Kitchen Exhaust Hood - Fixing Cold Air Infiltration

JustinS

Good morning everyone!

I was hoping that I get some thoughts on this issue I'm having.

I've got an exhaust hood in my kitchen whose exhaust ducting was improperly installed when the house was first built about 8 years ago. After we moved in and discovered this, I started a thread on this forum about how to insulate it.



I filled that box with fiberglass insulation and covered the top of it with rigid insulation. All of this helped some but the issue still exists. Not only is the Broan 643 Wall Cap equipped with a lousy non-spring-loaded damper but I believe that the hood's internal damper often open/closes in the wind.

As I see it, I have two options, aside from blocking it off and reconfiguring the hood to recycle air which I don't really want to do.

#1 - Leave ducting route as-is and install a better butterfly damper that has a spring and some insulation, something like this perhaps? Or possibly a motorized butterfly damper?

#2 - Abandon existing ducting and go up into the ceiling above the hood (instead of into the garage behind it) and turn left towards the back of the house. I don't really like this approach because it would involve tearing up the ceiling drywall and making another hole on the outside of the house. However, maybe it's the better course of action?



This issue is all compounded by the fact that I can't find any contractor willing to help on this.

Any thoughts? Many thanks in advance!

Tinman

I agree that the Broan 643 isn’t the best, but I can almost guarantee the $19.00 option from Home Depot won’t be an upgrade. 

There are options. 

https://www.famcomfg.com/product/hooded-wall-vent-with-spring-loaded-damper-gasket-and-screen-painted/?attribute_pa_size=91-8inch&attribute_pa_color=48-black&utm_source=Google%20Shopping&utm_campaign=Google%20Live%202&utm_medium=cpc&utm_term=577&gclid=CjwKCAiA_omPBhBBEiwAcg7smXQSEahFRIggUY3bw2wqxTOrlXsF6AvnOagIv3aY1mFkRJuSQZOlRRoCjyMQAvD_BwE

pecmsg

is it cold air falling into the kitchen or negative pressure drawing it in?

JustinS

Steve Minnich said:

I agree that the Broan 643 isn’t the best, but I can almost guarantee the $19.00 option from Home Depot won’t be an upgrade. 

There are options. 

Thanks Steve!

pecmsg said:

is it cold air falling into the kitchen or negative pressure drawing it in?

I'm not sure how to know the difference? I can say that if I put my hand underneath the hood, I can feel air lightly flowing down from it.

I had considered that I might have negative pressure and tried the 'crack a door and see if air comes in' trick. I sort of feel cold air but not an incredible flow, if that means anything.

I will say that my gas boiler doesn't have a fresh-air intake so maybe that's an issue.

Daveinscranton

I had this problem with bathroom fan vents years ago.  Windy location.  Ferocious wind.  Venturi effect.  Flappers etc did not stand a chance.

I added powered dampers.  24 volt as I recall.  Used the 120 volts on the fan to sense when to open and close.  120 volt primary winding on small relays.  Doesn’t take much.

Worked like a charm.  Zero maintenance in my application.  They are in the attic if I ever have to get to them.  Old laundromats sometimes can be a source of 6” or 8” powered dampers.  Type of thing you run across and find a use for later.  I used 4” ones in my application.

Best wishes.

PC7060

I’ve found HVAC quick to Be good source of quality discharge caps. I’m using a stainless steel 8” unit on my hood. 
https://www.hvacquick.com/products/residential/Kitchen-Exhaust/Discharge-Caps

JustinS

PC7060 said:

I’ve found HVAC quick to Be good source of quality discharge caps. I’m using a stainless steel 8” unit on my hood. 
https://www.hvacquick.com/products/residential/Kitchen-Exhaust/Discharge-Caps

Could you tell me which one you have specifically? Looking at these they seem to be gravity flappers and not spring loaded. Very similar to the Broan I currently have.

JustinS

Daveinscranton said:

I had this problem with bathroom fan vents years ago.  Windy location.  Ferocious wind.  Venturi effect.  Flappers etc did not stand a chance.

I added powered dampers.  24 volt as I recall.  Used the 120 volts on the fan to sense when to open and close.  120 volt primary winding on small relays.  Doesn’t take much.

Worked like a charm.  Zero maintenance in my application.  They are in the attic if I ever have to get to them.  Old laundromats sometimes can be a source of 6” or 8” powered dampers.  Type of thing you run across and find a use for later.  I used 4” ones in my application.

