Definition

A fresh air inlet is a pipe connected to the main house drain inside of the main drain trap, and extending to a point outside of the building where it is open to the atmosphere. Its object is to admit fresh air to circulate through the drainage system to keep the air within comparatively pure; also to act as a relief pipe to prevent compression of air within the system when a heavy flush of water, in descending, fills the pipe full bore, or when a strong gust of wind blows down the vent stacks from above the roof. If a passage for the escape of air were not provided for such occasions the compression of air in the drain pipes would force drain air through the seal of some of the traps. It is quite evident from the function of a fresh air inlet that any form of check valve or inlet fitting that prevents air escaping from the month of the pipe during heavy discharges or "blow backs" should not be used.

Connection To House Drain

The fresh air inlet should connect to the house drain by means of a T branch. It should never connect to the cleanout opening of a trap. In cold climates, such as the northern part of the United States and Canada, the fresh air inlet should be connected to the main drain from 5 to 15 feet inside of the main drain trap. If connected to the main drain near the trap the rapid circulation of cold air through the system will, in winter weather, freeze the water in the trap.

Location Of Outlet

The mouth of a fresh air inlet should be located at least 12 feet from all windows, doors, ventilator shafts or flues communicating with a building, and the end should be so protected that it cannot be obstructed by children or choked with dirt, water, snow, leaves or ice. In some of the large cities the fresh air inlet opens into the side of a box located at the curb, and is protected by a removable metal grating. When so located, the bottom of the box should extend at least 18 inches below the bottom of the pipe to prevent the mouth of the fresh air inlet becoming choked with dirt. The principal objections to this form of inlet are, first, the box is seldom, if ever, cleaned, and in the course of time fills up with dirt; second, during winter weather the grating becomes completely choked with snow and ice. City houses are built so closely together, however, that the only other available place to locate the fresh air inlet is on the roof of the building, and in some localities it is there located, although this method gives a poorer circulation than the other.

In detached houses the fresh air inlet generally is extended 15 or 20 feet away from the building, and the end protected with a return bend that opens facing the ground. When this form of inlet is used, it is good practice to locate the inlet in a clump of bushes or some other place equally inaccessible to children.

When sufficient space can be found in the foundation wall of a building, near the main drain trap and far enough from all openings to the house, the fresh air inlet can be located there, and the inlet protected by a metal strainer a, Fig. 16, secured to the stone work. This makes a very superior form of inlet. In fresh air inlet grates the openings should equal in area the size of the fresh air inlet pipe, and the size of the openings in the grate should be not less than one-half inch in their least dimension.

Size Of Fresh Air Inlets

Fresh air inlets should be the full size of the house drain for all sizes of drains up to 4 inches in diameter. For 5 and 6-inch drains it should be not less than 4 inches in diameter. For 7 and 8-inch drains not less than 6 inches in diameter, and for larger drains not less than 8 inches in diameter.