Replies

1. GBA Editor
   Martin Holladay | Dec 14, 2014 06:36am | #1

   Caroline,
   Embedding ductwork in a slab, or installing ducts under a slab, is always a bad idea. Many such ducts have either (a) filled up with water, or (b) begun to grow mold, especially if they are rarely used.

   I advise you to develop a different plan for routing your ducts.

1. 
   daniellecorwin | Feb 18, 2020 02:44pm | #10

   Hi Martin-
   If the exhaust duct mold issue is mitigated, what are you thoughts of using a downdraft vent in combination with our CERV2 unit? From what I understand the CERV2 will help with the harmful toxins as it will boost the ventilation volume until the space is rid of them based on thresholds we set on the control panel and sensor readings.

1. 
   JC72 | Feb 19, 2020 01:19pm | #11

   Downdraft vents simply don't work because they are incapable of overcoming the vertical velocity of the hot air, smoke, moisture rising off the cooking vessel. It's a poor compromise for homeowners who insist on having an island cook top.

1. 
   daniellecorwin | Feb 21, 2020 01:21pm | #12

   Thanks, John. It is difficult when a kitchen design works best with a cooktop in the island, especially when the alternative is a cooktop up against a windowless wall and the island faces an open plan dining room/living room. I have read some positive reviews from folks who have used these and have had no complaints. Also, I would think using a a CERV2 unit ( "smart" ventilation which can detect extra smoke, fumes & moisture), would make the ventilation more robust as it has the capacity to make up for whatever the downdraft system doesn't exhaust. Before I make my decision I want to see if there are any other success stories....

2. Expert Member
   ARMANDO COBO | Dec 14, 2014 06:03pm | #2

   In my opinion, designing a kitchen with down draft venting is a bad idea. Indoor air quality is imperative in all homes, but especially in high-performing homes. Smoke, grease, vapors and odors typically rise, and when not properly vented, can cause negative health issues, bad odors, and build-up on surfaces in the long run.
   Must all down drafts raise 8”-10” from the counter surface, making it very hard to pull air across the cooking surface, tall pots or pots from the front burners. In addition, the motor would have to be bigger or work harder to be able to pull down pollutants to the slab/joist level and outside. Essentially, you are fighting the laws of nature.
   If you have a flat ceiling, a good alternative is to install an overhead vent hood dropping down from the ceiling. If you have a really tall or vaulted ceiling, I highly recommend you to redesign the kitchen so you can install your stove in an outside wall.

3. Expert Member
   MALCOLM TAYLOR | Dec 14, 2014 07:12pm | #3

   Good point Armando.
   https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/hazards-cooking-gas

4. 
   albertrooks | Dec 14, 2014 08:24pm | #4

   Slightly off topic but... What do you think about a downdraft when there is no combustion from gas cooking? I saw the LBL presentation quoted above at one of the PH conferences (Is there really so much going in PH that I could now possibly forget which conference?) The issue really had more to do with combustion gasses rather than food byproducts.

   I'm asking since I'm starting a small personal deep retrofit and am challenged by the venting. A very very small space that the "space planning aspect" wants a down draft. I'm banning all fossil fuels from the project so I've got no combustion gas to worry about. I need exhaust venting because I remain -happily married to a really good cook.

   Here is my take on the situation:

   I saw the presentation about range hood collection effectiveness.It was great but the measurement model was for code homes. I came away from that with a feeling that make-up air is a big "unmeasured contributor" to the issue. Makeup air can channel the collection stream by placing the pollutant between the make-up air and collection. If somebody recalls differently, please set me straight.

   When Projects will also use a Zehnder system with a 36CFM exhaust in the kitchen along with a range hood, the lack of free flowing makeup will cause the room to be de-pressurized. This will slow down the effectiveness of the HRV exhaust port at a time that it can be drawing the gasses/smells that are high in the room that were missed by the down draft.

   And yes Caroline, I would avoid downdraft ducting through a slab if possible unless it can be held to the top of the slab in a short, inclined run after a long run in the toe space. Fabricating a flat-wide duct can done. Think about make up air quantity and placement to balance the flows. (My project is has a wood floor system below - Lucky me :)

5. 
   CASUDI | Dec 14, 2014 08:36pm | #5

   Thanks for your responses. If you know how to cook using a down draft vent you can eliminate the issues you mention. My issue with putting in the slab is for repair and maintenance, yet none of the down draft vents I have had installed in a crawl space have had any issues and needed any access over a period of 12 -17 years.

   Unfortunately the kitchen in this particular project is in the middle, and is an integral part of a very compelling spacial design so I guess plan B will be a better option; which is going straight up through cabinets and to the outside via an infrastructure channel which is situated in the center of the building. It will be rather a long run (20') but I have the space for the duct and a non invasive location on the exterior of the building for the vent grill. This will be more of a show kitchen (reservation kitchen) then daily use so the down draft focus suited the design better.

6. 
   CASUDI | Dec 14, 2014 08:44pm | #6

   Thanks Albert ~ I had not read your comment when I posted just now ~ this was very helpful regarding install in a slab:

   "I would avoid downdraft ducting through a slab if possible unless it can be held to the top of the slab in a short, inclined run after a long run in the toe space. Fabricating a flat-wide duct can done. Think about make up air quantity and placement to balance the flows."

   It's a long run as indicated in my comment ~ and I am learning more about balanced interior air flow from Zehnder.

7. Expert Member
   MALCOLM TAYLOR | Dec 14, 2014 09:36pm | #7

   Caroline wrote:
   "If you know how to cook using a down draft vent you can eliminate the issues you mention."

   I'm not following how the cook's behaviour influences the problems raised.

8. 
   CASUDI | Dec 14, 2014 10:00pm | #8

   Malcom > my comment was directed at this ~ sorry I was not clear ~

   "Smoke, grease, vapors and odors typically rise, and when not properly vented, can cause negative health issues, bad odors, and build-up on surfaces in the long run.

   Must all down drafts raise 8”-10” from the counter surface, making it very hard to pull air across the cooking surface, tall pots or pots from the front burners. In addition, the motor would have to be bigger or work harder to be able to pull down pollutants to the slab/joist level and outside."

   In other words > you can modify your cooking habits/style in order to cook with down draft venting and not encounter the problems mentioned~ however you are right my comment was a bit off topic on the actual question being answered which was about cook top down draft vent in a slab :-)

9. Expert Member
   MALCOLM TAYLOR | Dec 15, 2014 08:09pm | #9

   Caroline, It was my comment which was off topic. I was just curious.