- This topic has 5 replies, 3 voices, and was last updated 23 years, 7 months ago by
SylvanLMP.
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Anonymous
We built a new house out in the country & on windy days, winds blowing 30+ miles an hour, the water gets sucked out of our toilets til you can hear the wind blow in the stack!!! What can we do to stop this? I am sick of coming home from work at the end of the day to check the toilets for water!! Please help
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This is similar to venting problems for heating appliances; although gas appliances have a vacuum break in the form of a draft diverter to prevent sucking a flame. The chimney code requires that a stack be a minimum height above a roof of 2 feet higher than the nearest point of a sloped roof that is 10 feet away from the vent to prevent pressure or vacuum. Your vent may be too low. The same rule seems to apply in the case of a sewer vent. Perhaps a Licensed Master Plumber can give another successful anecdote to solve the problem.
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Harold one of the questions ALWAYS asked on the 2nd year apprentice test is how can a trap loose its seal?
Of the 6 ways one answer is always “Oscillation” due to wind effect.What is happening to LuAnn is a common problem.
As wind goes down the vent terminal it causes a negative pressure and back syphons the trap seal until the trap seal is gone (water empties out)
Now the height of the vent terminal has actually NO BEARING as some codes state
A roof for weather protection may have the vent terminal terminate at least 24″ above the roof.Besides on a boiler vent one could always use a “SWISS CAP” to prevent down drafts as this diverts the downward flow to outside the chimney plus on “plumbing” we don’t use a diverter on vent piping as we can’t have openings like we do in heating
Now the code also states that a promenade deck shall have the roof terminal extend 7 Feet above the roof. (Old code use to be 5 feet for all vent piping except acid systems.)
To stop this phenomenon all that is required is either install a return bend (180 degree fitting) Or a J. R. Smith 1748 Vent cap that allows the vapor out BUT prevents down drafts and it will not allow hoar frost to close the vent opening in the winter months.
Back syphonage is also caused by poor design when for example a toilet on the 47th floor of a building is flushed and it causes a negative pressure sucking out the traps on the lower floors along this soil stack.
The installing plumber most lightly did a soil stack vent set up (twined the vent into the soil) rather then a soil stack tie in to the vent thus causing all down drafts to effect the soil line and thus causing he trap seals to oscillate back and forth until the seal is gone or the very least weakened.
Wow plumbing can be so educational at times HUH Harold?
ILL bet hydro would just cap off the vent with a coffee can with a few holes punched into causing all the great sewer gases to enter the home and make everyone SICK But hey its CHEAP fix and no license required
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Great info, Sylvan. The question left is: If there is no specification for vent height to prevent wind pressure from forcing down or up as the wind makes an aerodynamic effect over the building, then why is a return bend or mechanical check valve necessary?
The impression is that the wind will target the vent pipe to go down the pipe and not create a pressure on the roof. Another impression is that the NFPA is incorrect in specifying the height requirement for a chimney, even though thay have found that a Swiss cap or a power vent can malfunction from wind pressure on a low termination.
The difference in the solutions stem from the differences between combustion venting and water seal. The furnace will depend upom milinches of pressure to flow vent products, the trap will depend upon the weight of inches of water to keep a seal. The trap may work at pressures 100 times the air pressure that would disturb combustion venting, so the solutions can be different.
It is good that a 2nd year plumbing apprentice is asked this, as he will confront this again in a different and lighter pressure when he discovers that he is also required to be a heating expert among his other plumbing skills. It is unfortunate that many states have no licensing or education prerequisites for heating contractors; in this case a plumbing apprentice has an advantage.
As for Nick Hydro, I think you are right. He would put a can on a short chimney or vent and come back many times until he finally got it right – if ever.
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Harold we do not use a “mechanical check valve” the cap is just that.
A cap to prevent vandalism of bums throwing debris down the vent terminal BUT because of its design it doesn’t allow wind to go straight down the pipe.The return bend again is designed to keep debris from being thrown down the vent piping BUT it would also prevents down drafts.
The vent height varies with each code BUT even the standard plumbing code states in part “All roof vent pipes that extend through a roof shall be terminated at least inches—“
EXCEPT other then weather protection the vent shall be run 7 feet above the roof.
The local Code I use says 24″ for weather protection ONLY
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Harold, the NFPA (54) is not wrong either is ANSI 223.1 Or the AGA National Fuel code when they specify chimney height off the roof.
What these code bodies do specify “chimney termination is based upon residential or low heat utilization’s and they shall extend at least 3 ft above the highest point where it passes through a roof of a building and at least 2 ft higher then any portion within a horizontal distance of 10 ft ” SEE National fuel code (NFPA 54) Article 7.5.2a
You also must take into consideration a “plumbing” vent pipe does not have the “draft diverter” of the gas counter parts to prevent spillage and the plumbing vent is working 24-7 unlike a gas/oil / coal / wood appliance that fires part time.
So the down draft condition is not as common as a much larger opening of a chimney/flue.
The plumbing vent is subjected to hoarfrost and negative pressures when a fixture is used.
When the waste water is flowing down a drain does cause a slight back pressure on the venting system (especially on undersized venting).
The plumbing vent system is designed to be able to function without any mechanical means or special draft hoods.
The cold in coming air is pulled into the Fresh Air inlet (FAI) located within 4 feet of the building trap and it sized at half the size of the building house drain Or 3″ which ever is larger and is it located 6″ above grade.
Then as this cold air picks up the warm sewer gases it causes a natural draft up to the roof terminals.
Knowing sewer gases can be carcinogenic and flammable is the reason the codes do specify the horizontal distance from any window or door or adjoining building AND that is why promenade roofs and roofs OTHER then weather protection only have these vent terminals extend a minimum of 7 feet and 2′ on weather protection roofs only.
Harold I am a federally certified low pressure boiler inspector (ASME and NBBI) and unfired pressure vessel inspector PLUS a member of the NFPA as I do have a Master fire suppression piping contractors license.
Granted I do not follow up as much as I would like on NFPA 54 as lately I have been studying NFPA 13 (sprinkler systems).
Lots of changes on designs and conflicts with other NFPA publications, like inspectors test station locations.
These is so much knowledge to learn out there and so little time.
Rather then discuss this on here your more then welcome to bring this topic up for discussion on the PIPDL list where we can get other professionals involved in a round table heated discussion.
Harold, as always a real pleasure reading your postings.
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