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NealTX 08-13-2011 03:55 PM

range hood duct
This is my first post here. I'm currently planning to install a range hood over my kitchen stove and am hoping I might get some good advice from you guys on a couple of questions I have. First of all, the range hood I have ordered is a Kobe model RA-0242SQB, wall mounted 42 inches wide. It has four settings...400cfm, 600cfm, 800cfm, and 1000cfm. The installation instructions say to use 8 inch round duct.

I have just come down from the attic (drenched in sweat now!) where I've been looking over the place where the duct needs to come through. From what I can tell, it appears that I will have exactly 6 feet to run duct between the ceiling and the roof, so I think it is a relatively short run. However, it will need to have at least one 45 in it for clearance.

My main question to you guys is...would it be unwise for me to do a transition from 8 inch round to 10 inch round duct, just above the range hood, in order to make the majority of the 6 foot run have a 10 inch duct? The reason I ask is because a friend of mine who has installed a few range hoods in the past has advised me to do this in order to get more efficient and quieter performance from the hood. I'm a little reluctant to do it because I'm not really sure if it is a wise or even safe idea. My house is not very air tight and I usually have one or two windows slightly opened. Also, I live in a place where there are no codes or inspectors, so I have no obligation to follow any rules....just want to be sure that what I do is safe.

Would there be a significant increase in efficiency and noise reduction if I transition to a 10 inch duct? Are there any other concerns I need to be aware of before undertaking this project? Any feedback you can provide would be much appreciated!

hvac122 08-13-2011 05:05 PM

The first thing you need to think about with a Hood that big is you will need make-up air. Without it you will could possibly back draft you appliances and bring carbon monoxide into your house. What do you have for a furnace and water heater?

NealTX 08-13-2011 05:57 PM

My furnace and water heater are both gas. This really wasn't my first choice for a range hood, but it is what I have ordered, so I suppose I will just have to deal with it.

My house is fairly old; it was built in 1946, but I have new windows and doors that are well sealed. However, during the warm part of the year, which last from about March through November, I have an evaporative cooler in a window. This thing pushes lots of air into the house, so I don't think air replacement will be an issue during those months. Also, I typically keep at least two windows cracked open a bit year round. One of those windows is located in the kitchen, directly across from the stove, about 8 feet from where the range hood will be located. I'm pretty conscientious about having fresh air in the house, so there is really never a time when I don't have windows open. Furthermore, I'm hoping that this range hood is efficient enough that I won't need to be running it on the higher setting much.

My stove is a vintage 1953 model that is 40 inches wide and I think it puts out a lot of btu. I have a tendency to cook foods that produce grease that has accumulated all over my kitchen and that is the reason I'm installing this thing. I just finished patching the drywall, painting and installing new cabinets and countertops, so I'm hoping that this range hood is going to significantly cut down on my grease troubles. I think that because I'm aware of the danger of backdraft, I can always make sure I have the kitchen window open if I need to run this thing on high. Hopefully that will be good enough. I appreciate you pointing it out to me as that is exactly the sort of info I'm seeking here.

As to the need to go from an 8 inch to a 10 inch duct, I'm still uncertain about the benefits vs. potential negatives. Perhaps using the 8 inch duct would allow less chance of backdrafting than the 10 inch? I guess I probably have more cfm than I actually need, so trying to get more efficiency out of the hood may not even be necessary. The noise level reduction is the only other consideration I would have in choosing a larger diameter duct. Would the larger duct significantly reduce noise? At 400cfm, it is rated at 1.4 sones and at 600cfm, it is rated at 3.5 sones. it is not until you crank it up to 800 and 1000cfm that the noise level increases to 7 and 8 sones. If i can get by with using it at 400-600cfm, then the noise level will probably not even be a concern. If it is keeping the grease off of my walls and ceiling at the lower settings, then I see no reason to crank it up to high.

Another consideration is this: My local hardware store has 8 inch duct in stock, but I will have to order and wait for delivery if I choose to go with 10 inch. If anyone knows of any really great reasons why 10 inch would be preferable, I would sure like to hear about it. Otherwise, I think I'm starting to lean toward going with the 8 inch. Thanks for your help!

hvac122 08-13-2011 06:10 PM

Where do you live. Are you willing to bring in 600 cfm during the hottest and coldest times of the year. That is what will be coming in through your open window. This air will also need to be conditioned.
This is the reason there are codes. What if someone else does not open the window? Just trying to get you thinking about doing this project properly.

