In the sense that it is possible, you certainly can route out the back of a range hood and go down through your existing downdraft's existing venting. This has been discussed in a number of recent posts, btw, and if you have not searched on this topic, try doing so.

Re-using the down draft venting strikes most of us as a last resort, but I'm not yet clear on what you are contemplating doing or why the duct length would be too long. I'm not saying you've given us a bad post, just that a bit more information could be helpful.

First, where does the existing downdraft go and how long is it? I'm picturing a standard pop-up that is a 4" deep x 30" wide box that sits at the top of similar box that goes in the back the cabinet and which has a plenum box at the bottom that feeds into some ducting. I'm guessing the ducting is round 6" diameter vent pipe or maybe 3" x 10" rectangular ducting, (I'm hoping the joints are all taped.) From the plenum box, I'm guessing the ducting it goes down through the floor, makes a 90-degree turn, and runs between the joists to an exterior wall. Do I have that part right?

So, how far does it travel to an exterior wall?

Seems to me that going out the back of a hood would add maybe 30" to the run. You will have to put some kind of 90 bend to go down from the back of range hood, but there are better ways to do that than using a straight 90-drop out the back. There's a rule of thumb that says a 90-degree bend is like adding 5 feet to the duct run. But, it will be much less if you use a radiused bend. Depending on how deep your back wall is, and what is on the other side, you might need to bump out the hood a bit for a radiused turn to fit. (Not a bad ieas, btw, as it gives you a bit better coverage over the front od the stove. Plently of prior posts on this, too.) .

Not sure how to answer you on in-line blowers and boosters but I suspect that it will wind up being a lot more expensive than you might think and probably won't help with your problem. Personally, I don't like in-line blowers for a kitchen, especially situation like this. That's because they gunk up and will be hard to get to when you want to clean it. Also, noisy for the reasons you've already identified.

Maybe there are links in some of kaseki useful postings or on his clippings page, if you haven't looked there already. Or maybe he will join in and comment.

In the meantime, I'm not getting a clear picture of that back wall. Does one end of this 8-foot-tall wall butt into an exterior wall? If so, my first inclination would be to simply go up to a run of venting along the top of the wall and box it all in sheetrock (basically making the whole wall 8'7" tall. Alternatively, instead of sheetrock, I might box it in wood (mayebe stained) so it looks like a beam. Or, if I were putting cabinets on that wall, just run the cabinets up to the top of the wall and run the venting along what would otherwise be the top shelves.

But maybe this wall runs longitudinally through the middle of the house and doesn't butt into an exterior wall? If so, is the problem with going up that a box-like chimney will be a big visual problem from the other side? I'd suggest getting some cardboard and trying a mock-up to check that out. Paint the cardboard the same color as the walls and see what you think after a couple of days.

Maybe others will now join in with better suggestions.

This post was edited by JWVideo on Mon, May 6, 13 at 23:32

My mother did this and she says it works. It goes up a normal hood, then down the wall into the crawl space duct and out.

I just don't know if her idea of "works" is the same as mine. I keep forgetting to check it out when I'm over. :)

The vent in the floor is:

PVC 6"x13' long with a 90 at the head end and 2-90s at the tail end. (3*12+12' = 48')

From what I can think of in my head, I'd need:

round 90 out of the vent (12)
round to rectangular 90 deg elbow (10)
5' rectangular down (5)
round to rectangular 90 deg elbow (10)
90 round down to the vent (12)
(49')

Squeaking by at 97' (plus the lengths to connect the connectors).

I've seen wildly conflicting equivalent lengths for each of these connectors. For instance, one doc said round to rectangular 90 deg elbow was 44', whereas another said 10'. If I go with the calculations from the conservative doc, there's no way I can keep my duct equivalency under 100'. The numbers that I showed above were according to the 'easy' doc.

>>But maybe this wall runs longitudinally through the middle of the house and doesn't butt into an exterior wall? If so, is the problem with going up that a box-like chimney will be a big visual problem from the other side?Its pretty much dead center of the house. All roads in the house lead to this wall. When the houses were being built 20 years ago, someone else closed up that space in their house so that they could put a vent through the roof. It really made that central point of the house dark. That's probably why the designer left it open.

