Lyndhurst Garden House

Lyndhurst Garden House
Lyndhurst Garden House

Friday, January 20, 2017

Range Hoods

Most of the range hoods I've seen don't make sense, I realize now.  I once liked the sexiness of curved glass range hoods with inner stainless block, like this one


though I have only been looking at the wall mount kind, to replace my fake (no exhaust) system.

But the capture area of such a hood is determined by the inner metal part.  The glass doesn't help much in capture, by the time the smoke or steam rises there it is probably lost, unless the fan is blowing harder than necessary.  What counts is more approximately the area directly horizontal from the air inlets, and not too far out from the last air inlet in any dimension.  Or, if the center is raised, capture area extends as far as the sides continue lowering.

Capture area is what you want to maximize with a range hood.  For efficient capture of the steam and odors, building scientists have determined that you need a hood at least as large as the cooking area, and increased 6" for each 30 inches of rise above the cooking area.

Conventional electric cooktops end around 22 inches from the wall (this specification is remarkably hard to find, and I'm guessing) and this is the spec that matters for range hood depth, because it shows to meet an efficient capture area a hood at 30 inches above cooktop should be 28 inches deep.  I have not seen that, though I do sometimes see hoods at 25 inches or so deep, called "professional" and having very hard to meet residentially 1000 cfm or higher.

There's been a kind of horsepower race in big ranges and range hoods, where the sellers of these things push people to large sizes ("everybody does it now", the familiar self-fufilling claim) so people feel inadequate without the latest 2000 cfm or higher for their 48" range.  (Meanwhile, lifelong cooks for large families say a 30 inch range is entirely adequate...)

If you have a range hood higher than about 100cfm/1000sqft of house space, some say (actually there is a code on this now, and I'm not exactly sure what it says, but possibly something like this) you need to explicitly provide make up air.  And this is still sometimes not done (though less often than before thanks to the code change in 2009).

Anyways, not only do you need makeup air, but it's expensive, so you don't want an unnecessarily powerful fan.

And to get the most clearance of cooking smoke and smells, you can do it one of two ways:

1) have the proper sized hood (as described above, quite deep, in fact unobtainium in residential equipment) combined with the rule-of-thumb sized fan, for example, 10cfm per inch of electric cooktop width (it's a btu number for gas).  So for my 30 inch wide wide range I need (or would need) 300cfm blower (which means I'd need makeup air, but not too difficult with 8 inch dampered duct).

Oh, and optimal hood also includes being 6" wider on both sides, so the hood should be 42" wide in my case... 

2) Have an undersized hood, and a stronger blower fan.  Well of course this is what everybody actually does, and for those that don't bother with such nicieties (and code requirements now) as makeup air, they aren't getting the nameplate cfm anyway, as the house turns to vacuum state and crud rushes in from everywhere.

The typical range hood is 17 inches deep, way inadequate even for electric, very inadequate for gas.

Well I may get get a 22 inch deep Broan Evolution 4 hood anyway, with 440 nameplate CFM hood (and you gotta look for the vertical 7" shaft number...the fan above is actually advertised as 600 cfm, but that's with a particular large rectangular duct only...another form of specification inflation).  I could possibly get by with the Evolution 3 and 330 CFM.

With 440 (instead of 300) CFM, the lack of proper depth and sides is somewhat compensated for, without going overboard.  Anything larger than the above would be way overboard.  I haven't seen reasonably sized fans or designs above 22 inches deep so far.  Also, the hood has to be raised to allow for taller people, especially as the hood gets deeper than 22 inches.

Perhaps a blank slate hood would be 34 inches deep, 7 feet high.  Such a large hood could actually use lower CFM than a typical system, and work as well.

And I'm doing the fan-sensor controlled makeup air system with controlled duct, and outlet hopefully above the refrigerator.  Cold air will sink onto the warm refrigerator coils, hot air will stay around the ceiling.  Muchly.  And it makes sense for the gain or loss to be felt in the kitchen, where the source of the problems is, and the operator can make needed adjustments if going too far overboard.

As long as it doesn't get too cold outside (combined with cooking in the early AM) make up air dumped above the refrigerator should be no big deal.  BTW, otherwise the kitchen is heated to 75 degrees as the rest of the house in my case.  If there were not such general heating/cooling, the makeup air dump there could be more of an issue...and would have to be relocated to some other place chosen to cause the least discomfort...bedrooms and bathrooms worst, living room(s) best.

Generally, in a fully designed system there is balanced ventilation, with fresh air routed to the central system, and continuous exhaust in bathrooms (I suggest also the attached garage have it's own continuous exhaust, which is what I do).  And the ultimate would use something like ERV to obtain energy efficiently, so indoor air temperature is less compromised by the inlet of outdoor air, which can range from far too hot to far too cold.

That cost...the cost of loss of good climate control...is the cost of using too big a range hood fan, ultimately the biggest cost (well, without makeup air, if you had unvented gas appliances, the occupants could suffer with CO poisoning and such...but after  those serious errors, the loss of interior comfort is the greatest reason for designing the range hood right and not using too much exhaust flow.

UPDATE: A week of more research has changed my range hood selection.  I was not reading the complete story (nothing new, sorry dear readers).  The Broan Evolution hoods are intended for under-cabinet mounting only, and that may partly explain the low noise, since in testing the "cabinet" that the range hood is mounted to absorbs most of the vibration.

