Internal, inline and external range hood blowers

There are three basic forms of range hoods blowers:

• Internal range hood blower - the range hood blower is located inside of the range hood canopy
• Inline to the duct run - the range hood blower is located somewhere along the range hood exhaust ducting
• External to the range hood - the range hood blower is located either on the roof or on an external wall

Internal range hood blower

External range hood blower

Inline range hood blower

If the cooking equipment - the stove or the range dicatate (based on the btu to cfm calculation that can be found on posts in this blog) that around 600cfm is required then there is not a great deal of benefit to be achieved by placing the range hood blower remote from the range hood canopy

Noise is one of the main reasons why remote blowers for range hoods are installed. If the range hood blower, whether inline or external is close to the range hood very little benefit in term so of noise reduction will be achieved.

An external blower mounted on the wall directly behind the range hood without any bends in the duct run will not help the noise a great deal.

For optimal noise reduction from an external blower there should be at least one 90 degree bend in the duct run.

For optinal noise reduction a duct silencer system can be placed in the duct run.

Duct silencers

What the Home Ventilation Institute(HVI) has to say about range hoods

PS31 SP Textured Black

 

Kitchen Range Hoods:

A kitchen range hood must move more air than a bathroom fan. about 50 to 140 L/s (100 to 300cfm) as a minimum. As a result, they are noisier, with the lowest rating about 4.5 sones, although they can be

relatively quiet on low speed.
The most useful units have a low noise rating, an energy-efficient fan, fluorescent lights, sound
insulation, anti-vibration mounts and duct connections. For heavy duty use, select non-corrosive
materials such as aluminum or stainless steel. High quality hoods may have heat sensors and a
safety shut-off.
Kitchen exhaust systems should discharge outdoors. Recirculating range hoods rely on filters to
capture some odours and grease. The filters are generally made of carbon which must be
replaced frequently to be effective. Grease will coat carbon, making it ineffective. With
recirculating fans, cooking moisture and odours will usually remain in the house.

Positioning:

Range hoods are most effective when they extend out over the stove surface and are close to the
stove top. Island units are less effective than wall units.

Cleaning:

Range hoods usually have washable, aluminum-mesh grease filters. Better quality filters have a
smaller diameter mesh over a larger surface area and can be cleaned in the dishwasher.

Fire:

There is always the possibility of a grease fire with a kitchen range hood exhaust. Smooth metal
ducting, preferably galvanized steel, is safer in a fire than lighter assemblies.

Installation:

Install fans and exhaust systems so they make the least possible noise, vibrate as little as possible
and leak as little air as possible.
Anti-vibration pads or foam tape can isolate the fan housing from wood joists and drywall. You
can wrap fan housings and some duct sections in rubber or vinyl noise barrier mats.
DuctsInstall exhaust systems according to the building code and manufacturer's recommendations.
Straight, short duct runs, with few turns, will result in the highest fan flow

Range Hood Width

The range hood should as a minimum cover the width of the range. The reason for this is that the hot vapours from the range expand as they billow up. Under ideal circumstances the range hood should be wider than the range by 3" either side - however this is not always possible.

Wall Range Hoods:
Wall range hoods should as a minimum be the same width as the range top

Island Range Hoods:
Island range hoods should as a minimum be 6" wider than the range top. The reason is quite logical - island range hoods are exposed to cross drafts in the kitchen from ventilation systems and even people moving around.

Here is a picture of a 12' wide range hood - the range top is a 30" induction cook top and we provided a 600cfm internal vent blower with stainless steel mesh filters directly over the induction cook top - 30" wide. The remainder of the island range hood underside was custom fabricated stainless steel panels with lighting. The lighting is controlled by two dimmable range hood light controls - one controlled the kitchen hood lighting above the induction cooktop and the other rangehood light controlled the island rangehood lights on the perimeter:

Link to range hood design information

Does your range hood perform as well as it should

Here are a few of the typical issues that consumes encounter with their range hoods and a brief look at why these issues may exist:

Problem = NOISY RANGE HOOD
Result = You don't use the range hood and the grease, vapors are spreading through your home

• The ducting is restricted
• This could be as a result of a back draft damper flap that is jammed in place creating back pressure on the blower - like all things in nature the blower responds and pushes back by increasing its effort
• The ducting is to small for the range hood blower
• Same situation as above - the rangehood blower works harder to get push the air out
• Filters are blocked - usually the case with mesh filters where the grease has congealed and the air cannot pass through easily
• Clean the venthood filters - if they are dishwasher safe - dishwasher otherwise by hand with a powerful detergent
• A cheap motor - as with most things in this world there are varying degrees of quality and a badly balanced and cheaply built range hood blower will make a great deal of mechanical noise

