What advantage is there in returning the main vent pipe into the stack below the lowest fixture?

It makes the work rigid and allows any scale or rust from the pipe or condensation to pass off, thus preventing ultimate stoppage of the vent.

Where should the main vent pipe be carried?

Either through the roof or into the stack above the highest fixture.

Why should the main vent pipe enter the stack above the highest fixture?

If entered below the highest fixture, waste from that fixture, in passing down the stack, might lodge in the vent pipe.

Why is the main stack carried through the roof?

To relieve any pressure on the trap seals, to ventilate the plumbing system, and to furnish a supply of air to the fixture traps when the main vent is connected with the stack.

Why is it wrong to run back air pipes into the chimney?

The collection of soot might close up the vent, and a downward draft or pressure might force gases and foul odors into the house through chimney holes, etc.

Why should a water closet located close to the stack be counter-vented?

The action of fixtures above might cause at least a partial syphonage of the fixture unless prevented. (See Syphonage.)

Why does the venting of a water closet located at considerable distance from the stack, make the syphonage of the trap less liable?

With a long line of horizontal soil pipe, the waste would move slowly, and a slight obstruction in the pipe might cause the water to set back sufficiently to fill the long arm of the trap, and thus produce syphonage, which venting makes impossible. (See Syphonage.)

Do you consider that a water closet located close to the stack, with no other fixtures entering the stack, needs to be counter-vented?

No, for there is nothing that might cause syphonage of the water closet trap.

What is to be understood by back pressure?

The pressure brought to bear on the sewer side of a trap seal.

How can the effects of back pressure be overcome?

By counter-venting.

What causes sewer pressure?

A sewer not properly vented may exert a pressure on the trap seals, by a sudden rise of temperature within the sewer, causing an expansion. This would be liable to happen if a great amount of hot water or steam were thrown into the sewer suddenly. The backing up and filling up of the sewer by tide water and other causes, also produce pressure.

When a fixture is located over 8 feet from the main vent, what course should be adopted for its ventilation?

The vent should be carried independently through the roof or enter the main vent stack above all fixtures.

Should long lines of vent pipe be increased in size?

After passing thirty feet, they should be increased one size.

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Cellar Plan Of Plumbing For Residence.