If the cooktop were centered on the wall, I'd just run the chimney up the center and be done with it. I can't move the cooktop however because of that pesky hole in the granite. I have an exotic granite, so I can't just replace that piece either. If it were tropical brown or something, I'd do that in a heartbeat.

I was thinking that, even with the extra ducting, having a hood vent to the downdraft duct would probably do a better job than the useless downdraft vent venting to the downdraft duct. I'd probably need a good quality hood with reasonably high CFM eh?

BTW -- I did try a search. I used google to search gardenweb and others. I found a little info, but it wasn't so fruitful. Perhaps I wasn't creative enough with my search terms.

Some days, the search engine works well. Some days, it inexplicably does not. I just went looking a for a couple of threads in which I participated a few months ago on the subject of re-using the down-draft venting and I can't find them either. Must be one of those days.

Those equivalent duct length numbers seem too high to me, but it has been years since I looked at any tables, so maybe they've really changed. My recollection was that the ICC and the hood installation manuals I looked at said something to the effect that a 90-degree bend in a 6" round duct was going to add 5 to 8 feet to effective length for a range-hood depending hood brand, and on duct material and interior surface irregularities

As for going with a higher cfm hood, that helps but only up to a point. When you start getting CFM cpacity that is much over 600-700 CFM, a lot of hoods need 8" diameter ducting. That stuff has wider bending radii which really whacks the aesthetics and positioning of the hood, plus reducing to small diamter pipe makes for more noise. There was some good discussion of this point in one of the threads I just looked for and cannot find.

Also, on going out and down, I'm not sure you want to go round out of the vent as indicate din your first line of the list. You're talking about out of the hood, right? Then you want to bury a vent duct in the wall behind the cooktop so that is why you want to go rectangular at that point? If so, I don't think you've got the space at the back of the hood without really pushing the hood out from the wall into the room. Afaik, the back vent on wall-mount and cabinet-hung range hoods is almost always a rectangular port, anyway. Seems to me that it would be much easier to go out with a sweeping rectangular 90 and just go down. behind the cabinets.

Speaking of that, to me it seems that there is an inherent conflict between keeping your countertops in place and the work of cutting in a vent extension. It probably would not be hard to cut out the drywall above the stove -- assuming that you don't have an existing backsplash to remove -- but what do you do about the stuff behind he counter and the cabinet back and drywall below the countertop? Of course, if this were my house, there would turn out to be stud or framing right in the middle of where I wanted to run that ducting. And probably a couple of electrical lines, as well.

All that starts making going up look a lot simpler. Put a simple sheetrock box around that riser duct and extend it out six inches over the top of the wall. It would look like a support pillar from the other side of the wall. If the asymmetry bothers you, you could put a matching sheetrock box towards the other end of the wall. Even one just running from the top of the wall to the ceiling would be pretty small. We're only talking 8 to ten inches wide and two feet tall. Those would still leaves plenty of room for the light to pass through.

Would you really have to fill in the top of the entire central dividing wall?

This post was edited by JWVideo on Tue, May 7, 13 at 12:45

Do what you can to NOT do that. The duct size is too small for the length needed. A more direct route up or back or up and sideways to the nearest exterior wall will be MUCH better. Especially since you can use the larger duct demanded by a larger CFM fan. Get a HVAC professional in to assess the situation. You may be surprised at what you can do and how simple it might be.

JW , I disagree with your assessment of in line blowers/ fans gunning up.

All fans are in line ! They are always AFTER the filter but before the atmosphere.

Blades closer to the filter like with 90% of hoods are much easier to get dirty than blades in an in line power pack mounted in an attic or craw space or remotely mounted on a roof or 2nd story wall.
An in line power pack may or may not be hard to access, it just depends.

Xedos:

Now that you called my attention to them, I see that my passing comments were unclear.

First, you are correct that all vent fans are "inline." For what I meant to say in the passing comment, better terms would be "remote mounted" and "mid-line mounted." And you are correct that my prejudice on mid-line mountings is indeed accessibility which is an "it depends" thing.

