It is possible, through suitable connectors, to join iron pipes of different sizes. Thus, the drains that feed into the building drain usually have a smaller diameter than the building drain. So this situation isn’t at all uncommon.
There are three possibilities: pipes of the same size joining; a small pipe joining a larger pipe; a larger pipe joining a smaller pipe. With pipes of the same size there is no problem, assuming, of course, that the pipe has the right diameter for the job it is supposed to do.
When Plano Plumbers join large pipe feeds into a smaller one, there is always the possibility of blockage and backup. The larger pipe can handle so much more waste so much more quickly that it may easily overwhelm the capacity of the smaller pipe. And so, as a rule, we don’t want to feed from a larger pipe to a smaller one.
It would seem that feeding from a smaller pipe to a larger should present no problems, and yet it does. It all depends on the ratio of the two sizes. If you have a very small pipe—that is, a pipe with a small diameter connected to one having a much larger diameter—you can get a rapid drop in the temperature of the waste liquid. The velocity in the large pipe will be much less than in the smaller one.
Consider also that this piping may be in an unheated area, such as a basement. The reduced temperature may help cause the formation of greases, which in turn could represent the beginning of a clog. There are so many factors involved it is impossible to make a positive prediction. But a clog is a possibility.
Slope of Drain
There are two ways in which you can increase the velocity of waste in a drain pipe coming out of the boilers. One method is to use pipe with a small diameter. There is a practical limitation, of course, for if the diameter is too small, the waste will just pile up inside the pipe. Another method is to give the drain pipe a downward pitch or slope. Pitches vary from 1/16 to 1/2 inch per foot of horizontal length, but a pitch of 1/4 inch per foot of pipe is commonly used.
The pitch must not be so great that the outlet from an unvented part of the pipe is lower than the bottom part of the trap on the fixture. If this is the case, there is the possibility of water siphoning out of the trap.
One of the problems with drain pipes that are of considerable length is that they may have an inadequate downward pitch. This, coupled with the fact that the pipe diameter may be somewhat too large, will permit the accumulation of grease and waste matter in the pipe.
The process just described is an accumulative one. Not only is there the buildup of a possible clog, but the inside walls of the pipe also become coated with grease and waste. The net result is as though you had replaced the pipe with one having a smaller diameter.
This, in turn, can cause siphoning of the traps of various fixtures in your home. But with this water siphoned out, sewer gases would have an opportunity to discharge into your home through your fixture drains. Whether this will happen depends on the amount of venting your plumbing system has. Of course, if the drain pipes in which this is happening originally had a small diameter; you will have the backup of waste right into your fixtures.
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