Kitchen Draft Hood: Recirculated air or vent to outside?

D107

the GC on my job is the carpenter. Good at what he does, but when he asked me for a decision on the spot and said he'd have to open up the wall down to the floor, it sounded risky to me since I'd been told about the various complications of balloon framing. I might have reacted differently if he'd said confidently he could do it. Perhaps I'll re-evaluate since we're in a holding pattern on this right now anyhow.

Thanks alot,

David

D107

any serious disadvantages of re-circulated option?

Kitchen is starting to take shape. When part of the wall was opened in the area above the stove where the microwave with draft hood will be installed we found a double vertical beam right in the way of the path of any duct to the outside. That would mean losing even more space in the cabinets above to an exhaust duct. So I'm thinking of just re-circulating the air, and making sure the filters are kept clean.

Cabinets are so expensive we don't want to lose so much space to a duct.

Suppose the duct was 8" wide instead of 12, thereby avoiding the beams? I know it's not rated for that but maybe that would just reduce the flow or make it noisier. Of course some cabinet space would be lost, unless there's a way to vent directly out of the back of the unit.

Another thought is that running into beams must be a very common occurrence in these situations since most stoves are 30" wide or more and with much construction 15 or 16 on center that there must be established techniques to deal with this.

Thanks,

David

bob young

redistribute load

cut back beams & rework with bridles. doing it correct always is harder & more expensive but is small item in scope of entire project. kitchen should last at least 25 years.

Brad White_185

Recirculated Exhaust Option

The only thing recirculating exhaust does is let you breathe it twice. Sure, recirculating hoods do capture some of the grease, usually on filters not much thicker than a window screen but you have to clean these regularly. The odors pass right through.

The "carbon" they use is at best cosmetic (to be effective, activated charcoal has to be much thicker and with much more surface area, pleated or panels in v-banks for example).

With recirculated air hoods, you also lose the ability to remove combustion products and smoke.

You mentioned a 10-inch duct size which tells me that this is larger than your average 30-inch NuTone or Broan kitchen hood. In that case especially, I would seek to vent it outside as it is probably relative to a larger cooking surface.

One more point: That amount of exhaust (600 maybe 900 cfm?) will require make-up air when ducted outside. That airflow rate can pull make-up air down chimneys and create a CO hazard.

Lastly, a variable speed switch gives you some options so if you are making stir-fry, crank it up but if making spaghetti, not so much. A trick I use in higher end houses is to intercept the fan speed variable voltage signal and use it as proportional to a make-up air fan. Easy-peasy.

D107

photos

Thanks for your very timely advice Brad and Bob. In my rush I had forgotten the CO issues.......

Attached are photos. First one has the template placed a little below where it would be located. (old slot from old exhaust is shown too.) Slot is 12 inch wide but manual says it's designed to mate with a standard 3.25" by 10" duct. Seems that if you use the rear vent option --as pictured--then the vent shouldn't have to go up through the cabinets which will be above the microwave, but straight out the back. Must be some kind of flex 'accordion' duct that can fold up as the microwave is placed against the wall(?
)

This is a range hood attached to the underside of the microwave and I believe has the variable speed switch going up to 400 cfm.

not sure where we'd fit the make up air fan, but I agree we should do it right.

Thanks alot,

David

Joannie_15

Opinion

I would give up the cabinet space if it meant not having to exhaust the oven hood back to me. And this is coming from a female person that LOVES lots of cabinet space.

My dislike for the lingering effects of blackened fish or broiled scallops wrapped in bacon override my love of cabinet space.

Rick Kelly_6

Range Hoods

If you are considering using a recirculating hood, save your money and crack a window instead. Most of us on this site are heating heads and so we don't normally use our Combustion Analyzers for testing the CO output of kitchen ranges. I instruct numerous classes associated with Home Performance and CO testing is just one area of interest that is part of the coursework. I have tested hundreds of ranges and have seen CO levels exceeding 900ppm. Outputs of 100ppm are fairly common. Ask yourself this question:

Are you comfortable with knowing that you (or your family) are breathing in high levels of CO inches from your face while you are cooking? If the answer is no, then do whatever is required to do the job right. If the answer is yes, then install yourself one of those mickey mouse recirc hoods and save the cabinet space for another can of beans and a box of pop tarts.