NealTX 08-13-2011 06:26 PM

yes, I do wish to do it properly. I'm really not experienced in any of this myself and have taken on this project and found myself learning as I go. What are the options for bringing in replacement air?

I live in the extreme western part of Texas near the Mexican border, in a desert region. It is hot here most of the time, but we do get a few days of extreme cold each winter.

I am the only person cooking in this house, so there's not much chance of someone else forgetting to open the window. However, I do understand your point about bring in all of that hot or cold air. Its starting to look like I may have made a poor decision in purchasing an overly powerful hood, so I suppose I should learn more about the options for replacement air.

I'm going to be leaving home in a few minutes and won't be back online for at least 2 or 3 hours, but I would sure appreciate if you could give me a quick rundown on what sort of options are available for replacement air.

hvac122 08-13-2011 09:21 PM

There are many ways to bring in fresh air. One way is to just have a permanent opening into the house. This could be conditioned or not.

One of the ways we do it here is to broin a insulated duct from outdoors and tie it into the return air. We install a motorized damper in this line and also run electrical to the hood. All of this is interlocked so that when the fan comes on, the furnace blower starts and the damper opens.

When we get inspected here there is no set way to stop back drafting. Our inspectors will start all ot the gas appliances (furnace, water heater, fireplaces) and start all bath fans. Then they will start the hood and make sure nothing back drafts. If it does they will just tell us to fix it but not how to fix it.

One other thing to think about is if your furnace and A/C is large enough to overcome the extra load on it.

To answer your other question, I would vent your hood per manufactures instructions. If they want 8" then that is what I would use. Make sure it is the venting instructions and not just the opening size you are looking at. Also do not use any screws into the duct when installing it. Just a good quality tape and straps.

NealTX 08-14-2011 12:12 PM

I really appreciate all of the input you've given me on this subject. Because all of this is new to me, I was not aware of the potential for backdraft when I started this project and ordered a range hood. Now that I have the range hood ordered, I think I'll just deal with the situation and make sure that I don't endanger people in my house with a backdrafting problem.

I started my kitchen remodel with a pretty low budget and have done all of the labor myself so that I could spend my money on nicer materials. However, I have come quite close to the end of my spending power at the moment. I've done some research on make up air units and it looks like a pretty big expense. Therefore, I think I'm going to have to just be very careful about how I use the range hood until I come up with some more money to install a make up air unit.

I'm thinking that if I run the hood only on the lower settings and make sure I have the kitchen window open, I should be ok for the short term. Also, from some of the reading I've done, it seems that the cfm rating on these hoods is calculated with 0 static pressure. I'm hoping that after taking into consideration the static pressure created by my vent duct and roof cap, perhaps the cfm moving through this hood will be considerably less than the stated rating. That's another reason I've decided to go with the 8 inch rather than 10 inch duct. I suppose the smaller diameter should create more resistance. Maybe I'm way off track here, but that is my current line of thinking.

Another thing I've considered is the fact that I don't cook on my stove more than about 6-8 times per week, if that much. Typically, I can get a meal finished in half an hour or so. For that amount of time, I can just be sure that my furnace is not running while I'm cooking. Of course, this really only applies during the colder months of the year because I have a swamp cooler that is pouring lots of air into my house during the warm months. It is located near the kitchen and is a giant hole in the side of the house whether it is running or not. It is usually running during times when I am cooking. My water heater is an on-demand type, so there is nothing other than a pilot light going unless I'm using hot water at that moment. With the kitchen window open and diligence in preventing backdrafting scenarios, I think I'll be ok until I can afford to install a make up air unit.

You did mention the possibility of simply having an unconditioned hole in the house as a quick-fix method. I'm wondering how much benefit something like that would provide in the short term. I suppose I could cut a hole somewhere and install a manual damper that i just open up if I feel the need.

Again, I thank you for your input as it has certainly made me aware of some things that I hadn't considered before.

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