Your 1st picture in the article covered several lines of text. Let me know
if this is my fault (screen resolution, etc) or just an editing mistake.
You mentioned the need to provide replacement air. Please elaborate on
proper size, placement, and locations. How do you keep cold outside air
from filling your kitchen to replace the exhausted air?
Thank You
Kitchen Exhaust Fan
Summary: Kitchen exhaust fans have to be sized properly for your kitchen. Overhead exhaust fans capture the smoke and grease mist that could coat your kitchen surfaces. Some of the cook exhaust fans come with built-in lights.
DEAR TIM: My new kitchen plans call for a new kitchen exhaust fan. To be more precise, a kitchen hood exhaust fan has been suggested. Is one kitchen stove exhaust fan more effective than another? Years ago the down-draft exhaust fans were popular. What exhaust fan is in your kitchen if you don't mind me asking? How do I make sure the kitchen exhaust fan I select will adequately ventilate my kitchen? Where does the replacement air enter the house? Kathleen K., Exeter, NH
DEAR KATHLEEN: You are asking all of the right questions about your new kitchen exhaust fan. All too often, I see builders and remodelers fall down here. Either the fan installed is not powerful enough for the size of the kitchen, the installer fails to vent it properly, or overlooks the need for makeup air.
You really need a good kitchen exhaust-fan system if you cook greasy foods and boil foods. The cooking process often creates both visible particles as well as an invisible aerosol mist of grease and smoke that can coat the surfaces of your kitchen if they are not vacuumed and exhausted to the exterior of your home. Even with a great exhaust fan, you can still develop a fine coating of grease on light fixtures, cabinets, walls and ceilings. This is the voice of experience talking.
PHOTO CAPTION: This high-powered kitchen exhaust fan is tucked up under a decorative hood. It is sized properly for the large kitchen. IMAGE CREDIT: Tim Carter
My kitchen exhaust fan is matched to the size of my kitchen. The fan is a powerful three-speed model that has brilliant halogen bulbs that are built-in to the fan. There are three removable grease-collector screens that we take out regularly and put into our dishwasher. When the fan is on the highest fan speed, it sucks 1,100 cubic feet of air per minute (CFM) from above our cooktop and pushes it outside.
The fan is connected to metal ductwork that extends from the fan all the way to the roof of my home. Each joint in the ductwork was carefully taped with special metal-foil duct tape by my ventilation contractor. It is very important that no air seeps from the duct to other parts of the house. If that were to happen, hidden spaces in your home could become grease-covered posing a significant fire hazard.
The exhaust from my fan exits the roof through a special roof cap that is made to handle that much air flow. It was easy to install so that rain does not enter the house.
Sizing a kitchen exhaust fan is fairly easy. Many experts simply measure the square footage of the kitchen floor and multiply that by two to arrive at the cubic-feet-per-minute of output for the fan. For example, since my kitchen is 350 square feet, I would need a fan that must exhaust at least 700 CFM of air flow. My fan can do that on its middle speed, and the highest speed produces the massive 1,100 CFM of air movement.
You are really observant to recognize that large kitchen exhaust fans like these have a voracious appetite for air. In today's modern homes sucking that much air out of a house can cause serious back drafting issues if a makeup air inlet is not installed. Back drafting can cause deadly carbon monoxide to be drawn back down a chimney or metal vent pipe and/or smoke or smoke odors from fireplaces.
Newer homes are so airtight that when air is sucked from a house by a powerful fan, it replaces that air with air from outdoors through the path of least resistance. That path could be a furnace or water-heater vent, a chimney, or other vent that is open to the atmosphere. Installing a makeup-air vent solves this problem in almost all cases as outside air can easily flow through this device into the home.
Before you buy a kitchen exhaust fan, it is always a good idea to get the written installation instructions from the manufacturer. These documents will often contain sizing guidelines as well as detailed step-by-step methods the manufacturer wants you to follow to keep the warranty in force. Reading these ensures that the fan you are considering is the right size and that you can satisfy the minimum installation requirements.
Resist the temptation to use smaller ducting for the fan. Some people think that the size of the exhaust piping is not that important. Believe me, you must use the exact pipe as called for, and be sure that you do not exceed the maximum length of pipe allowed.
Pay particular attention to the bends in the exhaust piping. The written instructions will almost always tell you to avoid 90-degree bends, and how many can be put in the exhaust piping. These hard bends in the pipe create significant restrictions that make it hard for the fan to exhaust the air from your kitchen.
