additionally, these CFM requirements are calculated for an average kitchen which is 10 feet × 10 feet × 8 feet. I, therefore, have included a calculator to determine exactly what you need for your kitchen. For homes with an open kitchen plan, this can be a bit tricky. It's best to consult a local expert in this case. But, if this is something you cannot do, I would recommend that you make sure that you have a kitchen chimney with a capacity that satisfies the other criteria because the CFM needed based on the volume of your kitchen is almost never larger than the CFM required to fulfill the other conditions.
CFM Based on Power of the Cooktop (BTU)
Apart from the dimensions of the cooktop and its location, you also need to take into consideration the rate at which it produces heat. The amount of heat produced is traditionally measured in British Thermal Units (BTU).
If you look at other websites that talk about kitchen chimney CFM requirements, you will notice that they assume that for every 1000 BTU you require 10 CFM (a factor of 100 lower). This is wrong and it must be 6.13 CFM instead. Following the wrong calculations almost doubles your CFM requirement! I have plenty of sources and detailed calculations to back up my statement. I do not believe in just throwing numbers around with no explanation on its origins. Let's do some Maths together!
Converting Between BTU, Watts and CFM
In order to completely understand the calculations, we need to go through some basic definitions and we will convert everything to the SI (metric) system as that is the system used globally for scientific calculations. Relevant sources for conversions, etc. are also provided.
- BTU: One BTU is that amount of energy that is needed to raise the temperature of one pound of water (0.45 liters of water) by one degree Fahrenheit at sea level. In the metric system, the calorie is used in its place and is defined as the amount of heat required to raise the temperature of one gram of water by one degree Celcius. Heat is a form of energy and one BTU is between 1054 - 1060 joules. It is widely accepted to be 1055 J. Stove output, however, is measured in BTU per hour. Therefore one BTU/hour = 1055 J / hour which equals 1055 J / 3600 seconds. Therefore, one BTU equals 0.293 J/sec or 0.293 Watts (see definition of Watt below)
- Watt: This is the metric unit of power and is equivalent to one joule per second. Therefore 1 W = 1 J/sec or 1 kg×m2/sec3 based on simple unit conversion as confirmed by Wikipedia.
- CFM: CFM is traditionally measured in atmospheric conditions at sea level and is technically called the atmospheric cubic foot per minute. This is, therefore (1 atm × cubic feet)/minute. But one atmospheric pressure in metric units is 101325 Pascal. The Pascal is a unit of pressure which is essentially Force (N or Newton) per unit area (measured in square meters). The Pascal, therefore, has the units N/m2. The Newton according to Newton's law of motion F = m×a further breaks down to be kilogram×meter/second2. Lastly, one foot equals 0.3048 meters. Combining all of this, CFM is essentially: (101325 N/m×0.30483 m3)/60 s. This equals 47.82 N×m/s. Replacing N with kg×m/s2 we get 47.82 kg×m2/sec3 or 47.82 Watts. Therefore one CFM equals 47.82 Watts.
We now have a conversion between BTU to Watts and CFM to Watts. But we are interested in converting BTU/hr to CFM and Watts to CFM. Let's move some things around to get these relations:
Watts to CFM
One CFM equals 47.82 Watts. Therefore, 100 Watts equals 100/47.82 CFM which is 2.09 CFM. This is similar to what is shown on the tables here.
BTU/hr to CFM
1000 BTU equals 293 Watts, but 100 Watts equals 2.09 CFM. Therefore 293 Watts equals 6.1237 CFM. Finally, we reach the conclusion that 1000 BTU equals 6.1237 CFM and not 10 CFM. Blindly following general rules of thumb instead of doing your Maths can be expensive! This is the same as seen on this calculator.
CFM to m3/Hr
By a simple conversion of feet to meters and minutes to hours, all we need to do to convert a given CFM to m3/Hr is multiply the CFM value by 1.69901082 units
Important Points to Note
There are a few points regarding kitchen chimney efficiency and use that the general public is not aware of. Please understand and keep these in mind when you select your kitchen chimney:
- Exhaust rate isn't everything: It is just one piece to the puzzle. One of the most important pieces I must add. The other is the capture efficiency which is determined by the shape of the hood and how far over the cooktop it extends. The broader your kitchen chimney (how far out towards you from the wall), the better capture area it would have. Therefore, the best kitchen chimney would be one that has a good capture area along with your required exhaust rate. Do not be led on by manufacturers that talk about having extra deep capture areas. This does not make sense. Area is two dimensional, having a really narrow hood with lots of depth is useless. You need a balance of breadth and depth. For more information on all of this, take a look at my article on determining kitchen chimney dimensions. Unfortunately, there is no metric that combines CFM and kitchen chimney capture efficiency that you could use to make the best choice. My kitchen chimney reviews try to cover all of this when possible.
