What is a counter-vent?

A counter-vent is a pipe through which a trap is supplied with air.

Why should traps be counter-vented?

To prevent partial or total syphonage of the trap, and to ventilate the drainage system of the house.

How does the counter-vent prevent the syphonage of a trap?

By supplying air to the crown of the trap no vacuum can be formed, and consequently syphonage cannot take place.

From which side of the fixture trap should the vent be taken?

From the sewer side of the trap.

Where should the vent from a fixture trap be carried?

Either into the main vent at a point higher than the top of the fixture, or directly through the roof.

Why should the back air pipe enter the main line of vent pipe higher than the top of the fixture?

If entered below, in event of a stoppage of the waste pipe, the waste from the fixture would pass off through the vent.

What should be the size of fixture vents?

The vent from a water closet or a slop sink should not be less than two inches, and the vent from all other fixtures should not be of smaller size than the waste pipe it serves.

What kinds of pipe may be used for main vents?

Cities differ on this matter. Most of them allow either cast iron or galvanized wrought iron pipe, while others prohibit the use of wrought iron pipe entirely, and still others will allow the plain wrought iron pipe.

What kinds of pipe may be used for fixture vents?

Cities differ on this point also. Some call for lead, others allow lead and galvanized iron pipe, and in some places where cheaper work is done, plain wrought iron pipe may be used.

In what different ways are water closets vented?

They may be vented from the lead bend, from the vent hub attached to the T-Y, and where a cast trap is used, from the crown of the trap; also from the crockery.

Why is it wrong to vent a water closet from the crockery?

The vent being rigid, any settling of the floor or of the house, or any movement of the fixture itself, is likely to break off the vent horn.

If the water closet is vented from the lead bend, from what part of the bend should the vent be taken?

From the top section of the horizontal part of the bend, never from a point where the flush will throw matter into the entrance of the vent, or where the waste will settle into the vent pipe.

Why should the use of rubber vent couplings be prohibited?

Rubber soon loses its life and cracks, allowing odors and gases to enter.

How does venting effect evaporation?

It increases evaporation by bringing in air upon the trap seal.

How are the top and bottom connections of the main vent with the main stack usually made?

The main vent is usually connected at the top with a T or back vent fitting, if of cast iron. If of wrought iron, through a tapped T. The bottom connection is usually made with a Y and eighth bend. Sometimes instead of re-entering the main stack at the bottom, the main vent pipe stops at the lowest fixture.