Kitchen Range Hood Wall Cap and Strong Winds
I am having a kitchen range hood and ducting installed shortly and am having some hiccups with finding an ideal wall cap.
I live in an area that has the potential for very strong winds and extremely brutal winters. I am having the duct work exit through the back wall of the house, which will require 1 elbow and about an 8′ duct run.
The kitchen range hood will have a gravity fed backdraft damper pre installed. The wall cap includes a gravity fed flap and an exterior guard. The manufacturer of the wall cap warned me that the wall cap flap is not the same as a backdraft damper and may not stay closed during very strong wind episodes, thereby allowing cold air in through the duct work. I thought about adding an additional backdraft damper in the duct line, near the wall cap, to serve as back up when the wall cap flap fails, but was told to be careful with setting up too many dampers (wall cap flap, in line backdraft damper, and range hood backdraft damper), as this may restrict airflow and build air resistance, which will reduce the effectiveness of the range hood.
Long story short, here are my questions:
1. Would adding a backdraft damper near the end of the duct line, by the wall cap, and eliminating the range hood backdraft damper be advisable? Having a wall cap flap and two downdraft dampers would be too much, no?
2. If installing a backdraft damper in the duct line close to the wall cap, would it still be necessary to insulate the duct work? Won’t the backdraft damper serve to prevent cold air from coming inside? Insulating would be overkill?
3. Should I not add any additional backdraft dampers and leave the wall cap flap (not the best in windy situations) and range hood backdraft damper and simply insulate? The backdraft damper pre installed in the range hood will do its job from preventing cold air from coming into the house? Only insulate to prevent duct condensation?
Thank you for your time.
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These questions come up about twice a year on our Q&A pages. One of the most authoritative recommendations for a high-quality termination came from Charlie Sullivan, who recommended this model: the Imperial Manufacturing VT0500, 3.25-inch by 10-inch R2 Exhaust Hood. Here is a link:
Yes, I did see that thread before posting my own question. I clicked on that amazon link Charlie provided and I did not feel comfortable using that product for several reasons:
1. I am using a 500 CFM range hood, which requires a 6 inch round duct. If I were to purchase this product you and Charlie endorsed, I would have to use a reducer (I had also contacted Imperial directly and they confirmed I would need a reducer piece). According to several range hood manufacturers, they stated it is important to not downsize the ducts (via reducers), as this will impact the amount of CFM that a single blower can exhaust away from the house, it can interfere with the flaps installed in the range hood body from opening to their full/ upright position, it can increase the noise of the hood, it can create grease traps which can potentially create a fire hazard.
2. It is also important to keep in mind that the side of the wall cap that attaches to the duct work (in my case, it is 6 inch round duct, which equates to about 27 square inches total circumference of the pipe), needs to be equal to the other end of the wall cap, where air exhausts out of. Otherwise, it reduces airflow and the performance of the hood. What happens with a lot of wall cap brands is that you will have 27 square inches on one end, but the other side will be smaller. One amount of air coming in one way should be the same amount of air coming out the other way. It should not be reduced at all. With this model you endorsed, amazon doesn't even provide the dimensions. So I had to call the manufacturer directly, who stated that she doesn't have those dimensions--the dimensions listed on amazon and on their website is the measurement of the outer portion that attaches to the siding itself-- so the measurements of the portion of the wall cap exhausting the air is even smaller than what is listed.
3. I asked the representative on the phone if they had tested their products for air flow performance (as most people using this product would end up needing a reducer and also with the discrepancy in the circumference/total square inch difference), to which she said they do not. She said they met industry standards but they were not tested for airflow output. In fact, a lot of companies do not test their own endorsed wall cap products. I know this because this is why I did not purchase the wall cap by the same manufacturer I purchased the range hood from.
4. This product also does not have a guard, so a bird can still create a nest, thereby creating a blocked exit. Having a damper alone is not enough to prevent birds from building nests at the edge--you need a guard, which this product does not have.
There are range hood companies that test their range hoods with reducers and not recommended wall caps, and they test airflow performance with a device. The result is that airflow is in fact compromised, sometimes by up to 50% reduction.
I researched wall caps for weeks. I finally found a wall cap that has equal circumference on both ends, is not angled (which can restrict airflow), has a guard and a gravity fed flap, but it is not considered a backdraft damper and will not hold up during strong wind episodes. So I posted on here about what backdraft damper suggestions you had. I am satisfied with this wall cap. It was tested for its effectiveness, meets all the criteria, as far as not restricting airflow from the range hood, but I need to address the high wind situation as I do not want a backdraft issue. The wall cap I purchased works great in most applications; however, they do say that their flaps can move in a high wind situation. There might be products out there with more secured flaps (spring loaded perhaps), but it doesn't mean their design is without structural defects.
So, I am planning on keeping my wall cap (I should've mentioned that in my original post), but I am inquiring about backdraft dampers and their placement, the removal of a range hood backdraft damper (research shows too many dampers can restrict airflow), and also insulation of the duct I will have installed.
Hopefully my questions make more sense now.
I wouldn't eliminate any dampers. Note that they're called dampers for a reason. They reduce flow, but don't block it off. Is the duct in a conditioned, or unconditioned space? If unconditioned, you probably want to insulate it so when it's extracting humid air it doesn't condense and freeze inside the duct. If conditioned, probably still want to insulate it so it doesn't condense water on the outside of it when not in use.
The duct is in an unconditioned space. However, there is a loving amount of insulation less than 4 feet away. Does that make it semi conditioned? My concern with duct insulation is that I don't know if I will have room for it. The space is going to fit a 6 inch duct, leaving very little space for anything else (we had soffits removed, and we discovered electrical wiring and plumbing, so the space for a large duct or duct with insulation is shortchanged). How thick is duct insulation? This is why I was asking about placement of the backdraft damper. If I place a spring loaded backdraft damper right next to the wall cap in the duct line, that will prevent cold air from coming inside the duct line, it will prevent rattling of the ducts during strong winds, it will prevent condensation in the ducts, etc. I am leaning towards having an extra damper placed, but people are saying that might be overkill. I don't know.
Wait, isn't 3 1/4 x10 32 square inches of area, compared to 28 square inches for a 6 inch round?
everything in the flow path is a restriction in some way. I would think that being 14 percent larger would be enough to ease some concerns about flow restriction, especially if it actually works in high wind conditions.
The representative over the phone said that those dimensions were not for the exterior portion of the wall cap that is the final exit point for the air. She didn't have those dimensions, which makes me think that the square inch value is less than what you calculated.
Also, both Victory and Vent A Hood had performed tests showing the importance of using curved versus sharp edges, as this can make a difference in air flow performance, especially with range hoods because of the high CFM rating. They all discouraged rectangular shaped ducts because rapid air flow has a harder time navigating through sharp edges.
Again, I am content with my wall cap purchase (special order from Canada), but I am looking for backdraft damper suggestions. I'm not looking to purchase this Imperial product.
This product might work for high wind situations, as far as minimal flapping and vibrations, but what good is it if it effects the performance of the range hood?
There are several reasons I chose not to purchase this product--square inch value is only one. But I'm not really inquiring about Imperial, but I appreciate the suggestion.
I'm interested in keeping my wall cap product, but I am also interested in installing an extra backdraft damper set. The question is if I place it near the end of the duct line, would it be overkill to still keep the range hood damper. I know someone said to not remove that one, I'm curious to know other's thoughts. And also duct insulation related question.
Thank you, I appreciate you taking the time to answer.
Thanks for the info you provided. Which wall cap did you buy from Canada?