Heritage Restoration, Inc.

Vent is bringing in the cold

My home is a 60 year-old ranch and I recently replaced the original range hood fan/vent.   I did this thinking it would help with the ‘open-window’ effect of the vent allowing cold air into the kitchen via the fan.  It hasn’t, and I’ve asked around but no one seems to be interested in solving my dilemma.  The exterior vent flap has been cleaned and closes flat, but is not ‘seal-tight’.  Any ideas so that this winter’s wind doesn’t give me more air exchange than I desire? 

North Kingstown

A stove or vent hood can be a major source of airflow.  The “flap” is a pretty simple concept, where a metal flap is hinged by slots and metal hems with gravity keeping it closed.  Yet debris, install, and quality can all be factors of function.

The first thing I would check is the flapper itself.  Does the flap move freely? Does the flap sit well against the vent hood back?  Is there gunk or something inhibiting the flap from sitting well?  What is the condition of the flap hinge?  Is the hinge broken?

First thing is clean debris around the flap. Cleaning vents are especially important for preventing fires and airflow. Keep your dryer door closed when you are not using it. But that wasn’t the question.

If the flap does not sit flat, then the vent may be out of level.  If the top of the vent is farther our then the bottom, then the flap will not sit hard against the back, allowing air to come in.  If the top of the vent is slightly back, then the flapper may have a better chance of staying closed.  

Like many things, there are good and bad exterior vent hoods.   Cheap has thinner metal and lacks design features like a good hinge system. If the flap floats then the it may be too light, which is especially true with cheaper vent hoods.  We use vent hoods where the metal’s gauge is thicker, hence the flap is a bit heavier.  A heavier flap is a better seal, although you don’t want one so heavy that it won’t open.  That is less likely to happen, unless you fan is undersized. The CFM (cubic feet per minute) of the fan will effect how well a heavier flap can open. Determining the proper CFM for a stove is dependent on several factors, including the range size, kitchen size, duct diameter, duct length, and how many elbows in the duct line.  

If the flap bounces from time to time it can be from internal pressure changes or the vent is out of level.   Internal pressure changes can happen when a house is fairly porous, with a lot of airflow through a house’s various cracks and openings.

Another issue may be the vent hood was installed with gaps around the duct hole through the building. You may not see that from the outside but you can from the inside when everything is removed. If the flap is closed and you still feel air, then you have a poor seal. Spray foam solves that quick.

If your flap is poor or have internal pressure changes you can install a good quality in line (inside the ductwork) back draft damper. 

Then there is jerry rigging.  While we call professional jerry rigging “improvising”, I often choose winging it at home.  If the flap is light, then you can put some real silver duct tape (with a removable paper to reveal the sticky on back) on the bottom of the flap.  You can tape a washer too if the tape is not enough.  This will work only if it is level or slightly back. Just be sure what you do DOES NOT restrict airflow or collect debris, be it oil for a stove or lint with a dryer.  

I would go through the steps of determining the cause, and hopefully it is a simple flap problem.  If it is a bit more complicated like the install or quality, you may have to take some things apart and solve it by better quality or the right install.  I always advocate doing it right for others.  I can personally settle for less.

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