What you need to know to install range hood ducting

Firstly we need to understand a critical element in range hood ducting – static pressure in the kitchen hood duct configuration

Follow this link to the author for additional useful range hood installation information

Static Pressure:
Inside every duct, there is a constant pressure being exerted at any point from all directions. When a range hood fan moves air through the duct, the air counters resistance from this pressure which is known as static pressure. Thus, kitchen venthood fan has to have the ability to overcome the static pressure in a duct so as to effectively duct stale air to the outside of your home. This can be done by calculating the equivalent duct length of any duct for the range hood ducting.

Calculating Equivalent Duct Length (EDL):
Simply measuring the length of a duct is not enough to know how much static pressure an exhaust fan has to overcome. Ducts may have one or more elbows, turns, or wall caps which add to the static pressure in a duct. Thus, you have to calculate the equivalent duct run and not the actual duct run so as to size an exhaust fan properly.

The static pressure in any duct run differs according to the material of the duct, number of elbows and turns, exterior wall cap and wall jacks etc. Listed below are the standard values for different duct components.

• Smooth metal duct:
• Actual duct length x 1 (eg. 3ft of duct = 3EDL

• Flex aluminum duct: Actual duct length x 1.25 (for 4”diameter duct)
• Actual duct length x 1.50 (for 6”diameter duct)

• Insulated flex duct: Actual duct length x 1.50 (for 4”diameter duct)
• Actual duct length x 2.00 (for 6”diameter duct)

• Wall caps/roof caps/back draft dampers: 30 feet for each cap (for 4”diameter duct)
• 40 feet for each cap (for 6”diameter duct)

      

• Elbows and turns: 15 feet for each (for 4”diameter duct)
• 20 feet for each (for 6”diameter duct)

For example assuming 6” ducting for the kitchen range hood:

 A rule of thumb is that a duct run for a range hood should not exceed 100EDL


Using the above values, you can calculate the equivalent straight duct length that an exhaust fan has to overcome so as to push stale air outside your home and counter static pressure effectively.

Inline and external range hood blowers

Remote range hood blowers – also referred to as external or inline blowers


The majority of range hoods have the blower located inside of the range hood itself – internal blowers.

Typically a remote blower is called for when a high cfm unit is required because the stove or range dictates it – link to blower sizing.
Exterior blowers are installed either on the roof or an external wall. Installing an external blower directly behind the range hood has limited value in reducing range hood noise – ideally there should be a bend in the duct run between the range hood and the exterior blower and it should be at least 10 feet distant from the range hood.

Exterior blower Fantech

Exterior Blower Abbak

Inline blowers are installed in the duct run – as far down the end of the duct run as possible. These are a great option for efficiency and noise abatement provided an inline silencer is used – the inline range hood duct silencer works in very much the same way as a car muffler does. An important element in inline blower installation is to ensure the ducting is well secured to eliminate duct vibration.

Duct Silencer

Inline Blower

Remote range hood blowers are more efficient than interior blowers purely because it is easier to pull air (as a result of creating negative air pressure in the duct) than it is to push air – this of course pre-supposes that the duct sizing is correct.
Due to structural restrictions, such as a range hood being on an internal wall it is not possible to vent directly outside with a short duct run and less than 2 elbows – no matter what size blower is recommended for the stove it is recommended that a remote blower be used – the reason being of you have a complex and long duct run an internal blower will be very noisy even on the lowest speed setting because today’s high efficiency actually pick up their work rate when they sense high static pressure in the duct run – resistance. This then of course leads to duct size issues – the beauty of remote blowers is that they typically require smaller duct sizes than an equivalent cfm internal blower



Range Hood Terminology

Common Range Hood Terms

A very common term used in ventilation is cfm – cubic feet per minute – this typically measures the volume of air that the hood can process in 1 minute and is an important metric in sizing the ventilation to your particular cooking environment.make up air. In the North of our great continent our new homes are almost 100% airtight – we make them so to keep drafts out. In the middle of winter everything is shut tight – we are cooking up a feast and we have our hood fan running on maximum and its sucking out 600cfm – that is 600 cubic feet of air every single minute – if our home is airtight where is the replacement air coming from – there is only one place it can come from and that is the exhaust pipes used by our furnaces, water tanks and fireplaces that are working at expelling carbon dioxide and a ton of other lethal gases – the result is that we fill our house up with poisonous gases – there have actually been a few deaths from this. The only way to combat this and still keep our beloved kitchen ventilation is to install a make up air system – a system that replaces the air in a controlled fashion as the range hood sucks it out. This is a pretty complex area and can also get very costly. There is no real regulation on this yet but authorities have set cfm limits that can be used in certain areas – typically very low at that – this is very frustrating for a ventilation person because we must match the cfm to the cooking equipment and if we can’t get enough cfm our whole effort at kitchen ventilation will be futile.More ventilation terminology- British Thermal Units – this is a measure of the heat output of the cooking appliances.Canopy – The decorative exterior of the system – sometimes referred to as a shell and can also be a wooden or stone canopy – see liner below



Duct or Ducting – The pipework that the collected is expelled throughExternal Blower - A blower unit that is mounted on the outside of the home.