D107

yes we have gotten co readings

of 6ppm numerous times in the last few months when cooking.
(with the NSI monitor). With all four stove burners and the oven going at once, draw is 65K btu, much more than our direct HWH.)

Seems if I keep duct rectangular to match microwave outlet, shouldn't have to up into cabinets or convert to round duct.

Thanks,

David

D107

CO PS

To clarify,a CO reading of 6ppm on the second floor could very well mean a much higher level at the appliance itself. So I am getting good advice indeed, and Joannie's point is also well taken,

Thanks,

David

Bill W@Honeywell

Lots of IAQ problems associated with recirc hoods!

CO has already been mentioned, they also sling grease all over the place, especially above the exhaust. Charcoal is useless, gets grease coated quickly, and therefore can't aborb any odors. "Grease" filters are little more than window screen, and usually disintegrate after a few washings.

Best to exhaust outdoors, but leave access so the fan blades and duct can be cleaned. Also realize that the exhaust cap will collect and drip grease down the side of the house if it is not kept clean. Make sure that the fan does not put the house under a negative pressure! As others have mentioned, that can defeat your chimney's draft, and pull CO and other products of combustion back into the house. Unfortunately, the "good" units that can be easily taken apart & cleaned are usually high volume "restaurant quality" range hoods that are very pricey. Grease buildup also is a fire hazard.

Brad White_185

So, Joannie...

"blackened fish or broiled scallops wrapped in bacon"

What time do we show up?

Or as my Susan would say,...."Say- is that... chicken?"

Joannie_15

Anytime!

Well, anytime after I have a nice hood for my stove.

I totally made myself hungry when I wrote that.

Mike T., Swampeast MO

David,

Looks to me like your best route is directly through the wall. You can get a horizontal (wall) termination (outlet) for nearly any exhaust hood.

400 cfm shouldn't be any problem at all in an older house unless you've tightened it incredibly. The CFM rating of most exhaust hoods is based on free air (e.g. no impediments to movement like ductwork) and typically actual air delivery is significantly less. With a short horizontal termination however, you'll probably get quite close to the rating.

If at all possible, I'd suggest venting to the great outdoors. I grew up in a house with a recirculating range hood and wholeheartedly agree with Brad that the carbon filters in them are slightly effective at best and when quite new. The grease filters (in all hoods regardless of exhaust or recirc. operation) are quite effective, but do need to be cleaned at least every year or so.

If it's a gas range, I would do anything possible to vent outdoors as they will soot up walls/ceilings over time. If electric, your only problems with a recirculating hood could be excess humidity and heat (mainly in summer) and odor. Of course I like to eat and cooking odors--even cabbage have never really bothered me.

Bob Harper

I wish all homebuilders read this discussion. Great points. I recommend you ensure it complies with ASHRAE 62.2 regarding MUA. Once done, perform worst case depressurization test for spillage of CO. You can install a current sensing switch off the fan circuit to a powered MUA unit.

Cut and head off the stud if need be but vent it outside instead of your lungs. Even a lot of high CFM/ high velocity fans do not capture all the aerols from the stove. I've investigated numerous cases of Black Particulate Matter where cooking aerosols escaped the hood and dispersed all over the home.

CFM is part of the equation but so it velocity. Best demostrated with a chemical smoke puffer to look for dead zones where it does not draw properly over a burner. Try this with and without forced air running. Ditto for clothes dryer just as in WCD Test.

HTH,
Bob

D107

Thanks Mike; trying to visualize

the direct connection from microwave rear to wall vent. Somehow as the unit gets pushed against the the wall, a connection is made that must be airtight. Interlocking or rubber gasketed so seal is created by unit being installed tight onto the wall? There is really no duct in this scenario,just two rectangular connectors(?) The manual diagram doesn't show this too well. And the wall vent should have an insulated damper to prevent cold air infiltration when not in use.

Thanks,

David

Mark Hunt_4

Bob

Excellent post!

Mark H

Bob Harper

Thanks Mark. I think we see things very much alike. Keep up the good fight,
Bob

Mike T., Swampeast MO

You'll probably have to go up a very short distance, then elbow back and through the wall. Mfgrs. often make fittings just for this purpose (including a transition from rectangular to round).

