Right Size blower over small size induction cooktop

15 years ago

Good practice:

1. Increase the width of your hood to more than the width of your cooktop

2. Position the hood as low as possible

- --- except for gas cooking because the flames' heated air prevents this from being applied

To calculate CFM, I searched the internet for Rules of Thumb and found many:

- The bigger the hood the more CFM

- -- Multiply the hood area (in square feet) by 75 (by 100 if island hood)

- Allow 1 CFM per 100 Btu's per hour (BtuH)

- -- Divide the heat output of the cooktop by 100

- Allow 15 air changes per hour (ACH) for the kitchen

- -- Divide kitchen volume (length x width x height) by 4

And another one that doesn't apply to me:

- Allow 100 CFM per linear foot of hood, for commercial type ranges

- -- Multiply hood perimeter by 100

This applies to cooking with gas. Heat is a byproduct of flames.

http://ths.gardenweb.com/forums/load/appl/msg1019521121608.html

-"Completed Manhattan Kitchen, Blue Star, Liebherr, Miele, Ikea"

http://ths.gardenweb.com/forums/load/appl/msg1219592831530.html

- 700 CFM for 6-burner gas rangetop, 36" wide

http://ths.gardenweb.com/forums/load/appl/msg0813221814359.html

- 600 CFM for a 36" gas rangetop

http://ths.gardenweb.com/forums/load/appl/msg0701240618682.html

- 600 CFM

There is certainty that induction cooking produces less airborne heat than gas.

There is evidence that induction cooking produces less airborne grease too. Strong evidence.

One example: http://ths.gardenweb.com/forums/load/appl/msg0514502325016.html

Quote " .... induction cooking does not mess up my wall cabinets as much as gas cooking did. The yellow greasy spatters that happen are concentrated much lower down, on my backsplash about 12" high and there doesn't seem to be any buildup on my hood. With gas, my hood would routinely need to be "de-greased". I can only surmise the reason: Without the gas/heat rising there is nothing to carry the grease upwards. Not only is the cooktop easier to clean, but the backsplash and uppers are cleaner too. .... "

What will reduce a blower's ability to produce air flow?

1. Duct length and elbows (friction losses)

- -- - Unknown: how to convert friction losses into reduced CFM's.

2. Undersizing a duct

- -- - Unknown: maximum CFM flow through each size of duct.

Today's assignment, class, is to right-size the ventilation over a 24" induction cooktop (7.4 kW).

In my building it is hard to know what to answer when asked about my duct size.

It's a multi-storey condo building with hugely oversized booster blowers on the roof.

Built in 1982. Each condominium apartment has a 6" duct going into a common duct.

Will that 6" duct have a maximum possible CFM associated with it?

Some say yes, some say no, most say they don't know.

I fear that GW pundits have a bias towards high-output ventilation.

I trust we can work around that.

To convert kW to Btu's requires one to use another rule-of-thumb.

All Four Burners on High means Absolute Maximum is 25,000 Btu's.

Using the rules of thumb calculations listed above this converts into 100 to 150 to 250 CFM and not more.

Miele says their 400 CFM power pack calls for a 6" duct.

Many inline blowers (or power packs) call for a 7" or 8" duct.

I think that any inline blower that can take a variable speed controller will do.

Because 99% of the time it will run at the lower end of its capacity.

For my size kitchen, my size cooktop, my size duct and my cooking habits.

If this thinking is valid, then all I want is a speed-controlled inline fan blower that works on a small duct. Unfortunately this is hard to find.

David

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Comments (16)

llaatt22
15 years ago
last modified: 9 years ago
Where is the duct located in your kitchen? What are the problems, if any, with your present cooking setup which is what?

While recirculating range hoods are not ideal, they are helpful. The Prestige brand is one of the best of that type according to some owners here.
davidro1
Original Author
15 years ago
last modified: 9 years ago
I"m gutting and renovating.

A 6" duct is embedded in the building, so its physical limits are permanent.

No need to consider recirculating. Miele can put more than 400CFM through a 6" duct and claim it is done quietly too.
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fandlil
15 years ago
last modified: 9 years ago
There are several brands that are claimed to be okay with a 6" duct. Shop around. Miele is not the only one.
llaatt22
15 years ago
last modified: 9 years ago
In what area of what room was the original inlet to the ductwork located?
davidro1
Original Author
15 years ago
last modified: 9 years ago
It's in a column in the kitchen.
ya_think
15 years ago
last modified: 9 years ago
Could you describe your condo's exhaust a bit more? The very first thing that came to mind is, if you're blowing more air into a common duct than is already being drawn out as it is, are you going to be blowing your fumes into your neighbors' units? I suppose if every line has its own backdraft damper that risk would be "somewhat" alleviated.