Problem =SMOKE AND COOKING FUMES ARE NOT BEING SUCKED IN BY THE RANGE HOOD
Result = Grease, vapors and odors spread through your home

• Blocked filters as above - this very rarely happens with baffle filters, it mostly happens with mesh filters
• Clean the filters
• The range hood blower system is under powered relative to the stove or range - in ventilation terms it is not drawing sufficient cfm(cubic feet per minute of air) to handle the volume of smoke or vapor being created by the range - very often happens over ranges with bbq's or griddles.
• Not much you can do except change the range hood blower - this has other implications - is the range hood ducting large enough to cope with the increased cfm - there is a correlation between cfm and range hood duct size - see other posts on this blog
• The capture area formed by the range hood does not cover the burners - when hot vapor rises it expands outwards like a funnel - if the range hood doesnt at least cover the burners you will have this problem
• Not much you can do again except a mjor reno and replace the range hood so that it covers the burners.
• A case in point - we were called to a consumers house who had a 48" wide by 27" deep wolf range with bbq - the client loved to bbq. However the hood above the range was only 14" deep - it was/is a beautiful 72" wide custom cabinet mantel hood. This was a big problem - not only are there no inserts on the market that are around 14" shallow by 48" wide there just is not the coverage - we came up with a solution for them - we bulit a custom 14" deep by 72" wide custom insert with baffle filters and only had baffle filters over the 48" space - we then added a 1000cfm external blower to the mix which creates a huge amount of negative air pressure through the filters - not a 100% solution but pretty close - most of the bbq work must be done at the back of the bbq. Picture doesn't show the whole story but there it is.

• Maybe you have a microwavehood - these typically suck in the air and then blow it back into the room - these are called re-circulating hoods - not much to say about these that is positive.

If you have any questions or live with these problems feel free to contact us - we provide advice and suggestions free of charge.

Questions and Answers

Here are some of the typical questions we get on kitchen range hoods and kitchen ventilation in general:
Q.
How often should i clean my filters
 
A.
We always recomend weekly or at least every two weeks. Why so often - well the purpose of the range hood filters is to collect the grease and solid contaminents in the vapor from the cook top. It therefore makes sense that regular cleaning will stop these contaminents from congealing on the filters. Most filters are dishwasher safe. Another tip is to ensure that the filters are "dried off" before re-installing them otherwise water will drip down onto the cooking surface or worse still if the fan is powered up before they are dry it could suck water up into the motor.
 
Q.
How wide should the hood fan or range hood be
 
A.
Ideally the hood should be 6" wider for a wall mount hood. For an island mount hood it is advisable to make the island deeper than the cook top as well - say 27" instead of 24". Earlier we discussed how vapor expands as it rises - thats why we recomend wider. Islands in particular are exposed to cross drafts and this further contributes to the challenge of capturing all those rising vapors.
 
Q.
Are baffle filters better than mesh filters
 
A.
In our opinion yes. Baffle filters are more effective, because of their unique design at capturing the smoke and steam than mesh filters and they are generally quieter than mesh filters. Quieter is really only an issue with high powered fans where we always try to minimize the noise of airflow. They are also more expensive!!!
 
Q.
Why do you need a range hood
 
A.
Cooking creates vapor that is laden with grease, moisture and of course some pretty powerful odors. Without an effective range hood those odors and grease laden vapor will drift through the home and attach to fabrics and surfaces. Allied to this is our homes have become extremely air tight - we dont have our windows and doors open anymore because of the AC or heating system - so all of these bad vapors get absorbed into the fabric, carpeting and onto the solid surfaces.
 
Q.
How Powerful should the exhaust blower be for my range hood?

A.
Typically the manufacturer of the cooking appliance specifies how powerful and how high the range hood should be above the stove. In the absence of this information call the stove supplier or you can use the industry rule of thumb as a guideline - 1cfm of range hood power for every 100btu of cook top output. This formula takes the maximum btu output of the cooktop - assuming the worst case scenario that all burners (gas) are on at the same time. So for example a 60 000btu gas stove top would need about 600cfm. Electric or induction cook tops are a little different - the rule of thumb here is 100cfm for every 10" of cook top - so a 30" electric cook top would need say 300cfm minimum. There are of course other factors involved not the least of which is the ducting. At Custom Range Hoods we offer hoods in 400cfm, 600cfm, 1000cfm and 1400cfm or multiples of these - yes sometimes 1400cfm is not enough - we recently completed a project with a 10' wide hood and 2400cfm external blowers.

