# Positive Pressure in Building Sewer



## bzolla (Feb 5, 2013)

We are doing a major TI in a 2 story medical building. During demo and while tying in to the waste system we experienced more blow back of sewer gas than I've seen in 30 years of plumbing. The building is less than 5 years, cast iron, and drains fine. The General Contractor, and Owners Rep. want us to explain why. The tenants have been complaining of smells long before we started the project.

Sure would appreciate any explanations or suggestions on how to diagnosis the problem.

Thanks


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## deerslayer (Mar 29, 2012)

Could be alot of reasons but usually I find that the building has a negative pressure and that makes it seem as if the sewer has positive pressure even though it is normal atmospheric pressure.


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## johnlewismcleod (Apr 6, 2012)

Too many exhaust/fart fans, rooms with exhaust fans or A/C returns and sealed doors. 

Hospitals and clinics are renowned for having airtight doors and are generally negative atmosphere factories...open a sewer line and they'll all but suck the shiite out of the pipes


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## plbgbiz (Aug 27, 2010)

HVAC is unbalanced and/or the supply ducts are undersized for the air volume bing drawn by the returns.

When you opened the sewer it gave the system unrestricted air to draw from effortlessly. 

I have seen this countless times in commercial kitchens when the exhaust hoods are running over the grill. They will often draw so much air through WH vents that it is impossible to keep pilots lit without sealing the rooms off.


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## DesertOkie (Jul 15, 2011)

See whats down the street. I worked at a clinic that had the same problem. A new canning place went in down the road and pushed so much water down the city line it pushed air into the clinic sewer. 

We installed a building trap and cleared up the issue.


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## SewerRatz (Apr 25, 2009)

This is one of the many reasons an AAV should never be used.


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## PLUMBER_BILL (Oct 23, 2009)

bzolla said:


> We are doing a major TI in a 2 story medical building. During demo and while tying in to the waste system we experienced more blow back of sewer gas than I've seen in 30 years of plumbing. The building is less than 5 years, cast iron, and drains fine. The General Contractor, and Owners Rep. want us to explain why. The tenants have been complaining of smells long before we started the project.
> 
> Sure would appreciate any explanations or suggestions on how to diagnosis the problem.
> 
> Thanks


 Any dental offices ... check to see where the vented the vaccum. I have found them tied into the plumbing vents.


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## bzolla (Feb 5, 2013)

I'm not sure what AAV is.


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## bzolla (Feb 5, 2013)

PLUMBER_BILL said:


> Any dental offices ... check to see where the vented the vaccum. I have found them tied into the plumbing vents.


Yes, there is a dental office in the building. Great call.


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## Gettinit (May 9, 2012)

bzolla said:


> I'm not sure what AAV is.


:blink: Air Admittance Valve.


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## DesertOkie (Jul 15, 2011)

Studer vent


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## AlbacoreShuffle (Aug 28, 2011)

DesertOkie said:


> Studer vent


cheater vent. :yes:


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