Fig. 146 illustrates a galvanized-wire guard placed in the hub; such a guard is generally used on conductor pipe, but is equally suited to the protection of soil and vent lines in mild climates. In Northern localities, the regular cast hood and thimble made for the purpose are better. The area of the openings of the strainer should aggregate at least 12 square inches for pipe 4-inch or less; and where frost trouble is feared, the strainer should be recessed several inches so that the frost will have to close the open end of the pipe instead of the grating.
The foul-air pipe should not have abrupt offsets at any point. The lodgement of foreign matter therein would be possible, and the function of the pipe perhaps thus impaired. This pipe not only ventilates the sewer, but offers egress for air when storm water is crowding the sewer, and at other times when air-pressure would otherwise drive the seal of the trap toward the house, enough ulti mately, in some cases, to lose the seal by waving out when the pressure is relieved.
When a trap loses its seal by waving out, the water, in flowing back to its normal position, gains momentum enough to throw some of it over the weir, and the balance is not enough to seal the trap. Waving out is always caused, first, by air-pressure on the sewer side, and then by gravity acting as described.
The operation of the fresh-air inlet depends on air from the open entering the house drain near the trap and filling the house system, passing out through the vent pipe above the roof. The inlet should be as large as the house sewer, which should never be less than 4 inches diameter, usually 5 inches. The same precautions taken against snow and ice and other obstructions to the foul-air outlet, are necessary to the fresh-air inlet. The difference in level of the inlet and the exit, together with the warmth of the building, causes an upward current through the stack. Even the taking a more exposed course and stopping at an elevation inferior to the outlet of the soil-pipe extension, when necessary to carry the inlet to the roof, will usually insure a draft.
Objection is often raised against the fresh-air inlet, for the reason that puffs of foul air are thrown out when fixtures are discharged. This is easily possible, but mainly the result of faulty installation. One feature of plumbing is no more likely to be satisfactory than another where ignorance prevails, or when merely the simple letter instead of the spirit of ordinary specifications is lived up to. House
main lines of the same size as soil-stacks (4-inch) will cause puffs of air from the fresh-air inlet if the horizontal run and the inlet branch are both short. It is well to remember that the air so puffed out is not sewer air. It is air which has just entered the house system from the open. And, if the fresh-air branch is of decent length, as de scribed, and as shown in Fig. 144, the puff occasioned by the discharge of a fixture in an ordinary house, even in an objectionable job, may not equal a third of the really fresh air in the inlet branch.
The chance of puffing under the action of fixtures can be avoided by a loop providing for simple revolution of air when fixtures are discharged. A soil-stack from the main horizontal line is carried up to the roof, with all connections as usual, except one. This is made above the highest fixture, and of the same size as the soil-stack, and is generally ried down and connected, as it should be, into the horizontal main several feet nearer the intercepting trap than where the responding soil-stack leaves the main. Some connections are so close to the point of exit that the vertical stacks are made to stitute the whole loop, as shown in Fig. 147, in which cases the direct stack E from X to Y should invariably be a portion of the vent. If the connection X is made in the zontal run, as before mentioned, stack F should be the vent, as a rule, instead of carrying the closet branches GG as shown. V and V are crown vents for the closets. The crown vents may in some situations be made into a separate smaller line leading into the soil-stack above the highest fixture.
By the loop plan, air is thrust before the water discharged from a fixture as usual; also, there is the same tendency to a vacuum behind the water so discharged. But, in stead of reversing the general current and drawing air from the roof to fill the void, the roof current in the soil-stack from the loop connection up, is merely checked, more or less; and the air already rising in the loop turns down the soil stack and fills the void. Without the loop, considerable compression would take place in front of the water before the current in the house main could be reversed. With the loop, this compression is confined principally to the stack. The void being supplied by the loop, the air driven in front of the water simply passes up the loop in response to the-call for air to fill the void behind the water.