Q.
How big should the ductwork be for the range hood

A.
Firstly one should size the duct as a minimum for the cfm that the blower is exhausting. Secondly one must factor in how many bends and how long is the duct run - it makes sense that a 200ft duct run of 6" will be less effective than a 20ft duct run of 6". Bends in ductwork seriously degrade the efficiency of the duct run - for example rather use two 45 degree bends slightly spaced than one 90 degree bend. Finally factor in whether an internal or external blower system is being used - external blowers are more efficient than internal blowers by about 20% - its easier to pull the heavy vapor laden air through the duct than to push it. Now there is as far as we know no scientific formulas or wizadry involved here. So you are now totally confused right - the answer is always oversize the ducting when in doubt - thats all we do - 600cfm = minimum 6" duct ideally 8"; 1000cfm = minimum 8" and 10" would be nice, over 1000cfm minimum 10" duct - or equivalents in rectangular ducting. If you have a really bad duct run and no other way to do it - go with and external blower.

Q.
How high should the range hood be above the cook top?

A.
The hood manufacturers specify this distance in their installation instructions.Code in most provinces or states specify a minimum of 30" distance to a combustible surface - we always advice our clients to set the hood at a minimum of 30" = if the filters get laden with grease because the filters haven't been cleaned for a year - that in our opinion constitutes a combustible surface. No jokes aside - 30" minimum is a convenient height and 34" maximum. Of interest to consider that for every 30" that vapor rises it expands outwards 3" - so if the hood is set at 60", which is totally ineffective, the vapors would have expanded outwards at least 6" in all directions - so the hood would in this case have to be 12" wider than the cooktop

Consumer discussion on magic lung vs baffle filters

Interesting consumer discussion and experiences on magic lung vs baffle filters

Follow this link to read unbiased consumer discussions on the magic lung filter system vs the baffle filter system - we run into this issue all the time with range hood filters - people are sold on the "magic lung koolaid" as someone put it, the less expensive hood and don't appreciate that baffle filters are by far the best form of filtering a range hood:


- Easy to clean

- 96% efficient

- Best fire spread retarder

- Realtively quiet



http://ths.gardenweb.com/forums/load/appl/msg0208411027007.html?22

Baffle filters - capture 96% of solid material



Remote range hoods 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.

Internal Blower

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.


 

External Blower Fantech



 

External Blower Abbaka



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.

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

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.

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

Make-Up Air Systems

Why do we need Make-Up Systems


1. Our homes have become extremely air tight with the result that fresh air replacement from outside via air seepage, can no longer be relied upon to bring in clean air. The result is that the air quality on our homes becomes very unhealthy. So in order to improve this air quality we need to find a way to bring in fresh clean air and evacuate or clean the stale air. This is achieved with make up air systems. You can liken the situation to living inside of a sealed plastic bag - without make up air - particularly in winter the air quality becomes very poor and unhealthy.

2. Now if you add a kitchen range hood that evacuates large amounts of air, the situation actually becomes far worse - we are exhausting the air from our sealed plastic bag example above and without any make up air bringing air back into the "plastic bag", the environment actually becomes depressurized - the pressure inside the bag rapidly becomes less than the pressure outside - the results can be catastrophic as our heating equipment such as fireplaces, hot water tanks and furnaces rely on balanced air pressure to exhaust the poisonous gases to the outside. When a home becomes depressurized these gases actually get drawn back into the environment with the lowest pressure - the "plastic bag" - our home.

So what do make-up air systems do. Very simply they are linked via pressure switches or electronic switching to suck air back into the home as it is exhausted and to constantly bring in a supply of fresh air. By way of example, when we use our range hood that is say exhausting 600cfm (cfm = cubic feet per minute - visualize a block of air that is 12"x12"x12" - that's 1cfm), now with a 600cfm range hood that is 600 of these blocks of air every minute leaving our "plastic bag" type home - under this situation the average home would attain a negative air pressure situation very very quickly and the dangerous gases from our appliances would invade the home and its occupants.

The make-up air system would be activated before this negative air situation has had chance to develop and would pull air back into the home to replace the exhausted air - make-up air system must of course also heat this air before bringing it back into the home in winter and cool it in summer.

So that very simply is how make-up air systems work and why we need them.

Below please find copy from the Uniongas website on make-up air:

http://www.uniongas.com/business/gastechnology/productsservices/iaq/iaqMakeupAir.asp

In most cases, the need for ventilation air is achieved by the addition of a make-up air unit(s). These units are essential to a building's mechanical operation and to the health and comfort of its occupants. By providing a controlled quantity of clean, conditioned ventilation air to common areas of the building, they:

•Reduce drafts, dust and dirt entering the building;

•Replace air exhausted by equipment such as kitchen or bathroom fans or process or industrial exhaust fans;

•Reduce odours.

