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Vent Hood Question

Posted by subonim (My Page) on
Mon, Jan 6, 14 at 21:39

1.) Is a 27" deep hood too deep for a 25 13/18" range top? While the range top measures over 25" in total, the front burner ends right at 24" and the last 2 inches is just metal.

I'm going with the 36" Thermador 6 burner cooktop and trying to decide between the 24" (PH36HS) and 27" (PH36GS) Thermador vent hoods. Both hoods are 36" wide (I don't have room for a 42") and use excellent 10" ducting with option for horizontal or vertical exhaust. I like the look of the 27" hood better and it offers heat lamps in addition to the halogen lighting. Unfortunately, neither has LED lighting.

I know the rising effluent will expand and so the 27" hood will offer better capture, but I'm afraid that it sticking out over the range top will interfere with my head. I'm 6'2" and even mounted 36" above the range top it will be at forehead height. Has this been an issue for anyone?

2.) For the exhaust, my range top will be on an exterior wall with a wind on either side. I plan on just directing the exhaust horizontally right out the back wall for the shortest possible duct run and using the vertical duct cover for looks only. With the nice wide 10" duct I figure this will maximize my efficiency and minimize noise.

Any reason to run the duct vertically a few feet and then turn out the back wall?

Thanks in advance!


Follow-Up Postings:

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RE: Vent Hood Question

The more effluent captured and contained the better, in my opinion, so 27-inch is better than less than 27-inch.

Draw a stick figure with your dimensions and bending point next to a drawing of the counter and hood to scale, and see what clearance is necessary to not hit your head. Also check sight-lines to back-burner pots. Alternatively, use cardboard to mock up the hood over the range and test with a live you. The bending point is critical as it lowers the head as it swings under the hood. However, toe-raises while waiting for water to boil will be an unallowed exercise close to the counter at your height.

If you have a back wall and side cabinets to direct effluent upwards, then inching up the hood, if it proves to be necessary for head clearance, can be done without much loss of effluent, given a 27-inch hood and use of back burners for the most effluent generating cooking.

I probably should add that the typical 22.5 degree half angle effluent expansion reported in the literature is from hot surfaces used in commercial cooking, and for our purposes should (a) be considered nominal angles with reported variability with surface temperature, and (b) to begin at the pan bottom surface, which can be more or less than the burner diameter. In a wall configuration with significant air flow, there will also be a tendency to bias the expanding cone toward the center of the hood (warped tornado look) that may be equivalent to an inch or so of overhang.

I see no reason to make the duct slightly longer than straight out. Long ducts with silencers in them can reduce noise, but a short duct will not provide space for a silencer or much difference in flow rate than one a few feet longer. Note that the major noise sources, assuming minimal duct and hood flexibility (no rattles), are the blade tip turbulence at the fan and turbulence at the baffles (or mesh). Duct turbulence would be next, so slightly more duct turbulence from slightly more length will not dominate the noise. Turbulence noise from the primary and secondary sources will be obvious at maximum blower operation. Whether they are also obtrusive is another question that is difficult to answer or predict.

Many hood lights have LED equivalents that may work, depending on various configuration issues.

kas


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RE: Vent Hood Question

I have a 27" x 36" hood over my 36" range, although my range is 28.5 deep. I mounted the bottom of hood at 36" above the tops of the grates, which puts it at 73" above the floor. I'm 5'-11", and there is plenty of clearance for me. A mock up is a great idea, I made one out of 1/8" plywood veneer, and hung it before committing to an expensive custom hood. Perhaps a cardboard box in the location will help you get the feel.


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RE: Vent Hood Question

Thank you both. I hadn't thought about measuring from the tops of the grates, I've been measuring from the countertop! That might give me an extra inch of clearance.

Kas, would your recommendation change at all if I do not have side cabinets to help direct effluent? I have the back wall only as I have a window on either side of the range top and hood. I know this is less efficient but was a necessary compromise with the wife.


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RE: Vent Hood Question

BTW, mine is an island hood, 1000cfm remote blower on 8' duct. Works fine over my Bluestar with griddle.


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RE: Vent Hood Question

A wider hood would be desirable without side cabinets, but I guess your windows restrict you there. (Narrower windows would have allowed a 42-inch wide hood.) Sometimes there are too many constraints to optimize a sub set of them.

You will likely get grease on the windows. Hood side skirts would help, but in the context of the look of the overall kitchen may be better or worse than grease precipitation on the windows.

kas


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RE: Vent Hood Question

Good news! Just left the build site and the windows were accidentally framed too large, they will be 24" instead 36" so I will have no problems fitting a 42" hood.

Now I just have to decide the depth, height and cfm. With such a short and straight 10" duct, I think I can get away with 600 cfm, what do you think Kas?


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RE: Vent Hood Question

90 CFM per square foot of aperture, times a factor for pressure losses. Maybe for your case 1.2 would be a large enough factor, depending on your MUA situation. (There is always an MUA situation.)

kas


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RE: Vent Hood Question

Hmm, the larger the aperture the better the capture but the more CFM required to provide suction across all that space?Is that to estimate your minimum CFM, or the maximum to take full advantage of the aperture?

