# Venting DC outside with air admittance valve on plumbing fixtures



## Todd510 (Jun 26, 2014)

I vent my DC outside creating a negative pressure environment in the shop. I'm in the process of putting in a sink/toilet and planned on venting using a studor AAV. Going through the roof would be expensive and a pain. Thinking about this a little more (maybe over thinking) it seems like if someone uses one of the fixtures while the shop is under negative pressure, the valve might not work. I'm not sure how strong the negative pressure in the sewer lines is to counteract the negative pressure in the shop, so was curious if anyone else had a similar setup and if they had any problems.


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## clin (Sep 3, 2015)

I do not have experience with this question concerning drains and this type of product. Assuming you have an adequately sized, return air path in the shop, there's probably not that much negative pressure to matter. Remember, breezes create all kinds of pressure difference in homes, and I've never noticed any drain issue.

If you do not have a dedicated air return for the shop, by not having a dedicated return path, you are making the DC work harder. You just might improve your DC performance by installing a dedicated return air back into your shop. This of course would also address the potential issue with the drain system. Again, this is if you are not already doing that.


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## jmos (Nov 30, 2011)

You could also have a problem with any fired appliances (water heater, furnace, fireplace) if the house pressure is too low; they won't exhaust properly, and you could possibly draw flue gasses back into the house. As Clin said, make-up air is important.


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## Knockonit (Nov 5, 2017)

Why even chance it , sewer gas if left unchecked is a very dangerous thing. bite the bullet and vent properly and there will be no worries, late at nite w hilst you slumber.

do it right, and go forward.
jmo
Rj


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## a1Jim (Aug 9, 2008)

Could explain how you're going to get negative pressure in your closed sewer line, your toilet has a built-in "S" curve to keep gases from infiltrating into your indoor spaces and a sink has "P" trape for the same purpose. I hear this negative pressure thing online all the time, but unless you're going to run you DC unit all the time 24/7 It's not an issue without inlet venting. I've had my DC unit outside for 8-10 years with no problems.


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## rwe2156 (May 7, 2014)

Are you assuming a lot or do you have actual measurements proving "negative pressure"?

If you're worried about anything it should be heat/ac loss. Could be an issue with large DC/small shop.


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## a1Jim (Aug 9, 2008)

Every time I suggest venting outside these two issues are always brought up "Negitive pressure" and " Heat loss". I still say unless you're using your DC for most of the day then both of those issues are not relative. My shop's has gas heat and it does not come on from heat loss even after planning wood for a couple hours with my DC on all that time with more than one unit venting outside.


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## Gene01 (Jan 5, 2009)

> Every time I suggest venting outside these two issues are always brought up "Negitive pressure" and " Heat loss". I still say unless you re using your DC for most of the day then both of those issues are not relative. My shops has gas heat and it does not come on from heat loss even after planning wood for a couple hours with my DC on all that time with more than one unit venting outside.
> 
> - a1Jim


Your preaching to the Choir, Jim. I've been venting to the outside for 18 years. My experience mirrors yours.


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## splintergroup (Jan 20, 2015)

I've installed one of those valves on an extended drain line, basically a check valve that seals tight enough to keep sewer gas from creeping out.

Too much negative pressure keeping the valve from opening would be effectively the same as having your vent capped. This could impede the flow in the drain and suck out the water in the trap under these conditions, but it is highly unlikely that any DC in a shop would be strong enough to prevent the valve from doing its thing…


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## Kazooman (Jan 20, 2013)

> Could explain how you re going to get negative pressure in your closed sewer line, your toilet has a built-in "S" curve to keep gases from infiltrating into your indoor spaces and a sink has "P" trape for the same purpose. I hear this negative pressure thing online all the time, but unless you re going to run you DC unit all the time 24/7 It s not an issue without inlet venting. I ve had my DC unit outside for 8-10 years with no problems.
> 
> - a1Jim


Probably preaching to the choir about plumbing issues, but for those not so well in the know…..

I don't think the OP will have any problems, but it is not just about sewer gases infiltrating the indoor space. The flow of water down the drain does create some negative pressure (a.k.a. "suction") when you pull the plug or flush. That is why every drain line in the house is fitted with a vent through the roof to allow air to enter behind the flow. If you have a bathroom with a toilet and a sink and PLUG the vent on the roof and then flush the toilet you will hear the glug, glug, glug, of air being drawn through the P-trap on the sink. Heard it in my Mother's house and knew to call a plumber to clear the (as expected plugged)vent line. The type of vent the OP is referring to is for situations where you cannot easily run such a vent through the roof. I have one installed on a tub sink in my basement workshop. You need a vent, but cannot have an open pipe to the sewer gasses. The valve he is referring to is designed to maintain a seal under no-flow conditions, and open to allow air into the line when water is flowing down the drain. That balance is dependent on the ambient air pressure at the valve. In theory, a low enough negative pressure in the room could prevent the valve from functioning, but I doubt the OP can create that much suction with his DC. The issue would be a poorly operating drain due to being starved for makeup air, not the problem of sewer gasses being pulled back into the shop.


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## a1Jim (Aug 9, 2008)

Kazooman
Thank you for the very good explanation, I'm familiar with that type of vent but not that name ,if I would have looked at his link that would have explained what he had in mind(old guys what are you going to do with us ? LOL). I can't fathom that much negative pressure unless the DC and the vent where in a very small closet-sized room.


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## Todd510 (Jun 26, 2014)

Thanks for the responses. I've never used one of these devices before and was probably being overly paranoid about the idea of potentially filling the shop with methane.


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