First, you might want to confirm that the Broan damper actually closes all the way; sometimes an errant screw or distorted duct wall will obstruct the damper from opening or closing.

Second, thought should be given to adding a damper at the point of egress from the house, particularly if the draftiness is correlated with the exterior wind speed. A weighted damper on a side wall may be sufficient. Boiler flue bypass dampers are of this type. Otherwise, one could use a powered damper such as Broan sells for use with make-up air supply ducts. (The damper in this case would have to be controlled from power to the hood fan and not from sensing air flow, because there wouldn't be any flow until the damper opened.) Honeywell also sells (perhaps only in larger sizes) controlled dampers.

I can't easily evaluate from here whether damper addition is more or less trouble than rerouting the entire path. The damper issue might be unaffected by the new path. As for the 7-inch restriction, it may not be a big deal, so long as a proper smooth transition from 8 to 7 and then 7 to 8 is used on each side.

If the issue isn't drafts coming from the outside, but is just cold air settling into the space above the hood damper and chilling the hood, I don't see how rerouting the path will significantly reduce the effect.

kas

Hi

I thought that the damper closes completely when the wind isn't blowing but I'll have to look more closely...

I have weighed down the Broan flap with opposing magnets on either side of the metal piece... has had some impact but overall, not that much...

By egress from the house, you mean from the main heated part of the house and not where the Broan damper is on the exterior of the garage, correct?

As for the ducting restriction, I would only go 8-7 and then have a 7" duct on the exterior because there would be no room to go back from 7" to 8"...

The idea about re-routing the duct to address cold air settling is that doing so would bring the ducting into the heated space of the house (as opposed to sitting in the unfinished area above garage)... right now, the duct has gotten cold enough to have frost just inside the kitchen wall...

Thanks for the reply

Ah! If the existing damper is at the exterior wall, then a damper is missing from the top of the hood where there should be one to resist just what you are observing. In any case, if the hood metal is frosty from conduction to the garage area, then I concur that a warmer straight up to the roof path is better. It would accomplish a more important mission also, keeping the ducting warm enough to reduce grease accumulation.

What I don't understand, though, is why, when the house was under construction, the space for the duct wasn't increased when it became apparent that it needed to be?

In general, I would argue that such duct spaces should be designed to accommodate 10-inch ducts as this size is needed if one is to upgrade to serious ventilation to deal with serious grease-emitting cooking projects involving grilling, frying, or wokking on larger cook-top surfaces.

kas

There is a 2nd damper built-in to the hood itself... but it doesn't appear to do much as far as stopping air from coming out the bottom of the hood, nor does it affect the conduction of cold along the ducting itself into the hood and its associated hardware...

Your question as to why the space for the duct wasn't increased once it was discovered is a very good one...

The 2nd floor was already on when we discovered that there was insufficient space to go through the ceiling and exit through the side... the builder never suggested moving the joist and I guess I wasn't experienced enough to know that it was even an option... he said that going into the garage was a good solution but clearly he didn't know what he was talking about...

In addition, going up through the ceiling to the roof isn't an option as the master bathroom is right above the hood... no space to box off a 8" round duct...

Personally, I am very disappointed in how he implemented this, especially since he's basically washing his hands of it - saying that "he just installed what we supplied"...