Second, when I expressed a personal prejudice about inaccessible mid-line blowers, I then moved too quickly to what I saw as cleaning issues with the particular design proposal. So, let me clarify: when talking about the need for cleaning, I was intending to comment on the specific set-up --- that is, venting downwards immediately out the back of the hood and then putting a blower at the bottom of the drop. It is THAT location that seems likely to me to create a gunk-collection problem which, in turn, raises concerns about accessibility and cleaning of the blower unit. I did not mean to suggest that mid-line blowers will have this issue when venting upwards.

Clear enough?

This post was edited by JWVideo on Tue, May 7, 13 at 12:42

>>Also, on going out and down, I'm not sure you want to go round out of the vent as indicated in your first line of the list. You're talking about out of the hood, right? Yes

>>Then you want to bury a vent duct in the wall behind the cooktop so that is why you want to go rectangular at that point? Yes

>>Afaik, the back vent on wall mount range hoods is almost always a rectangular port, anyway.I didn't know that the output on range hoods were rectangular.

>>Seems to me that it would be much easier to go out with a sweeping rectangular 90 and just go down. behind the cabinets. Yes, I could do that. I don't care what in the heck I do to the cabinets above the vent or the wall they're attached to (on the kitchen side).

>>Speaking of that, to me it seems that there is an inherent conflict between keeping your countertops in place and the work of cutting in a vent extension. It probably would not be hard to cut out the drywall above the stove -- assuming that you don't have an existing backsplash to remove -- but what do you do about the stuff behind he counter and the cabinet back and drywall below the countertop? If/when I do this, new upper cabinets and backsplash will be installed. Currently the cabinet above the cooktop is too high for a vent hood to work properly. Plus, we're just going for a new look up top. When the new cabinets are put in, the backspash has to be removed because there is a decorative design above the cooktop that will be encroached upon. We will have free reign from the top of that wall all the way down to the bottom (I don't mind cutting out some of the back of the base cabinet). Yes, of course, if there's a horizontal 2x4, I'm in trouble.

>>All that starts making going up look a lot simpler. Put a simple sheetrock box around that riser duct and extend it out six inches over the top of the wall. It would look like a support pillar from the other side of the wall. If the asymmetry bothers you, you could put a matching sheetrock box towards the other end of the wall.That's an awful lot of pillars for an 8' wall. Don't you think it will look like pillar-ville? They have large crown at the top which will just enhance the effect.

Okay, so that wall is 8' long by 8' high beneath a 10' high ceiling? Personally, I'd settle for one pillar and not be bothered by asymmetric positioning.

But, with the your other explanations, I think you could get a decent range hood with a 6 inch vent (say one of the Kobe 600 to 700 cfm models) and route your ducting down to the existing ducting with fewer bends.

I suggest something like this:

1. Out the back with a 90-degree rectangular (3"x10") duct piece. (Better if you can get a sweeping 90 as it will impinge less).

2. Down 2.5 to 3 feet with 3x10 rectangular ducting inside the wall to a point below the level of the countertops.

3. Add the rectangular to round coupler.( Although the round part will bulge out from the wall, you are now unseen below the countertop inside the base cabinet so not having the aesthetic problem it would be above the countertop.)

4. Adjustable 6" diameter elbow/coupler angled slightly outward from the wall so it projects into the cabinet space. Maybe a 15 to 20 degree angle. You're angling out to get ducting outward enough to line up with the existing ducting in the floor in front of the wall. You could jog over with twin 90s or twin 45s elbows, but we're trying to get as gentle an angle as possible to minimize impingement on the air flow.)

5. About 2 feet or so of straight 6" round duct that stops a little bit above the connection to the existing 6" PVC at the floor level.

6. Another adjustable elbow/coupler which connects from the down-tupe to the PVC in the floor,

This reduces the number of additional 90s from three down to a single 90 and a small and much less obstructive angle out from the wall. Maybe 20 foot equivalent length (or less) by the calculators you were using. No need for additional booster blowers, either

Curious about why the current run's tail end has twin 90s? Does it have to jog around something or go up over a foundation wall?

