Range Hood Ducting through roof - roof cap?

2 years ago

I am having a new range hood installed (previously had a pop up island down draft that was worthless) The new hood is a Zephyr 400cfm and will be ducted up through our cedar shake roof

The HVAC company showed up with the chimney shown below - its huge 9” diameter double wall with the top cap over 12” in diameter I’ve seen other roof caps look more like the second picture. I’m sort if dreading a giant, shiny chimney next to my outdoor patio, if it is overkill. I feel like I’ve only seen these type of chinney’s used for gas fireplaces or VERY HOT exhaust.

Does anyone have pros/cons or advice for what to do? He said he would look into the smaller option, but I don’t want to do something that will cause issues down the line.

Thanks!

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Comments (4)

Jennifer K
Original Author
2 years ago
My guess is if will be located about here:
kaseki
2 years ago
last modified: 2 years ago
The down-roof cap is conventional unless one is using a down-blast or up-blast commercial blower or blower style. Your 'side-exhaust' cap image is interesting, but I have no idea how it treats greasy air, whether its pressure loss is unnecessarily high (maybe not for the likely 260 CFM you will actually get from a 400 CFM rated blower, or whether quasi-self cleaning is even possible. If it is actually a gas combustion appliance roof cap, its use in this application may not be approved.

Also, double wall is totally unneeded for hood ventilation. The duct has to be steel. Normally, adjustable elbows are used to change direction and/or offset the path.

The impression you leave is that this HVAC company has never done a kitchen hood exhaust path. However, I can't read their minds from this distance, so some rationale for their deviation from standard methods would be useful.

You can operate with a six or eight-inch duct. The proper diameter is that which keeps the full power hood CFM (actual) to a duct velocity of 1000 to 2000 ft/min for grease impingement vs. grease condensation balance. In cold climates, leaning toward 1000 ft/min is desirable.

Consideration for how the exhaust flows toward occupied decks is also worth doing.

A damper at each end of the duct path is desirable.

A look a Broan's web site for hood exhaust configurations may help.
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I stand mostly corrected by foodonastump. According to page 31.7 of the 2003 ASHRAE Handbook - HVAC Applications: 'Model building codes, such as the IMC, and standards, such as NFPA Standard 96, have set minimum air velocity for exhaust ducts at 1500 fpm. ... ASHRAE research (Kuehn 1998) indicates that there is no basis for specifying 1500 fpm minimum duct velocity for commercial kitchen ventilation, but rather supports reducing NFPA Standard 96 recommendations to 500 fpm. This change allows flexibility for design of variable-speed exhaust systems and retrofitting older systems, though current design practice for new single-speed systems will generally continue to use design duct velocity around 1500 fpm.' The point here is that at the minimum variable speed setting (for commercial kitchens), the flow velocity should be 500 fpm. Most residential variable speed systems will go down to practically zero fpm, so at those velocities, one would be advised to not be broiling hamburger. Grease effluent consists of various sized particles and vapor. The vapor will not condense out until sufficiently cooled. The particles will either be affected by the centrifugal action of the baffles, or not. In the latter case I think the turbulence in the ducting would have to be extreme to compete with the baffles. I don't have Kuehn's paper so subtleties and assumptions should be checked for relevancy to residential use. A synopsis of the paper's results are available on line. I still need to find time to digest it. kas Here is a link that might be useful: New Rules for Kitchen Exhaust
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HU-867564120
2 years ago
I agree with kaseki: Double wall duct is used with gas fireplaces - totally unneeded in your application. Your bottom picture is what you want, keep pushing for it.
kaseki
last year
Down-roof kitchen ventilation blowers from Wolf, Broan, etc. follow the pattern shown below. These contain the motor, fan, and damper in one package. Although I don't have snow blockage with mine here in S. NH on a 5/12 roof, one might want a pedestal mount for areas needing snow clearance.

Note that the second photo in the OP's message is from Proline, also a kitchen hood manufacturer.

There may be municipalities that require kitchen ventilation roof caps to be of the "up-blast" configuration. Even so, a double walled design seems to be overkill because residential hoods with acceptable filters (which act as fire stops) tend not to grease up their ducts.

As always, code enforcement gets final say, so they should be consulted for local requirements.

Range Hood Ducting through roof - roof cap?

Comments (4)

Jennifer K
Original Author

kaseki

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Range Hood Ducting through roof - roof cap?

Comments (4)

Jennifer KOriginal Author

kaseki

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Jennifer K
Original Author