Inline blower – a blower that is installed between the range hood and the outside – typically in the duct run

Liner or Insert - A non decorative range hood that is installed into the underside of a decorative wooden canopyInternal Blower – A blower that is inside the range hood canopy.

You may also hear of a term static pressure – very basically this is like horse power but the measuring and applying it gets pretty complex and is therefore mostly ignored.

Standard range hoods are available in widths of 24, 30, 36, 42, and 48 inches. For normal cooking needs, select a size that will as a minimum match or closely approximate the area taken up by the burners. (With high-performance cooking equipment, the manufacturer may recommend some overlap, perhaps 3 inches or more on each side.)

In deciding how powerful a fan you will need in the range hood, consider three things: the amount of cooking, the type of cooking generally done in the kitchen and the power(know as btu - British thermal unit - a measure of energy for gas or watts for electric) delivered by the cook top. The more frying and steaming that is likely to occur, the higher the cfm (cubic feet of air per minute) you will want handled. Fortunately the ventilation industry has developed a rule of thumb for cfm required - 1cfm for every 100btu's of cook top output - this can be calculated, by looking up in the owner’s manual for your particular cook top, the given btu's of each burner/cooking spot and add them ALL together - this presupposes the worst case ventilation scenario - all burners are on at the same time and running on the highest setting.

Finally we have a relatively new term –

BTU

How high should the range hood be installed

What height do you need?

Determining the proper height for your rangehood is a simple matter as long as you know the height of your ceiling.  Let say for example that your ceilings are 9ft high, that’s 108 inches.  You would then take the total 108 inches and deduct 36 inches for the height of your range leaving you 72 inches.  Then you will need to determine how high you would like your range hood off of your cooktop.  Modern-Aire recommends hanging your vent hood 30-34 inches above the cooking surface for optimal performance, so let’s just say that you have decided to hang it 33 inches above the cooking surface. You have 72 inches remaining, so deduct the 33 inches for the distance between your cooking surface and the bottom of your range hood and you now have a total hood height of 39 inches.

Ventless or Vented range hoods - sometimes you have no option

A ventless hood is the last resort if you cannot find a way to get the fumes from cooking on the range to the outside. While such a hood will trap grease and will kill odors (while the charcoal in the filter is still active - very important - charcoal filters have a limited lifespan) it will not reduce the condensation and smoke caused by cooking. This is especially a problem if you cook with gas that generates a lot of condensation. Depending on how much you cook you may need to replace the carbon filters every 2 - 6 months to deal with the cooking smells.

While it may cost a little extra, over the years you will benefit enormously from a properly vented range hood - you will also save money by not having replace the carbon filters. http://www.customrangehoods.ca/

Ventless range hoods or Vented Rangehoods??

A ventless hood is the last resort if you cannot find a way to get the fumes from cooking on the range to the outside. While such a hood will trap grease and will kill odors (while the charcoal in the filter is still active - very important - charcoal filters have a limited lifespan) it will not reduce the condensation and smoke caused by cooking. This is especially a problem if you cook with gas that generates a lot of condensation. Depending on how much you cook you may need to replace the carbon filters every 2 - 6 months to deal with the cooking smells.


While it may cost a little extra, over the years you will benefit enormously from a properly vented range hood - you will also save money by not having replace the carbon filters.




http://www.customrangehoods.ca/

Range hood Noise and how to manage it

The critical factors in noise reduction are:

-          Type of filters

-          Quality of duct work – appropriate size for the airflow, length and number of bends

-          Quality and size of blower used

Filters:

There are in essence two types of filters – mesh and baffle. Baffle filters are quieter, 96% efficient and excellent fire travel blockers. Mesh are only recommended in lower cfm applications around 400cfm to 600cfm and over electric style cook tops.

Quality of ductwork:

The most critical element is the size and quality of duct material relative to the maximum airflow you try to move down that duct – for 400cfm internal blowers we recommend 7”, for 600cfm internal blowers 8” and 1200cfm internal blowers 10”. Inline blowers between 600 and 1000cfm need 8” duct and external blowers of 1000cfm 8” duct. Rigid metal duct is strongly recommended – flexible ribbed ducting will be noisy because the air is “tripping” of the ridges and makes noise.

The average duct length and number of bends found today is not too much of an issue – if bends and length is excessive then external blowers and more powerful blowers should be used – if a blowers is trying to “PUSH” a high volume of air down a long and wavy duct run the motor needs to work harder and of course will be noisier.

Blowers:

These  come in all forms of quality – the quality blowers have less mechanical noise because they are well made and balanced.

An internal range hood blower

An external range hood blower

roof or wall mount

An inline range hood blower

http://www.customrangehoods.ca/