Best wishes.

I must live in a wind tunnel or something because it's so incredibly windy nearby. I've actually had my propane grill blown over before!

So it seems like replacing the wall cap with a spring-loaded damper might be a good place to start?

Solid_Fuel_Man

A motorized damper is the only solution.

Weighted or spring loaded dampers flap in the wind and cause more restriction to the inches of water colum power of the blower. 

A 120volt motorized damper wired in parallel with the hood switch is the easiest solution. If that isn't possible, then a current transformer on the power wire to the fan to sense when it's on is the next best thing. That would require some electrical knowledge and access the romex that feeds the hood and nothing else. 

This may be an involved task now that it is all buried. 

FWIW, I just live with my kitchen hood leaking some air. If it isn't bothering the range or making someone feel cold I'd leave it alone. 

Bathroom fans are more of a problem, in that the cold air can fall right on someone in the bathroom.....the wife.....and a motorized damper keeps that from happening. 

JustinS

Solid_Fuel_Man said:

A motorized damper is the only solution.

Weighted or spring loaded dampers flap in the wind and cause more restriction to the inches of water colum power of the blower. 

A 120volt motorized damper wired in parallel with the hood switch is the easiest solution. If that isn't possible, then a current transformer on the power wire to the fan to sense when it's on is the next best thing. That would require some electrical knowledge and access the romex that feeds the hood and nothing else. 

This may be an involved task now that it is all buried. 

FWIW, I just live with my kitchen hood leaking some air. If it isn't bothering the range or making someone feel cold I'd leave it alone. 

Bathroom fans are more of a problem, in that the cold air can fall right on someone in the bathroom.....the wife.....and a motorized damper keeps that from happening. 

That's the thing, it's making the wife feel cold while she's in the kitchen/livingroom.

Thankfully, the exhaust ducting is somewhat accessible in the unfinished space above the garage. Still not easy but if putting a motorized damper there could fix the issue, it'd be doable. Not for me, but I'd have to find a contractor which has proven difficult to-date.

kcopp

Is there a place to put a magnetized flap/ seal over the inside that can be pulled down before turning it on?

JUGHNE

I realize all the above discussion is for existing houses.
However if anyone reading this post is doing new or remodel they could consider ducting/venting down inside the walls and then out thru the rim joist.

We have 4 bath fans, kit hood and dryer that do this.
2 of the WC/bath areas have soffit over WC with 2 X 6 inside wall adjacent to the fixture. 4" duct down that inside wall thru plate into basement then out rim joist.
1 WC 1/2 bath has small fan wall mounted with 3" vented down inside 2 x 4 wall and then out .
Basement 3/4 bath easy vent out rim joist.
Kit hood uses the 3 1/4 x 10 (increased to 14) inside wall then 6" in bsmt to outside.
Dryer has 2 x 6 wall behind it for down and out.

No cold drafts or condensation as the cold air will not migrate up the wall.
Dryer feels a little cold as it is close to the outside wall.

All this vents out the north wall, we are known for wind here.
You can hear the dampers slam shut sometimes but do not feel any cold air coming in (other than a little in the dryer).

JustinS

Could I use a fresh-air damper for my installation?

Is fresh-air vs exhaust just a question of mounting direction?

JustinS

Followup question - we had an HVAC company come by and offer their input.

They raised concern about installing any sort of damper in the ducting that is "airtight" (i.e, foam insulation around the edges). Their position is that aerosolized grease/oil could build on this surface to the point where the damper would no longer open, leading to motor burnout.

Thoughts?

Bob Harper

Fix the house and the hood will work. Investigate all sources of exfiltration first and eliminate or mitigate them. The code requires powered makeup air for exhaust fans over 450 cfm slaved to the fan. If the backdraft damper works too good, your fan may struggle to generate sufficient force to open it thus reducing the air entrainment, which is the reason for the hood in the first place.
If you inspect the duct annually and service as required the grease issue becomes moot. I would not install an inline damper with foam seal. This will severely restrict air flow and could constitute a fire hazard by absorbing grease.