I have no idea how condos are set up. Just thoughts off the top of my head. But I have been in some high end, modern NYC buildings where there is decidedly no air forced from the kitchen into the building's ducts. What the building supplies is what you get, 24/7, no more, no less.
davidro1
Original Author
15 years ago
last modified: 9 years ago
The advisory from my condo Board is that the common duct and the building's blowers are so oversized (built in 1982) that any size blower in the apartment will be fine. That's better than being told the opposite, but it's still not clear.

What we all have right now are low-end hood fans with blade motors. Everybody upgrades when they renovate.

Before going back to the building's resident experts, I'd like to figure out what the physical limits to a 6" duct are. My building's experts don't know. They are just people who learn on the fly just like any other owner renovator.

I'd also like to know whether I can find an inline blower or something just as good. VAH, Thermador, Best, Broan, Nutone e.g.

David
davidro1
Original Author
15 years ago
last modified: 9 years ago
In my building every 4 individual apartment ducts are combined into a common duct.
Oversized external blowers take the "mass" from these big common ducts.
I just need to get air over to that common duct; but i have to go through a 6" duct.
The 6" duct cannot be increased in size. Mine runs about 4 feet until it dumps its air in the bigger duct.

Three years ago we put in our bathroom a remote inline ILF fan in an ILFK kit. Nutone Broan.
Love it. It's such a nice little air noise pulling a lot of air. Like taking a deep breath very slowly.
Love the little button shaped hole in the ceiling that we can close when we want to.
http://www.nutone.com/product-detail.asp?ProductID=10476
I called them and asked for a Variable Speed Control for it.

Then I asked about the HLB3 in the kitchen and a Variable Speed Control for it too.
Got that too: VS64 rated for up to 5 Amps; more is better; I could also get a 3 Amp one.

The outstanding question is whether silent/quiet operation is still going to be in the cards
given that the output vent diameter would be reduced from 8" to 6" over a few feet length.
I'm not sure how to determine the increase in noise and the reduction in strength that will
come from installing an 8" inline blower on a 6" duct.

Later I'll get part numbers for filters and cover grills to suit SO's preference,
if we go with this blower.

David
clinresga
15 years ago
last modified: 9 years ago
Any chance you can use an inline blower sized for 8'' ducting and just use transitions to hook into the condo's 6'' duct? The Fantech FKD 8XL is rated at 836 cfm at zero inches of pressure. It's a great fan based on my experience with it. Fantech does not have an inline unit sized for 6'' duct. Now obviously you'd not achieve the 836 cfm due to duct resistance, but it would certainly outperform any dedicated 6'' inline unit, not that I know of one offhand.
davidro1
Original Author
15 years ago
last modified: 9 years ago
I get it now: fantech.net is local; fantech.com is in Australia!

Inline Centrifugal Fans working on a 6 inch Duct are FG6x.
The FG6XL, 483 CFM, draws 1.5 Amps.
The FG6 is 303 CFM drawing 0.7 Amps.

They are designed for the duct, and they pull enough air.
I'll keep exploring the accessories and other fans. Looks good.

David
moose_2008
15 years ago
last modified: 9 years ago
futuro futuro vent hoods also use 6" ducting. Check out their website
davidro1
Original Author
15 years ago
last modified: 9 years ago
Fantech and Nutone-Broan make inline blowers. I haven't found any mention of variable speed controllers on Fantech's web site.
--Other companies make hoods with fans in them. Futuro Futuro, Thermador, Miele, etc.

-david
clinresga
15 years ago
last modified: 9 years ago
davidro1: we have fantech with var speed control. Any rheostat will work as long as rated adequately. See link below for Fantech's own controllers (scroll down to "Variable Speed Controllers."

Here is a link that might be useful: Fantech accessories
kaseki
15 years ago
last modified: 9 years ago
You want to examine the fan flow versus pressure drop curve to be sure that the fan can operate into higher pressure than it might have to with a larger duct. Broan, for one, publishes such curves on their website. Fantech also publishes the data.

Generally, it is easier to build a fan to blow air against back pressure, than pull the same flow rate from a similarly restrictive duct, so a hood mounted blower should do well in this case (other than noise). However, the fan blades have to be designed for this purpose, which can be inferred from the fan curves.

kas
davidro1
Original Author
15 years ago
last modified: 9 years ago
Yes, I have seen the curves on Broan and Fantech. But I am not yet able to figure out how to transform duct size and distance into an estimated increased air pressure. Although one can then see the estimated air displacement, no information about additional noise generated is provided and might never be.

I am confident I will find an inline blower that works since the the distance is only a few feet and the smallest inline Fantech fan is made for 6" duct to begin with. Also, Fantech sells silencers that can be added to the line before I close the ceilng.

David
kaseki
15 years ago
last modified: 9 years ago
I believe that there are some on-line calculators for pressure drop in various configurations of duct shape and size. I don't have time right now to hunt for them. The pressure loss through whatever is used as an outside rain shield will also need to be considered. In any case, the point of the exercise is to determine whether the fan is in the right ballpark, not which seat will be the recipient of a home run.

kas

Right Size blower over small size induction cooktop

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