How can I figure out which hood can be installed?

eleenaMarch 30, 2012

Right now, we have a cheap-looking and pretty useless vent over the cooktop. It is just a rectangle box sitting right under the ceiling. IDK what it is as it came with the house.

I cannot take pix b/c it is hidden behind a brick "cove" wall around the cooking area.

When we were first going to remodel the kitchen, I saw a great floor model SS vent at a local store for a good price. Thank goodness, I had enough "sense" to check first with the contractor who was fixing the roof that very day. :-) He said I couldn't use that vent but I cannot remember why as it was about 3 years ago. Something about the opening, I think.

So, short of calling a contractor (very hard to get around here), is there any way for us to figure out what kind of vent can be installed?


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Was it something to do with the duct size diameter he said would not fit ????

    Bookmark   March 30, 2012 at 9:03PM
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Crawl up in the attic and measure the diameter of the vent duct. And get your ladder out and take a look at where it exits the roof. If it doesn't go straight up and out, you need to figure out it's path. That may mean removing some drywall or starting at where it does exit and working your way back. You WILL need a contractor at some point, and if you aren't comfortable doing the crawling around, then start calling some to come out and give you an estimate on upgrading what you have.

Most hoods will need at last an 8" duct to be able to properfly vent anything above about 300 CFM. How much CFM you will need will depend on which cooktop or range you purchase, it's features, and what you cook.

    Bookmark   March 30, 2012 at 10:11PM
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No, there are some 600 CFM hoods (e.g. Kobe) that will work with 6" ducts.

    Bookmark   March 30, 2012 at 10:16PM
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If you can see who made your exterior vent (use binoculars and write down manufacturer's name and the numbers typically on it), you can google the specs for them and get a good idea on the duct size attached to the fan and also its CFM-cubic feet per minute- output (minus loss). If the roofer saw your vent, it suggests you have a squirrel cage (centrifugal fan) with motor assembly, obviously called an "exterior vent".

I am about to do what GreenDesigns suggests: get up in the attic and crawl behind to where this squirrel box is on the side of my roof, and then measure the duct size coming off into the attic space before it heads south. I'm hopeful mine is true to what the spec says @ 10" duct. My exterior motor and fan still works well, but the kitchen backsplash wall switch which controls on, high, medium and low is an "infinite" series and the company says their new vents are no longer that switch. However, other companies do use infinite switches. The point is your switch may help tell you about your system too.

Take the baffels or mesh out of your vent hood if you have and can reach them with a ladder carefully when someone's home, and with a bright flash light (and electrical power off) look around up there. It's amazing what you can see and you will begin to get a diagramatic mind's eye picture of your vent system.

Good luck. 600 CFM's is a good amount of air movement for most ranges.

    Bookmark   March 30, 2012 at 11:37PM
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Thank you!

You, guys and gals, are the best!

    Bookmark   April 2, 2012 at 4:59PM
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So, I had an electrician over and he said we had a 10" duct. The problem is that it is not centered above the cooking area but is located on one side, nobody knows why. Also, the cooking area is surrounded by a brick cove and is quite narrow (~12"-13" from front to back). He said if I wanted to keep our current configuration, I needed a custom in-line vent - better two, one on each side of the cooking area, with ducts joined in a upside-down Y fashion.

What I did not understand is why I could get only 450 cfm max on both combined and that one would only give me ~150 cfm. Is it limited by the motor I have to order?


    Bookmark   April 24, 2012 at 11:16AM
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The custom vent hood is not a problem we have done that in the past for customers. The blower situation is what i don't understand

    Bookmark   April 24, 2012 at 11:51AM
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The wall behind the 60" cooking area is (real) brick and so are the side walls around it.
The walls are ~4" thick and go all the way to the ceiling.

They do not come all the way to the front (~24"-25") but stop at ~19" (from back to front).

The front of the "cove" is arched wall with the top of the arch being ~47" above the countertop (and going to the ceiling). I do not remember the exact height but know it is more than the recommended distance from the cooktop to the hood.

There is an ancient ducted vent box under the ceiling. The ceiling inside the 'cove' is only ~12"-13" wide ("deep"), so I cannot really reach that box to clean the filters. And it is very "weak" anyway. Plus, it is not located right above the cooktop but to the left.

IDK how to explain better and cannot take pix b/c it is dark inside the "cove".

The electrician wanted to put an in-line vent so we could go to the attick to remove the filter for cleaning. He also said the distance between joists was ~13", I think (no more than 15" for sure).

But why only 150 cfm?

    Bookmark   April 24, 2012 at 1:01PM
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In my future kitchen the chase that the duct runs in is offset 23'' to the left of the centerline of the hood. That was done so the duct would not run through the center of master bath sink. Had too flip the vanity and shower around to get everything to work. Ain't it nice to sort out these problems on paper before the first nail is driven.

The offset may require some custom duct work but for a good ductman, it shouldn't be a problem. It is just sheetmetal.