D107

Decision made

Well,just talked with the contractor. For a few hundred $$ more he was willing to open up the wall down to the floor, and cut the beam and rehead it and support the header from the floor. Money's not the issue here. I thought about it and recalled that this being balloon framing, there are a number of cross braces in this area.(if that's what they're called, beams at a 45 that make an X in the framing over quite a number of beams. Maybe they're firestops (?)) That would make things more complicated and I don't think he could guarantee that even then some cabinet space wouldn't be lost.

His plan B is to offset the duct coming out of the TOP of the microwave, moving it over about 6 inches to go through the bottom of the cabinet. So losing a bit of cabinet space and avoiding structural work (that would also loosen up and lose alot of that cellulose insulation I had blown in) seems the wisest choice.

Thanks for the all the input.

David

Mike T., Swampeast MO

In balloon framing, the diagonals are generally let into the vertical studs so with standard 2x4 framing the cavity is only about 2" deep where the diagonal crosses.

Firestopping is made with staggered horizontal pieces between the studs. You'll usually find it below ribbons supporting upper floors, the purpose being to help prevent the walls from acting like a chimney between floors.

Be aware that the ribbon (usually a 1 x 4) let into the inside surface of the studs with upper joists resting on top of the ribbon is a very important structural element and should not be removed unless an alternative method of support is added. While certainly not infinitely strong (like many old-time carpenters sure seemed to have thought), the ribbons are absolutely essential elements of balloon framing.

Bob Harper

balloon frame bracing

You're discussing diagonal "let-in" bracing. This can be 1x4's or metal ribs. The let-in brace has been replaced by plywood and is for shear wall strength. This keeps the building from racking side to side and yes, it must remain intact.
An alternative to the 1x4s was a metal rib. You cut a saw kerf along a chalk line, tapped it into place then two 8d nails per 2x3 or one 12d sinker with 2x6's. The 1x4s were set into a dado ploughed 3/4" into the studs with (2) 8d nails per stud.
The IRC gives you many choices for fireblocking these open stud bays in section 602.8

HTH,
Bob

Metro Man

Where to turn for range vent solution

Thank you, Brad and others, for this discussion.

I like what you said: "Lastly, a variable speed switch gives you some options....
A trick I use in higher end houses is to intercept the fan speed variable voltage signal and use it as proportional to a make-up air fan."

Where, please, can I turn, to get a decent solution for our piddly 24"-wide (ugh! -- limited choices) range hood? Something like what you are talking about. Maybe a duct-mounted or exterior fan, variable speed, and some option, as you suggest, for automated make-up air? Our family hates our cheapo fan, and tends not to use it... and wearies of opening the nearby back door for intake.

I am SO happy with our sophisticated Panasonic bath fan that I would buy a Panasonic range hood fan -- IF they offered one.

Should I just look in the yellow pages under Ventilation Supplies, or something similar?

Thank you!

Brad White_187

Fantech

Look up Fantech on line. Based in Sarasota FL, they have a line of fans ideal for many situations, including sidewall fans, in-line fans and controls.

Good luck!

Metro Man

Fantech it is

Thanks very much, Brad!

D107

solution in progress I hope

Well given I didn't want to try cutting balloon frame beams, bracing etc, the price I pay is losing a chunk of closet but seemed wiser at the time.

Photos show mockup of the 6" ductwork in what I assume is the plan. Coming out of top of microwave, up through closet, change to circular, then over to the right about 6" via 45 els, up and out through back of cabinet, gasketed damper before outside piece and another damper in outside hooded piece. Seeing that long straight piece on the side makes me think it might be easier to run a straight piece to the right on a 45 then connect to go out, but that may be his plan.

As a reminder, the reason for the 45 is that the whole vent needed to be moved over 6" to the right to get in between beams. Any comments?

Thanks,

David

bob young

skirting the obstruction

a competent carpenter would have re-framed that in no time with modern techniques like teko's & drill screws. probably took about an hour. and you would still have your cabinet. you made a poor choice

Sam_15

Code

Check the required cfm of the appliance that it would be venting. (check in the warranty section of the range or cooktop)

D107

it's variable speed

max of 400 cfm.

thanks,

David

D107

oops you mean required cfm for stove

I read the whole manual and nothing about required cfm. however if all burners and oven was on at one, I think maximum draw is 65K btu.

they do have interesting info on recommended heigh clearances from stove to bottom of cabinets above.

Thanks,

David