JustinS

Bob Harper said:

Fix the house and the hood will work. Investigate all sources of exfiltration first and eliminate or mitigate them. The code requires powered makeup air for exhaust fans over 450 cfm slaved to the fan. If the backdraft damper works too good, your fan may struggle to generate sufficient force to open it thus reducing the air entrainment, which is the reason for the hood in the first place.
If you inspect the duct annually and service as required the grease issue becomes moot. I would not install an inline damper with foam seal. This will severely restrict air flow and could constitute a fire hazard by absorbing grease.

Hi Bob

Thanks for your comments.

I can tell you that the fan max speed is precisely 450 cfm.

If the damper doesn't have a foam seal, wouldn't there still be air leakage?

PC7060

Typical dampers leak a bit but usually its mitigated by the two inline dampers; one at the hood and one on exterior of house. Inspect both to make sure they flapper is fully seated.

mattmia2

It will require occasional service. Maybe you don't need it to be so tight that it is sealed with a gasket. You can interlock the range hood with it then you would know if it isn't opening.

reggi

JustinS said:

Followup question - we had an HVAC company come by and offer their input........ Thoughts?

Question : Did the HVAC Company's input include a solution as long as they came by to evaluate it ? 

JustinS

reggi said:

JustinS said:

Followup question - we had an HVAC company come by and offer their input........ Thoughts?

Question : Did the HVAC Company's input include a solution as long as they came by to evaluate it ? 

By "evaluate it", do you mean some sort of annual check-in on it? If so, no they didn't... the technician's position was that it wasn't a good idea to install something that could fail in 6 months or whatever.

mattmia2 said:

It will require occasional service. Maybe you don't need it to be so tight that it is sealed with a gasket. You can interlock the range hood with it then you would know if it isn't opening.

I had thought of having the damper auto-open when the hood fan is turned on but it sounds like your suggestion would require some sort of manual switch to open the damper first?

PC7060 said:

Typical dampers leak a bit but usually its mitigated by the two inline dampers; one at the hood and one on exterior of house. Inspect both to make sure they flapper is fully seated.

There is a damper internal to the hood but we can hear it flapping when particularly windy. The exterior has just a flap of metal that flaps with little to no wind.

mattmia2

JustinS said:

mattmia2 said:

It will require occasional service. Maybe you don't need it to be so tight that it is sealed with a gasket. You can interlock the range hood with it then you would know if it isn't opening.

I had thought of having the damper auto-open when the hood fan is turned on but it sounds like your suggestion would require some sort of manual switch to open the damper first?

Depends on what the switch for the hood is like. If it is variable speed and the speed control and on/off on the switch are connected internally then it would be difficult to do, but the idea would be that the switch would open the damper then the end switch on the damper would trigger a relay that turns the fan on.

JustinS

mattmia2 said:

JustinS said:

mattmia2 said:

It will require occasional service. Maybe you don't need it to be so tight that it is sealed with a gasket. You can interlock the range hood with it then you would know if it isn't opening.

I had thought of having the damper auto-open when the hood fan is turned on but it sounds like your suggestion would require some sort of manual switch to open the damper first?

Depends on what the switch for the hood is like. If it is variable speed and the speed control and on/off on the switch are connected internally then it would be difficult to do, but the idea would be that the switch would open the damper then the end switch on the damper would trigger a relay that turns the fan on.

The hood has a two buttons (+/-) on it to control the fan. Pushing either repeatedly adjust the fan speed in that direction.

ariccio

Daveinscranton said:

I had this problem with bathroom fan vents years ago.  Windy location.  Ferocious wind.  Venturi effect.  Flappers etc did not stand a chance.

I added powered dampers.  24 volt as I recall.  Used the 120 volts on the fan to sense when to open and close.  120 volt primary winding on small relays.  Doesn’t take much.

Worked like a charm.  Zero maintenance in my application.  They are in the attic if I ever have to get to them.  Old laundromats sometimes can be a source of 6” or 8” powered dampers.  Type of thing you run across and find a use for later.  I used 4” ones in my application.

Best wishes.

Do you have any recommendations as to which motorized dampers to use? I'm not sure which to pick, since grease is a concern, I don't know if I'm comfortable using any old damper.

JustinS

ariccio said:

Do you have any recommendations as to which motorized dampers to use? I'm not sure which to pick, since grease is a concern, I don't know if I'm comfortable using any old damper.

This might not apply for you but I ended up moving my hood from a wall-mounted one to a roof mounted one on my garage. Did this just by turning the exhaust pipe upwards instead of outwards in the unfinished space above garage.

Did this about a year ago and everything seems much better.