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Comments:
Robert Johnson 06 May 2008, 08:58
Charles 06 May 2008, 10:50
Tim, I see 2 major problems with your answer to this lady's question:
1. Range hoods should be sized to the range, not to the kitchen. There is absolutely no need for any residential range to require 1000 CFM. About 150 CFM is adequate. 2. There is no such thing as a passive air inlet that admits 1000 CFM. There isn't even one that admits 150 CFM. If you want to prevent backdrafting, you must open a window.
Gene 06 May 2008, 15:44
Tim/Charles; there are problems with both of your approaches to the kitchen
exhaust fan:
1. The exhaust should be designed with the intent as a commercial kitchen exhaust system. 2. Although low cfm will exhaust vapors, the real culprit and fire hazard is grease. That is why the velocity should be high. Generally, something in the vicinity of 1,500cfm is ideal, This has the effect of scrubbing the grease off the walls of the duct. Unfortunately, this "air scrubbing" isn't quite enough, but; it helps. 3. As much as possible, the duct should be seamless, preferrably welded so it is watertight at all joints. Remember, build it as if to expect a fire. Also, install it with the same clearances and protection from combustibles as for a red hot wood burning stove going through your walls, floors, roof, etc. 4. Make-up air works well passively if a separate air supply duct introduces the air at the hood-air boundary. This allows the exhaust fan to pull the air into the exhaust stream while minimizing the amount of heating or cooling energy loss...it's more energy efficient.
tom 08 Jun 2008, 23:45
Hi
My question is I want to install a small range under my window in my basement suite. Do I need to have a exhaust fan and if so can I install one as part of my ceiling which is about 48" above the stove? tom
aldrin abueg 13 Jul 2008, 13:52
hello im working as a hvac designer here in dubai..since i only been 6
months in my company i dont have enough experience..i want to ask how to
cumpute the exhaust needed for a kithen and how to compute makeup air
needed for residential kitchen
Grey G 19 Jul 2008, 06:17
Ok...that's twice. Hit wrong key with a splinted finger in the middle of my
mssg and caused both to send and can't retrieve either...I give up.
Maybe in a day or so I'll try again. Sorry for the false start(s) and appologizre to anyone eager to answer if they only had the entire question..basically NEED to use downdraft exaust and need some help/ House is 70yr old sngl story cottage with crawl space and raised deck attached to outside wall. Stove is 4 burner gas. Kitchen dims are approx 10x15 open to 10x12 DR and partially open to 20X25 LR through full wet bar. Can't go straight up or up and out.\] What's my solution ?? Thanks in advance. Grey G
Vindra Baijnauth 04 Aug 2008, 16:58
Hello,
We are currently having our kitchen renovated and the tradespeople want to use an existing roof vent for the kitchen fan exaust fan vent. Should this be done. How will the attic of the kitchen be vented? Please advise. Many thanks
Pat 13 Sep 2008, 13:25
I want to install an exhaust fan in a small office to vent cigarette smoke.
There is currently a small exhaust fan (like in a bathroom), but it is not
adequate at all! The room is 11 x 14 feet and the present hole in the
ceiling is 10 x 10 inches. It is a 2nd floor room and the fan currently
vent through the attic to the outside.
Any advice is greatly appreciated.
Gene 13 Sep 2008, 17:56
Hi Tim,
Read your article again about exhaust fan sizing and ducting the vapors outdoors. Your suggestions are definitely a good idea. As I said before, it's getting the velocity in the duct up high enough that does the most good. There are formulas that will give the proper size duct related to hood size related to the correct velocity that will give the correct cfm so that the correct fan can be selected. Maybe I didn't read close enough the first time because I see you had an HVAC contractor put foil tape around your duct seams. I've seen this before and it is a mistake. The foil tape used for HVAC applications is not suitable for a grease exhaust system application...a misapplication. That metal foil tape is aluminum which burns at about the same temperature as cardboard. If you can't or don't want to weld the seams in a continuous watertight welded joint, I would at least suggest assembling the joint with high temperature fire-rated sealant slip the joint together and secure with sheet metal screws or steel pop-rivets. Then apply another coat of the high temperature fire rated sealant over that. When grease is hot enough to burn, it runs through everything like water and igniting everthing it comes in contact with. By the way the duct should be made of either 18 ga stainless or 16 gauge sheet metal, not the 24 gauge vent pipe stock. Also, regular cleaning (say monthly) will prevent a lot of the potential for a grease fire.
sal 08 Nov 2008, 08:08
I would like an exhaust fan that is quiet. Would an in-line fan work.
Any disadvatage to an in-line system. View all comments |
