Vent Hood Back Draft Danger

[Jim Koepke]

After seeing this, my thought was that others here might find this informative or at least interesting to mull over if they have plans to install a vent hood over a stove or grill.

The 1200-cfm capacity of one of the hoods seems to be a bit much for the average home kitchen, but maybe some folks send up more smoke than the LOML and myself.

http://www.greenbuildingadvisor.com/blogs/dept/musings/makeup-air-range-hoods

jtk

[Eric DeSilva]

Cool. So now I don't have a drafty house, I have a "safe" house.

[Jim Rimmer]

There is a restaraunt on Galveston Bay that was destroyed by Hurricane Ike. They rebuilt and LOML go there often. On about the third visit I noticed they had an auxilairy A/C unit out front. I ran into the owner and asked him about it. He told me it wasn't a failure of the A/C system, it was the commercial range hood. It pulled the cool air out of the dining area into the kitchen. They had to do some significant design changes to the building - new doors between kitchen and dining area as a start and I don't know what else. Last visit the auxilary unit was gone and it was comfortable inside. Somebody overlooked the makeup air requirements for the commercial hood which I am sure has much higher cfm than even the upscale home unit.

You definitely don't want the cool air sucked out of a restaraunt dining area in Houston in the summer time.

[Rod Sheridan]

It's amazing how many times you run into this problem with clothes dryers, ventilation fans, woodworkers who exhaust their dust collector outside or even ventilation fans for workshops.

A few years ago I replaced all the windows and doors in my house, it was now too airtight for the natural gas clothes dryer, so I had to provide a makeup air intake into the basement.

regards, Rod.

[Ben Franz]

Makeup air requirements are part of the UMC and any competent HVAC contractor should know how to deal with it as a normal part of his/her work (nice P.C. touch there, huh?). New forced air units where I used to work typically required 2 sources of makeup air from outside the conditioned space envelope. Most times we had a screened opening down to the crawlspace and a rectangular duct up into the attic. Gas water heaters also need dual sources for makeup air. Large kitchen vents can be very dangerous if nobody plans for them.

A 1200 CFM hood is on the large side for most kitchens. If the cook does a lot of high heat saute work using heavy duty burners or a wok burner, it isn't overkill. We grill inside using a cast iron grill pan and I'm pleased with our hood at 800 CFM - no smoke or smell in the house from grilled fish or my wife's stir fry with canned sardines :eek::eek::eek:.

[Ben Hatcher]

I had a home energy audit performed on my place earlier this year. Part of that audit was a blower door test where they actually measured how much my house leaked. I was a bit surprised that my relatively new 2300 sq ft house leaked 2100 cmf! My furnace has a makeup air duct that draws from the outside but it didn't matter if it was open or closed. The leak was the same either way. According to the auditor, ~1 cfm/sq ft was the target range for a healthy house. That seemed like a lot to me and made me contemplate venting my DC outside. If it is going to leak almost that much anyway, why not have clean air, right?

[Callan Campbell]

My wife wanted a fancy stove with the top grill section back in '96, so the recommended spec for decent smoke extraction when grilling on these "stone brickette" style grills is pretty high as you'd expect. We installed a dual fan unit rated for 1200 CFM. Make-up air wasn't a problem then since I have an old house, leaky enough to allow make-up air unless we're grilling steaks or other higher fat content meat.:p Then, all that's needed is to crack open the enclosed porch/outside door a few inches to get more air flow into the house.
One thing changed all of this however. We added a bathroom exhaust 5 years after installing the kitchen upgrade. Now, with the kitchen fan set on High/Max, and no windows or doors open, it pulls the plumbing stack vapors down the vent pipe of the bathroom fan and into the bathroom since the installers of the fan put the vent pipe pretty close to the main stack vent on the roof[:mad:] and whatever vent flap is inside the bathroom fan isn't that tight for stopping air flow.
To stop THAT from happening, I simply crack the bathroom window open a bit and all is well. Besides, since I got my Big Green Egg last year, we don't grill inside anymore...:D
So yeah, I can really see how these things happen with a very tight house that's much newer and tighter than my 1910's 2 flat.