As for what size blower: Add up the BTUs of all your burners and divide by 100. That will give a starting point in CU FT per minute. To that you will have to add in loss caused by drag inside the duct -- figure adding 1 cubic foot for every foot of duct plus 10 feet for every 90 degree bend.

    Bookmark   April 24, 2012 at 1:09PM
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Thank you!

I do not understand either why he said i couldn't place a hood in the middle.

In terms of BTU vs. CFM, I am trying to solve the problem from the other end, so to speak. I was going to get a BS range. However, if I am limited to 450 cfm 6' above the cooking area, that is out of question, and I need to put a cooktop that requires less CFM, e.g., induction, which is not a problem for me.

I will demo the brick cove if I have to but is the last resort. It does add something to the kitchen and makes it unique. Everyone who sees it goes 'wow, this is nice'.

But what is limiting the CFM here, I do not understand.

    Bookmark   April 24, 2012 at 2:19PM
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I found this picture on HOUZZ.

If you take the "cabinet" right above the range and extend the side walls all the way to the floor, it will give you a configration similar to mine. The only difference is that they seem to have more space between the back wasll and the front wall of the "cabinet", so the fake cabinet is actually concealing a hood while mine only has a useless box sitting right below the ceiling.

Here is a link that might be useful: Similar configuration

    Bookmark   April 24, 2012 at 5:38PM
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What is the total length of the duct?

    Bookmark   April 24, 2012 at 8:34PM
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Do you mean from the ceiling to the outside? That is pretty long b/c the kitchen is in the front of the house and the subdivision rules allow venting to the back only.

If that is not what you asked, could you please specify b/c i am pretty clueless about these things. :-(

    Bookmark   April 25, 2012 at 3:56PM
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Yes, how far is it from the hood to the point that the duct exits outside. Measure along the path that duct follows

How many turns does the duct make? Each turn adds to the effective length.

    Bookmark   April 25, 2012 at 7:05PM
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You have a post where you first talk about CFM, and you indicated that you can get 150CFM with one duct to the vent, or 450CFM with two. Where did you get those numbers? Is that for a single inline blower, two inline, or something else?

Can't you use a flash camera or a lamp to take a photo of that "hood box".

Are you saying that the present setup has filters that you cannot remove for cleaning? Is that just because you can't reach them easily, or is it really impossible?

    Bookmark   April 25, 2012 at 7:50PM
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There is really nothing to see there. It is just a rectangular box sitting under the ceiling to the left of the center of the cooktop.

So, after several attempts, I was able to get hold of the electrician again and "interrogated" him about the details. Took a few rounds of repeating the questions but here is what I understood.

The current opening is a ~18"x16" rectangle that "turns into" a 6" round duct that becomes an 8" duct and then a 10" duct before it goes out of the roof. The entire distance, including the turn, is about 25'.

What I need is to go to the attic and take pix there, I think.

The location of the duct is limited by the studs that are ~17.5" apart. There is one going right above the cooktop, that is why the vent was un-centered.

If I don't remove the brick, I need an external motor and one in-line vent (or two tied together above). The motor is ~$950 and work is $850, plus tax, so it comes close to $2K.

He said that I can get 400-600 cfm and I still do not understand why he is giving me a range instead of one number and why it cannot get any higher. If it is sitting so high above (we have 9' ceilings), I need at least 900 cfm, even if I don't have a super-powerful cooktop, right?

I guess I need to call him again?

If I demo the brick wall, I can use any hood I want, it just needs to be re-ducted, as I understood, and the labor would cost the same.

Removing the brick walls will give me more cooking space but take away the decor element, which is a trade-off. Plus, I am not sure if the labor involved and getting a regular hood would be more cost-effective than working with what I have.

Does anyone know?

    Bookmark   April 26, 2012 at 11:35AM
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Eleena, you may wish to cross post this over on electrical wiring and see if an expert there may help you determine why your electrician gives a range for one at 150 cfms, dual at 450 cfms.

I wonder if the turn-on switch involved in your vent system is a limiting factor: an "infinite" vs "variable" switch. If I'm recalling right, our squirrel cage roof blower (actually a centrifugal acting motorized fan) produces one constant cfm amount (1000). It's our "infinite" type wall switch (high, medium and low) that controls how many cfms are gated through the duct, reduced by our 25' duct run and turns. Newer motorized fans may have variable cfm's associated with the motor itself, not just one constant cfm blower ensemble as our older one. So if your vent hood switch is an "infinite" type this may explain the 150 cfm and 450 cfm. It may be a 2 way switch then: off, on (one) and both (two). A change out would require different wiring.

Very limited in knowledge, it seems to me that the initial 6" duct coming out of your inset hood is perhaps also what is partially limiting the obtainable cfm. True, the ducts get bigger, but it starts with that small 6" duct.

