The pressure drop needed to open a damper is not large, and once open the pressure drop is even less. Check to make sure that none of the moving parts are binding. Variable motor controls typically progress from off to full on to reduced power. This aids both motor starting and dealing with such loads as initial damper opening. For systems with large motors and long spin-up times, it can be useful for the operator to "dawdle" a bit changing from full on to the desired lower setting. kas

Your second plumber is correct, you should not use an "AAV" (Air Admittance Valve) in this situation. Sewer gasses are primarily composed of Methane, Hyrdogen-sulfide, Carbon-Dioxide, and Sulphur-Dioxide. All of these gasses are toxic and some are extremely flamable or in even low concentrations and can be explosive in higher concentrations. (Methane is the base component of Natural Gas). In addition, there are a number of airborne pathogens that are known to commonly be present in sewer gasses. Some of the gasses present in sewer gas are lighter than air while others are heavier than air and the concentration of these gasses will continually change day to day or even hour to hour. When the mix is predominately heavier than air the gasses will settle downwards and ultimately go out your sewer line to the septic tank or municipal sewer. On the other hand, when the predominance of the gasses is lighter than air the direction of flow is upwards and out the roof vent thus all vents should allow a two way flow of air. As water flows through a drain pipe it pushes the air in the pipe ahead of it. This can then cause a negative air pressure behind the water flow and that negative air pressure will result in sucking the water out of the fixture trap. This then leaves the trap dry and allows sewer gasses to enter the structure. For this reason the codes are very explicit in regards to the length of pipe in a fixture arm measured from the trap to the vent. An AAV has a spring loaded diaphram that will allow air to enter the line to prevent sucking the water out of the traps, however it will not aleviate the problem of the lighter than air sewer gasses collecting in the line. In addition, because the AAV has a spring loaded movable diaphram it is subject to malfunction in which case the sewer gasses would again enter your home. The Uniform Plumbing Code limits the use of AAV's to a maximum of one per structure and only with the expressed written approval of the local inspecting authority. (Generally they will only approve an AAV for use as a kitchen Island vent and even then most inspectors would prefer to see an Island Loop Vent rather than an AAV). The International Residential Code requires all structures to have one "Main Vent" which must run undiminished in size from the house main drain up and out through the roof. Once the main vent is created they will permit terminating a vent; 1. in the attic space (very bad idea because it causes excess moisture to build up in the attic insulation and sewer gasses venting into the attic can infiltrate back into the living space.) 2. Through the wall (subject to a number of limitations). 3. May use an unlimited number of AAV's however AAV's should not be used where a common atmospheric vent can be achieved) In your case the atmospheric vent already exists so installing cabinets or changing a room decor would not be a justification for discontinuing the vent and installing an AAV. You could very easily drill a hole in both the top and bottom of the cabinet and extend the vent line through it.

Hey guys! Thanks for your help with my last question regarding whether or not I could use the vent as a dwv. I cut the stack and installed a wye and ran the new dwv up to the 2nd floor. It's going to work beautifully! Now, here's my new dilemma... Right now, there's a really 3" old cast iron vent that goes up through the roof. I have 2 questions about this: 1: Can I attach the new 2nd bathroom vent into the existing 3" vent? Or would that be overloading the vent? I would do it correctly and T in all of the other fixtures about 12" above the highest drain. Or rather, I guess a better question would be - can everything in the house use this vent, or do I need a 2nd one? 2: What are the rules for vents regarding bends and gravity and such. As long as I have the correct size pipe, can I twist and turn the vent as much as I want? Can I have angles tighter than 90 degrees? The tight turns would be close to the roof... thanks!!!

"The manufacturers of Vent Hoods are miseducating the consumers." Yes. Somewhat. Not really. They are in business to make money and they do that by making hoods as inexpensively as possible and then selling as many as they can for as much money as they can. Their goal is NOT your health or the IAQ of your home - that is up to you. Telling consumers that they'll also need to spend money on MUA if they buy hood X will negatively impact their sales (ignorant consumer will just go buy a hood from brand Y instead because it doesn't say you need MUA even though it has an identical need for MUA) so they do not want to do that. MUA is also not something that fits well within consumer hood manufacturers world except to the extent that they might offer a hood w/ integrated front curtain ducting. It is much more of a pure HVAC thing. It can be supplied to the return ducts of an HVAC system, the supply ducts or ducted directly to appropriate locations. Incoming air can be heated by gas, electric or hydronic. Incoming air may need to be humidified or dehumidified. It often needs to be electrically integrated with the HVAC system controllers to function properly. Consumer hood manufacturers - ARE NOT IAQ or HVAC people or engineers - they make something decorative that by nature must include air movement so they reluctantly include air movement in their product. Commercial hood manufacturers - ARE IAQ and HVAC people and engineers. Their customers are much more educated than consumers. The one singular purpose of the product they are selling is IAQ - so they design and sell systems that provide good IAQ. Aesthetics is quite secondary for them. Residential HVAC companies in the U.S. - ARE NOT IAQ people (they should be though). A tiny few, less than about 1%, know it well and a few more know it a very little but the vast majority know just enough, based on what they've been told at a 4 hr CEU course, to sound like they know what they're talking about but they don't really understand it. Most residential HVAC people do not understand air movement or why things are done the way they are - they only know how to use tables that tell them to do this or that and as soon as something is outside of defined parameters they're totally lost (though sometimes don't even realize that they're lost). Licensed Professional Engineers do (or most do, some don't) understand air movement and other elements. They are the people who create the tables that HVAC people use. They can think independently. Defined parameters for them are not tables but the physics of air - it's movement, temperature and components. Unfortunately, most U.S. engineers do not have a good understanding of human physiology (except perhaps for biomedical engineers). They do not, for instance, understand how CO2 functions in our bodies and how high levels of CO2 due to poor ventilation affect us. Or how poor ventilation resulting in high levels of VOC's, PM, Carcinogens and Pathogens affect us.