We have a similar setup, so we had to route our duct down from the kitchen through the basement and then out to the side. We have an inline blower in the basement, which lessens the noise. The system works fine, but you should aim to get the highest CFM possible, because you will lose a lot of flow from all the bends in the ducting. This also means you will have to use 8" duct to accomodate the high flow. But I am very happy with our setup. It provides great ventilation and is relatively quiet.

>>Curious about why the current run's tail end has twin 90s? Does it have to jog around something or go up over a foundation wall?I suspect because the person who laid the duct before the slab was poured wasn't paying attention to where the slab would be back filled to. The outlet of the vent out of the slab is 1/2 buried in dirt. There's a 90 out of the slab, perhaps 18" or so of duct, another 90, then the cap. This is right next to my back door and patio. Not so attractive I must say.

So, the PVC venting is buried in a slab-on-grade foundation? Nothing you can do about that a snorkel vent on the end, then. No way you can go to an 8" duct, either. Well, shucks.

Still, I think a hood with, say, 600 CFM capacity, could work reasonably well for you provided you limit the bends in the additional ducting as suggested above.

This post was edited by JWVideo on Wed, May 8, 13 at 17:34

That sounds awesome. It will be great to have a vent that actually works when I use it! 20yrs of lots of noise, but no cooking exhaust actually exiting the house is quite enough.

Here you go. I did the best I could. I couldn't get the whole wall head-on top-to-bottom because the hall is only about 5' wide; I couldn't back up far enough. I went as far as I could into a T hall to still keep the columns in frame (you can see the corner on the right).

Correction: the wall is only 70" long. So not even 6'.

This post was edited by barbja on Fri, May 10, 13 at 13:44

I really haven't followed all of this, sorry, but PVC caught my eye. I'm pretty sure that all ducting from a cooktop hood to the outside has to be steel, even if PVC buried in concrete is probably not a fire hazard. Given a fire, there could be a risk of the PVC slagging and blocking the duct path. I hope that none of the PVC is exposed within the house; that is, the ducting is metal to the concrete.

kas

I'm almost certain that what is exposed in the base cabinet is steel. (I'm picturing it in my head and that's what I see) What happens between the base cabinet and the entry to the slab, I don't know.

I'm 100% certain that what comes out of my slab on the outside is PVC.

I see what you mean about "Pillarville". I didn't realize you had columns at the ends - I was thinking about a non-load bearing wall.

So you don't have your stove centered on that 70" wall? Why not?

I think I'd be tempted to build the wall up to the ceiling and vent straight up. If you can hardly get a photo of the top of that wall, I don't think you'd miss the opening all that much.

It's interesting that there's crown molding on the columns and a lot of detail on the wall, but no crown in the kitchen. Is there crown molding elsewhere in the house?

>>It's interesting that there's crown molding on the columns and a lot of detail on the wall, but no crown in the kitchen. Is there crown molding elsewhere in the house?Oh my, have you hit on a sore subject. Crown molding was only included in the formal areas and master bedroom. They also crowned no sloped ceilings. Adding crown to other areas was $250 per "room". $250 now, not so much. 20yrs ago when we're building a house and running out out money, it was.

Crowning just that hallway and the adjoining family room cost $500 because it counted as 2 "rooms" even though it was only as much crown as a bedroom; the exterior wall is sloped so the family room is really only 1/2 crowned (one wall not crowned and open on one side). I didn't crown the kitchen because the only wall that would have received crown is that single wall (because again, the outside wall is sloped), and it would have cost me $250. Don't think so.

I did the crown in my office myself (come on -- a box) along with the sloped ceilings in the formal living room. If *I* can do it, why in the heck couldn't a carpenter? New cabinets will go up to the ceiling in the kitchen. They put tile crown where I cared in the master bath when it was remodeled.

What I should have done was made them crown the hallway to the bedrooms just to punish them. It has so many compound angles it would make any carpenter cry.

Kas --

Had the same thought about the PVC but found that the ICC and various state adopters say that Schedule 40 PVC is permitted for downdraft range venting when run through a slab-on-grade concrete foundation. I've linked to the Massachussets standards which were the first ones I ran across.

Here is a link that might be useful: Massachussetts rules on range hood vent materials