A make-up air unit consists of a fan and a heat-exchanger component as well as filters and dampers. Gas-fired units are available in many sizes to meet building ventilation needs and are supplemented by a variety of control components (both temperature and air quality) and operating strategies. Control systems are important as they can be designed to handle 100% outdoor air or to mix outdoor air with re-circulated indoor air. Mixed air systems can be electronically controlled. Heating can be provided at fixed or variable heating rates and control strategies for these systems can also vary from simple manual systems to computerized options.

Depending on the application, make-up air units can be installed either on rooftops or indoors. High-rise buildings generally install their units on rooftops, where they are mounted on either a curb or metal roof frame, then connected with the building's electrical and duct work system. In walk-up apartments, units are often installed in utility or attic rooms and conventionally ducted to feed air to the corridors and common areas.

A variety of features are available that will enhance the performance of make-up air units, including:

•Heat reclaim coils or heat wheels, to recover waste heat;

•High efficiency filtration to maintain air quality and automated controls governing both temperature and air quality to minimize energy consumption - including capacity control utilizing two-stage or modulating gas valves that will match the unit's output with your building's needs.

Electric make-up air heaters can also be converted to natural gas by installing a heating coil in the duct, which is heated by hot water that is supplied by a gas-fired boiler.

How much cfm do you need for your cooking equipment

What type of ventilation is required for your coking equipment? In order to determine the proper amount of air flow required for your cooking equipment, you must first know how many BTU’s (British thermal units) your gas range emits. Then, with a simple calculation you can determine the amount of CFM (cubic feet per minute) required to properly exhaust your cooktop. For example if your cooktop emits 90,000 BTU’s you would divide 90,000 BTU’s ÷ 100 = 900 CFM. This means that 900 CFM or more would efficiently exhaust your range. For electric and induction cooktops choose a blower with a minimum of 200 CFM per 12 inches of cooktop width. Remember, it’s always a good idea to round up versus down.

Baffle or Mesh Filters

The question is often asked what is the difference between baffle and mesh filters. Apart from the appearance baffle filters are more efficient than mesh filters because they permit the easier flow of air through the filter. When air passes through a baffle filter it must make 2 x180 degree turns and in so doing must speed up to make that turn - when the air speeds up it deposits the grease and moisture onto the bend in the filters. Baffle filters are also better at eliminating the posibility of a fire running up into the ductwork.

Baffle filter










Mesh filters on the other hand consist of anywhere from 3 to 6 layers of fine mesh laid one upon the other - so the air is forced to pass through tiny holes creating more noise and typically less efficient than baffle filters.

A Modern Aire Mesh Filter













Baffle and mesh filters are normally dishwasher safe and it is very important to wash them both on a regular basis to avoid not only a fire in the filter but also to ensure grease is not sucked through to the blower and inside of the hood.

Not all mesh filters are created equally - Modern Aire uses high quality stainless steel that has holes punched on the outside - this protects the finer mesh from damage. Most of the mesh filters on the market do not have this protective layer of stainlees steel and the finer aluminium mesh gets easily damaged when placing in the dishwasher.

Re-Circulating Range Hood Venting

Recently i have come across a fair number of people doing kitchen Reno's' who have no way of venting the hood externally - i therefore decided to write a few sentences on the topic to try and help them out.

No way of venting or not allowed to vent
I guess the first question on most peoples minds is - Why should i vent the hood in the first place - this article will help you with that question. Typically folks with this kind of problem are in condo environments where they are not allowed to punch a hole into the external wall or even go up through the roof. They really only have one option - they must re-circulate the air. This entails sucking in the air via a range hood that firstly captures the grease and moisture with the grease filter, it then ideally passes the filtered air through a carbon filter and then returns the cleansed air back into the kitchen. So as you can see this is a two stage process of removing the grease and moisture from the air and then the carbon filter removes the odors. So when planning or purchasing your range hood ensure that the hood has at least 5 layers of fine mesh grease filter to collect the grease and moisture and an optional carbon filter - I am sure you have seen those grease filters that have big gaps in them - this will not do the job and is typically found on the lower end of the range hood scale. The grease filter must be washable - i recommend placing the grease filter (which is typically layers of aluminum mesh with a stainless steel frame) in a dishwasher weekly depending on cooking style. The carbon filter is a replacement item that should be replaced after a month or two depending on cooking style. The most important thing in the grease/carbon filter relationship is washing the grease filter otherwise the grease ends up getting sucked into the carbon filter and negating its value.

If anyone has questions or needs advise please feel free to visit me at:

http://www.customrangehoods.ca/

go to the Contacts Page, send me a question or just post a response to this blog.