What I need (or at least wanted and still want) is a wall-mounted range hood.  Figuring this to be the need, I've already had the cabinet that the previous non-ducted hood was mounted to removed.  It would had to have been relocated (upwards) anyway to be compatible with my new range.  And while I could, in principle, reattach the cabinet about 8 inches higher than it was, the effect of different height cabinets with an under-cabinet hood would be unsightly in my opinion.  Though I think fancy designer homes go overboard nowadays with fancy detailed and oversized range hoods, the hood is a very visible part of the kitchen and should look nice--and impressive.

The Evolution hoods were also only 20 inches deep, and the correct size for a 24 inch deep residential range cooktop is exactly 24 inches also, for the capture area reasons I mentioned above.  Indeed it now seems that all the highest end residential hoods are 24 inches deep, and that's how you can tell them from the one-step-or-more-down hoods.  24 inch deep wall mounted hoods are made by the likes of Wolf, Viking, and Imperial, among others.  I'm now planning to buy the Imperial 24 inch deep hood with the smallest dual-blower option: 635 CFM.  That's a little more CFM than I'd like, but it's ok, and the sound level is rated at 1.5-4.6 sones depending on speed.  Because it's variable speed we can guess that the low speed is about 1/3 the maximum, or about 211 CFM.  (Looking at specs for high and low speeds, when they are actually specified, they always fall like this and I think it's because of the nature of the triac speed controls and possibly also the induction motors used.  If they used DC motors, they could have a low speed much lower than 1/3, which I think would be desirable in a lot of occasions.  I think the low speed should have been around 100 CFM, but 200CFM isn't much different.

Before deciding on the Imperial hood, I briefly looked at what would be possible from a Broan-owned brand, BEST hoods.  BEST makes a wall mounted hood with 20 inch depth and a very nice button system for speed control.  It has a button for each of the 3 speeds, and when you press the button, the bezel around it lights up.  Then you can press the same button again to turn it off, or press the on/off button which will turn everything off (light and fan).  This is so much nicer than the typical one button (where you just have to keep pressing to get the speed you want, and guess whether you have gotten there yet) I had deeply fallen for this.

But the Imperial (and Wolf and Viking) hoods use an even better arrangement: a rotary control with "infinite" settings.

The downside of the BEST hood (and the model I'm describing is their best model) is the relatively higher sound level.  The minimum level goes up from 0.3 sones (the likely optimistic rating of the under-cabinet hoods…where the cabinet does most of the sound absorbing) to 1.5, and the top level goes up to 9 sones.  That top level is really too loud IMO (though other hoods can be even louder).  Now looking at the charts, the Broan Evolution 4 under cabinet hood maxes out at 7 sones (with vertical stack) and 440 CFM (never mind the 600 CFM rating for horizontal ducting which I can't use). If you could imagine an increase of Sones with CFM, if you could increase the Evolution 4 to the higher speed, it would also be around 9 sones or more.  At the same 400-ish speed you would imagine the Evolution and the BEST to have roughly the same noise level, with the primary difference being that the BEST simply goes faster--and louder, and the Evolution does get slightly quieter at the lowest speeds (though how much that is just specsmanship related to the cabinet we can't know without actually testing them side by side).

So while you could argue that the BEST is not louder than the Broan Evolution at the same middling CFM levels, the BEST is only louder because it goes to an actual higher CFM, it nevertheless got me looking for something quieter.

Now as far as I can tell, the Viking hoods do not give any soundness level rating.  While the Viking hoods have the same or better features than the Imperial (Viking hoods can also come with heat lamps, and fancier lighted controls), I've seen a number of negative reviews related to the quality of construction (non-deburred metal sheets), the difficulty of removing the baffle filters (and all of the top hoods have non-cloggable baffle filters btw, which the cheaper hoods don't), and the 430 grade stainless steel.  Imperial specifies the sound level for every hood they make, and also specifies that they use only 304 grade stainless which they describe as the best.  (I'm not sure that 316 might not be even better, but 304 is very good, and very few other makers use it apparently, even the very high end Viking hoods).

The heat lamps had me desiring the Viking for a few hours, but I'm thinking that when it comes time to cleaning those heat lamps are going to be an issue.  For one thing, they get in the way of the baffles for cleaning, and for another the bulbs themselves (and their holders, etc) will ultimately need cleaning.  At the end of the day, it might simply be more convenient to use the warming elements and modes on my electric cooktop instead.

Imperial seems to have been thinking about hoods much as I do from the beginning, putting an emphasis on making quiet hoods.  Their 4.6 sones rating for top speed is the lowest top speed rating I have seen anywhere.  They also have a full makeup air kit, actually slightly nicer than the Broan kit (it comes with a pre-wired sensor duct, with the Broan kit, unless your hood already has the sensor in it, as the Evolution and top BEST hoods do, you will need to install the sensor in a duct yourself, which doesn't look hard, but your installer may be reluctant, etc).

Imperial also specifies 18 gauge 304 stainless shell.  At first that didn't sound very thick.  But most cheap hoods (rattle off all the names you know) are 22 gauge.  The Viking is 18 gauge, but in the cheaper and rust-prone 430 stainless.  Only the Wolf hoods ($$$$$) are a slightly heavier 16 gauge with 304 stainless.

I also like the fact that Imperial is made in the USA.  I have not noticed any other hood makers making that claim.










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