Does that mean that larger hoods always need to be run at a higher CFM to make them effective? While providing more effective clearance of effluent, are larger size hoods always louder?

For a 42 x 27" hood I would need about 850 CFM [(3.5'x2.25')90x1.2]
and for a 42 x 24" hood I would only need about 750 CFM.

For the MUA situation I was hoping that with the windows to either side simply opening on these up a few inches would provide a nice direct stream of MUA. Unfortunately, I think I'll still need a MUA duct with a damper due to local codes. More money into the pot I suppose...

Kas, you are a wealth of knowledge, thank you.


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RE: Vent Hood Question

Unless one had a way to block the part of the hood not capturing from a given burner, then the flow has to be the same everywhere in the aperture. Hence, yes, larger hoods need more duct flow rate to achieve the same velocity everywhere in their aperture.

In reality, many hoods do not have equal flow everywhere. Also, a very large hood dealing with central burners will have an advantage that when the rising effluent arrives at the baffles and the velocity there is a bit weak, the partially "reflected" effluent flow will also have a horizontal component allowing other baffles to potentially capture it. The details are non trivial, and generally serious manufacturers (usually for commercial applications) use Schlieren photography to see what the rising effluent actually does as a function of air flow. Nowadays, computational fluid dynamics on a powerful enough machine could also be utilized, but may cost more overall than the photography.

kas


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RE: Vent Hood Question

I've hit another snag. My wife has just informed that the aesthetics of the 24 and 27" deep wall mount hoods are not compatible with the rest of our kitchen. She really wants the chimney look, however this is a slightly inferior product at only 21.5" in depth, mesh rather than baffle filters and a smaller caliber 8" duct which will require a few feet of vertical run with a 90 elbow out the exterior wall.

If I go with the 1000 cfm 42" version of this hood I'll have adequate aperture laterally but poor coverage of the front burners. Classic form vs function dilemma. Kas, do you think this will provide adequate ventilation for a 6 burner 36" rangetop? We almost never fry but do sear and high heat cook occasionally.

This post was edited by subonim on Thu, Jan 9, 14 at 16:48


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RE: Vent Hood Question

Look at Modern-Aire and Prizer, both companies will build you chimney style hood to any dimensions and specs you want. I went through this a year ago, I know.


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RE: Vent Hood Question

Good suggestion ctycdm, but in order to make Thermador work for me I have to take advantage of one of their vent systems as it is free with the package. If I go off brand, it becomes cost prohibitive.

I do have an alternate package with Bluestar, Bosch, Electrolux and either a Zephyr or Kobe range hood which comes in at a bit less $$ but with the added burden of using multiple online vendors, potential shipping issues and having to store the units at my current home and then figure out how to get them to the build site when its ready. With the Therm, I have a local vendor who will deliver to the site whenever I need it and who will help me deal with any service issues.

It's been a tough decision overall, I'd rather have the bluestar range top, the dishwashers are a total wash (har har), I though the ovens were a wash but reading about the blue porcelain chipping issues and recent changes to Elux warranty have given me pause, plus with the therm I'd be doing a single oven plus a steam convection which gives more utility and the benefit of separate units should one fail.

The Therm fridge beats the Elux handily I think, but the hood may be a issue. That's why all the posts, if I'm going to spend a little bit more for the overall package, I don't want to end up with a vastly suboptimal vent hood and smelly, greasy kitchen. I'll settle for adequate though...

This post was edited by subonim on Thu, Jan 9, 14 at 22:43


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RE: Vent Hood Question

Go with the deeper hood and baffles. Tell your wife to deal with it :-)


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RE: Vent Hood Question

I like the way weissman thinks ;)


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RE: Vent Hood Question

Weisman may be a divorce lawyer :)


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RE: Vent Hood Question

I have the benefit of being the cook, so my wife knows better than to get involved in such kitchen decisions....;)


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RE: Vent Hood Question

In all seriousness, do you think the 42"x21.5" hood with a 1000cfm blower and 8" duct will be sufficient for a 36"x24" 6 burner rangetop?


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RE: Vent Hood Question

A point we may have failed to make is that capture (as opposed to containment, which it precedes) depends on the hood overlapping the rising and expanding effluent. So, for any likely duct size and flow rate, what happens with the front burners depends on whether the hood overlaps them. If it doesn't overlap them, or the pans on them, then the hood might be deemed sufficient if the front burners are not used for greasy or odoriferous cooking.

In this case, sufficiency may depend on where you want to be in your ventilation trade space.

kas


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RE: Vent Hood Question

Thanks, Kas. Those were my thoughts as well - as long as I use the back burners for the copious and greasy effluent producing meals It should work just fine.


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RE: Vent Hood Question

Two other points are worth noting for purists:

o The hood fan has to be turned on; forgetful DWs and DHs can overcome any amount of ventilation/MUA planning

o A wall oven opened after broiling, or broiling with a range that allows broiling with the door open, will have poor capture no matter what one's hood configuration is, except for the commercial porous ceiling type in an otherwise closed room. (In which case one might consider the entire room to be the hood interior.)

So there will be times of poor capture that will have to be endured. Under those circumstances, the best MUA is whole-house opened windows to clear the greasy air as quickly as possible before it precipitates on fabrics and walls.

kas


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