- There is no law or building code as far as I know that asks you to meet the requirements of exhaust rate based on the volume of your kitchen. In many cases this is the number that puts kitchen chimneys over the top and you would never need to really use this power.
Additional Power Due to the Ducting
So far we have seen the base exhaust rate that you would need. However, when you duct a kitchen chimney, you're going to need additional power to compensate for the losses. The volume of your ductwork (area of the cross-section of the duct multiplied by the length of the duct), the number of bends and the presence of a roof-cap play a role here.
There is no way to program an online tool to determine the exact losses due to the ducting because of the many varied factors that go into it: The equivalent area (area if the duct were a circle), exhaust rate of the blower, the static pressure, etc. play a role.
There is no need of us to get into the nitty-gritty details because there's a simple way to do estimate your losses. According to Canfilters and PlanetNatural, there is approximately a 7% loss in CFM for every 25 feet of ducting if you use flexible ducting and it is 3% for metal (non-flex) ducting. Also, there is an additional 3% loss for every 90-degree bend. There is an additional 3% loss if you use a roof cap.
Most websites say you lose 25 CFM on a 90 degree bend. This is simplified too much and is way off the mark most of the time (3% of 400 CFM is just 12 CFM and not 25). Following these websites which are written by non-tech people who don't seem to bother that they are providing wrong information that costs people would simply blow up the CFM you require.
My calculator works by first determining the exhaust rate you need and displaying these requirements to you based on the different criteria. There is also a specific output highlighting the minimum and recommended capacity using the HVI suggestions.
You would not necessarily be picking the maximum exhaust rate based on the different values. For example, you may originally calculate the exhaust based on the volume of your kitchen, but later decide that it just blows up and is not something you would want to stick to.
Therefore, when determining the losses due to duct efficiency, I do not give you the ultimate losses or the excess exhaust rate you would need, because I do not know which value you decide to go by. Instead, I give you a factor that you need to multiply the chosen exhaust rate by in order to determine the actual output you would receive if you were to buy a kitchen chimney with that particular exhaust rate.
For example: If the CFM is determined to be 400 and you have 12 feet of non-flex ducting with one 90-degree bend and a roof cap, the calculator would do this:
You lose 3% of the capacity for every 25 feet of ducting, this means you lose 1.44% for 12 feet. The actual CFM you receive is now (100 - 1.44)% of the original that is 98.56% of 400, i.e. 394.24 CFM. Due to the bend, you lose an additional 3%. The total loss so far is 98.56% times 97% (since you lost 3% from 100%). Therefore, the output is now 97% of 98.56% or 95.60%, this results in a CFM of 382.41. Finally, there is an additional loss of 3% due to the roof cap. The total output is, therefore, 97% of 95.60 i.e. 92.73% or 370.92 CFM. This is a loss of 21 CFM.
The output of the calculator for the losses, in this case, would be 0.9273. All you do is take this number and multiply it by the CFM you need, i.e. 400 CFM to get the resulting 370.92 CFM. I give you the output in m3 and you would still only have to multiply it by the factor I give you there are no changes to the procedure.
For the tech people out there, check out this page on the Engineering Toolbox to help you determine the precise losses for your installation. This page talks about the perfect duct diameter for minimal losses and noise. Please make sure you follow manufacturer recommendations and do not make any modification to the duct diameter unless you know what you are doing and are aware that this could void your warranty.
Finally, you acknowledge that this calculator is developed to serve as a guide and you are advised to speak to a local expert to determine the exact exhaust rate that you need. I am not responsible for the wrong use of this calculator nor if the output is ever erroneous nor if the methods used are faulty. Relevant links and verifications have been provided for every step of the process and the results are good for use to the best of my knowledge and understanding.
kitchen chimney Exhaust Rate Calculation: Some FAQs
How do you size a kitchen chimney vent?
If you are building your own kitchen chimney, you would want to make sure that you size your ducting such that the flow rate is at a certain optimum (within certain limits) so that noise and unacceptable friction losses are avoided. I will not reproduce data that the engineering toolbox has done very well. This page focuses on determining the CFM and not the ducting. However, if you’re buying a kitchen chimney follow the instructions in the manual for the dimensions of your ducting.