For your consideration, my vent ducts under the hood ensemble starts with two adjacent 10" (l) x 3.25" (d) vertical vent ducts which feed up to a short (30" h) run and a long (52"h) run of galvanized steel before making it's first 90 degree turn. SInce your cove inset is a rectangle, I'm wondering if you might look into similar rectangular beginning run ducting: they are not all circular in design. At some point I believe my rectangular duct run becomes a 10 inch circular duct but I have no idea where this occurs prior to the roof jack. Lastly, I'll put in a pitch for a back draft damper for safety reasons when you get this all worked out.

Loosing the stone surround seems a loss to me and a lot of work. I suggest you go slow here: getting a second opinion from an HVAC ventilation specialist for an older home may be very helpful.

Hope this helps some.

    Bookmark   April 26, 2012 at 4:03PM
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I will have to re-read your post with a dictionary or Google the terminology (as I never took Venting 101, unfortunately), LOL.

Upon some reflection on both conversations with the electrician, I am pretty sure he was sort of "making up" the numbers as he did not want to commit to any, thus, the 150 vs. 450 dilemma. Today, the 450 was increased to (possibly) 600, so 150 was probably his lowest estimate (for one vent) and the 450 (or 600) for both was the upper end of the range.

I am going to get a GC and/or another electrician come over, go to the attic with them, and have them explain everything again, so this is not all 'foreign language' to me (a picture is worth a 1000 words, right?).

if you don't mind me asking, what is a squirrel cage roof blower?

Is it something to keep squirrel away?

We live right next to a wet-woods area and have a terrible problem with squirrels getting into the attic all the time, no matter how many times we patch the holes.

Also, how do I get an HVAC ventilation specialist?

Thank you very much!

    Bookmark   April 26, 2012 at 4:40PM
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I agree about Ventilation 101. As yourself, I've had to education myself on these matters, but unlike you, I can stare up my range hood vent, measure and picture.

Any way, a squirrel cage is a type of cage in which my roof centrifugal fan/motor ensemble sits. I'm not sure of the word's origin but it resembles a squirrel's tail in shape, so perhaps it is that simple.

From Wikipedia: "A centrifugal fan (not to be confused with blowers) is a mechanical device for moving air or other gases with a very low increase in pressure. These fans increase the speed of air stream with the rotating impellers.[1] They use the kinetic energy of the impellers or the rotating blade to increase the pressure of the air/gas stream which in turn moves them against the resistance caused by ducts, dampers and other components."

"Centrifugal fan is a constant CFM device or a constant volume device, meaning that, at a constant fan speed, a centrifugal fan will pump a constant volume of air rather than a constant mass. This means that the air velocity in a system is fixed even though mass flow rate through the fan is not.
The centrifugal fan is one of the most widely used fans. Centrifugal fans are by far the most prevalent type of fan used in the HVAC industry today. They are usually cheaper than axial fans and simpler in construction.[3] In automotive industries, fans are used for cooling internal combustion engines. The fan derives its energy from the power generated by the engine. It is used in transporting gas or materials and in ventilation system for buildings.[4] They are also used commonly in central heating/cooling systems. They are also well-suited for industrial processes and air pollution control systems.
When one fan can�t afford the necessary flow and pressure, the fans must be run in series two or more, in order to achieve the goal. The pressure of the centrifugal fan is high, so it is widely used in the production and has high using frequency, the centrifugal fans in series are often applied in practical production. According to the relevant statistics, fan power consumption accounting for 12% of the total electricity consumption.
It has a fan wheel composed of a number of fan blades, or ribs, mounted around a hub. As shown in Figure 1, the hub turns on a driveshaft that passes through the fan housing. The gas enters from the side of the fan wheel, turns 90 degrees and accelerates due to centrifugal force as it flows over the fan blades and exits the fan housing."

One thought I have is do you need more than one fan to increase your cfm venting. Still, there is no doubt the 6" duct is a rate limiting step, as is the height away from your range.

If you could do away with the hood insert vent type, and custom make or find a inverted T type vent hood with enough pipe length from your 9" ceiling to 30" above your range and which accommodates your duct(s) (6" or 8") with some fiddling, you may reach a happier compromise to allow you the range type you wish. I think I saw you mention a BS or CC. Very nice.

Glad your getting some additional input. Remember to address make up air (MUA) at some point and cfm. It's a smart idea to consider the basics.

Hope this helps. I'm just learning this all along with you.

Here is a link that might be useful: Centrifugal fan link:

    Bookmark   April 26, 2012 at 5:46PM
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I just bought a 600CFM roof mounted Broan blower on eBay, new never opened for $233 delivered. They are widely available online for something in the range of $311 and up. The 900CFM version costs maybe $100 more. They sell variable speed controls for them, which can be mounted somewhere near your range. I think that the price of the control is about $48 (I'm using the control in my Broan hood, so I'm not certain of the separate control price).

You can find online calculators for pressure drop as a function of duct size and CFM, then see what effect that the pressure drop (static pressure) has on the fan performance by checking it against the fan curve in the specification. You can search online for the 331H Installation instructions.

Here is a link that might be useful: External Fan Specification

    Bookmark   April 26, 2012 at 8:15PM
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