Kitchen Exhaust Hood Leaky


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Old 02-11-14, 06:10 PM
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Kitchen Exhaust Hood Leaky

Hi everyone

I recently built my new house and installed a GE Profile Chimney Hood - we love the hood in terms of function and appearance but unfortunately, there is significant cold air infiltration from it...

I think that the cause is two-fold - we have a Broan 643 8" damper that doesn't appear to be the best in terms of staying closed, particularly in even moderate wind...

In addition, we discovered once the house was framed that the 8" round ducting was too large to fit in between the ceiling joists directly over the hood (the space was a little over 7", if I recall correctly)... as such, the builder recommended that we go out into the garage (unheated) which is directly behind the hood's location... so the ducting goes up the wall, turns 90 into the wall, goes about 4 ft, turns 90 to the left and goes out of the house after about 6 or so ft...

The ducting has no insulation around it, other than a drywall box built under it to protect it from the garage below... otherwise, it is completely open to the unfinished space above the garage...

Some have suggested insulating around the ducting but others have said there would be no benefit since it's 100% inside an unheated space...

Others have suggested possibly re-routing the ducting so it doesn't go into the garage but instead into the ceiling and turning to immediately out of the house after 6 ft... doing this would require going from 8" round to 7" round... GE says that it needs a 8" round duct but I am wondering how stringent this requirement is... while it's not ideal, perhaps it would be OK especially since we'd have one less 90 elbow and less straight run ducting compared to now?

Any suggestions on how to best address this?
 
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Old 02-11-14, 07:30 PM
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A similar scenario was part of one of my punch lists a while back. I'd suggest that you look into this product: Broan MD8T 8" Automatic Make-Up Air Damper

It's an inline damper that can be incorporated into your ductwork to help electronically ensure that you don't have any air coming back down the duct when the vent fan is not on. As it says in the description, "Synchronized to only open during Fan or Range Hood operation."

Installation instructions (see page 3)
http://www.ajmadison.com/ajmadison/i.../99044539C.pdf

It's possible that when your furnace kicks on, it is sucking cold air into the house through the vent as part of the makeup air. Adding this damper will help reduce that sort of air infiltration and ensure that it's only venting "out" when the vent is actually powered on.

You are right that insulating the duct out there probably won't help, but the entire duct should be insulated and if it is enclosed with drywall, that space should also be insulated and should have a vapor barrier behind it as per their instructions. (any drywall on the exterior wall "inside" that chase (what was formerly the garage side of the drywall before the chase was built) should probably be removed so that heat from the house can more readily get into that space) The duct should be fully insulated in the chase that runs to the exterior of the home as well. The chase also needs to be fire stopped. If cold air can follow the duct back from your rim joist, that could also be exacerbating the problem.

You will likely need the help of an electrician to wire the unit.
 
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Old 02-12-14, 02:00 AM
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Hi

Thank you for the suggestions - to clarify, this electronic damper should be installed as close to the house as possible, correct?

By "chase", you mean the box or enclosure that was built around the ducting?

You stated that insulating the ducting probably wouldn't help but then recommended that it be insulated as well as the chase... were you saying that insulating alone probably wouldn't solve the problem but in combination with the damper, it might make things better?

BTW, I have attached a picture of what the ducting box looked like prior to drywalling...

It probably doesn't matter but we have a hot water boiler, not a furnace...

Thanks!
 
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Old 02-12-14, 04:32 AM
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Okay, that's not what I was picturing. For some reason I figured it was built after drywall was installed... doh! Insulating that area will certainly help. What I would probably do is remove the drywall from the bottom lid and install that electronic damper in that line. According to their instruction, a poly VB should have been installed all around that duct. Maybe a duct wrap insulation would qualify. Fireblocking should have been added behind your ledger board that meets the 2 walls. And the interior lid of the chase above the pipe should have been drywalled or covered with plywood. The idea is that fire should not be able to jump from your walls, into the chase, and straight into the attic without some resistance to airflow.

It also would have been good if the duct itself could have been caulked to the back of the sheathing to prevent any wind from coming into the wall at that location. Fiberglass is not as effective if air is blowing over it.
 
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Old 02-12-14, 05:02 AM
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The top of the chase is actually completely open to the unfinished space so I can get at it from above... shouldn't have to remove the bottom drywall, although, maybe a contractor would do that to install electronic damper...

Since the house is new and was inspected, I believe that the wall between the garage and the kitchen is done with fireblocking... you're saying that it should be in the ceiling of the garage, right?
 
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Old 02-15-14, 04:35 AM
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So you have used this in an exhaust cooking ductwork, even thought it's an inlet not associated with cooking?
 
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Old 02-15-14, 06:52 AM
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Yes. Not me personally, but the HVAC and electrical subcontractors working under my direction. You simply have to orient the damper in the correct direction, with the arrows going in the correct direction for the airflow, which will be OUT instead of IN. (follow airflow illustration on page 3, and ignore the notations for interior wall register/outside air.)

In the instructions, they frequently call it a "make-up air damper". But that's just semantics. It's just an electronic damper that can be hooked to the switch of any device... in this case, it's your exhaust hood switch that opens and closes it.

The instructions indicate to use this damper only for Best or Broan exhaust hoods. That does not mean you can't use it for other brands of exhaust hoods, it simply means Broan will not warranty its use in those other situations, because it's not their exhaust hood. An electrician will be able to tell you if your exhaust hood is compatible or not.

The only reason to use this damper inline with your exhaust is to solve a problem with cold air coming back down the damper, for exterior dampers that let air back into the home. Which is why I suggested it. If you don't feel that this would benefit your system, then by all means seek some other advice.
 
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Old 02-16-14, 03:00 AM
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Thank you for the information - I consulted an electrician to install it and he commented on that... granted, he's not HVAC so he was just pointing it... mostly, I think it was whether or not it was suitable for an application that might have grease in the air...

It probably would help with the backdraft but I think that I need to insulate around the ducting first to see just how much of an impact that will have... especially since the cost of the damper and installation looks like it would be rather substantial (~$800)

Thanks again!
 
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Old 02-16-14, 05:59 AM
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That makes perfect sense to try that first.
 
 

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