[Pat Germain]

I plan to install a really kickin' vent hood over my new gas range. I'm counting on my doggie-door to provide all necessary air flow. If I see the flap being sucked inward, I'll know it's working. :)

[Callan Campbell]

I plan to install a really kickin' vent hood over my new gas range. I'm counting on my doggie-door to provide all necessary air flow. If I see the flap being sucked inward, I'll know it's working. :)
Funny that you should say that. We have a cat door on the door out to the enclosed porch, it just doesn't move for the 1200 CFM fan.. I don't want to know how much CFM it DOES take to move it either[;)]
but then again it's got magnets on it to help keep it closed once the cat has popped through to the other side[which is always the wrong side for her]

[Van Huskey]

That seemed like a lot to me and made me contemplate venting my DC outside. If it is going to leak almost that much anyway, why not have clean air, right?


That is interesting particularly when applied to stand alone shop...

[Jim Becker]

This can be a valid issue in many of today's "super sealed" homes that don't incorporate air exchange. I don't have a problem in this house when I run our 1200 CFM range hood fan full-blast during certain types of cooking exercises, but that's because there are a variety of ways that replacement air gets in due to it being an older home...including a section built in the 1750s.

[Callan Campbell]

This can be a valid issue in many of today's "super sealed" homes that don't incorporate air exchange. I don't have a problem in this house when I run our 1200 CFM range hood fan full-blast during certain types of cooking exercises, but that's because there are a variety of ways that replacement air gets in due to it being an older home...including a section built in the 1750s.
Ok,your house is way older than mine! Is the 1750s section still plaster walled? Any horsehair in the plaster?

[Jim Becker]

Ok,your house is way older than mine! Is the 1750s section still plaster walled? Any horsehair in the plaster?

Yes, there is plaster in the 1750s section. I have no idea if it has horse hair, but I doubt it...I don't think it's the original plaster in many places.

[Callan Campbell]

When I was younger, I dreaded and hated the Plaster walls of the spaces I found myself living in. Now, after all these years, they're like old friends, much denser and quieter than many drywalled houses I've been in. Of course, you get all the cracks, lost or losing plaster keys, or water damage too.

[Chris Padilla]

I just installed a new vent hood over a new stove. I'm thinking I need one of those flapper cover thingies. When the vent hood is not on, cool air leaks back into the house. I guess I need to look into this...any pointers?

I put a Zephyr Typhoon (http://www.zephyronline.com/v2/products.php?col=power&prod=22&prodname=Typhoon) 850-cfm unit in. Very nice unit but the designer needs throttled as it was a pain to install.

[Chris Padilla]

It's amazing how many times you run into this problem with clothes dryers, ventilation fans, woodworkers who exhaust their dust collector outside or even ventilation fans for workshops.

A few years ago I replaced all the windows and doors in my house, it was now too airtight for the natural gas clothes dryer, so I had to provide a makeup air intake into the basement.

regards, Rod.

My washer and gas dryer (and water heater for that matter) are located in the garage...pretty common here in California. :)

[Jamie Buxton]

.... I'm thinking I need one of those flapper cover thingies. ...any pointers?..

Sometimes the flapdoodle is built into the hood. You didn't notice one while you were installing? The other place the damper can go is at the duct's exit from the building. There are sheet metal fittings that penetrate through the wall, and have the flap built in. They generally have a hood over the flap, to keep rain out of the duct. Look wherever you got the duct.

[Callan Campbell]

I just installed a new vent hood over a new stove. I'm thinking I need one of those flapper cover thingies. When the vent hood is not on, cool air leaks back into the house. I guess I need to look into this...any pointers?

I put a Zephyr Typhoon (http://www.zephyronline.com/v2/products.php?col=power&prod=22&prodname=Typhoon) 850-cfm unit in. Very nice unit but the designer needs throttled as it was a pain to install. I installed a vent flap on the outside of the house where the duct exits for my Kitchen. I also rivted stainless steel hardwire screen over the opening to prevent any creatures from building a nest in the opening. So, I have the flaps directly above my blowers that are part of the hood itself, and the single wide flap that's part of the vent on the outside. I don't seem to get cold bleed back with this set-up. Hope this helps you out.