Dishwasher drain - to disposer or P-trap?

7303 Views 4 Replies 4 Participants Last post by techpappy, Dec 28, 2012

Chokingdogs

Discussion Starter · Dec 27, 2012

The dishwasher in the house currently has it's own dedicated P-trap for drainage, it's on the end of a horizontal waste run, and I can see it in from the basement.

The trap itself is tucked in-between the floor joists, so it's clearly lower than the lowest drain on the DW.

Is that a "good" way to drain a DW? I assume there's no high loop, unless it's directly behind the DW. I cannot possibly see any siphon effect happening, due to the fact the drain is lower than the DW, and with the P-trap in place, no sewer gas.....I guess, of course, if I don't run the DW for 6 months the water may evaporate....

I have no problem running the/a high loop to the disposer drain....but if it isn't broke and the drain is there now, I'd be just as happy to keep on using it.

Thanks

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Ishmael

· #2 · Dec 27, 2012

Chokingdogs said:
The dishwasher in the house currently has it's own dedicated P-trap for drainage, it's on the end of a horizontal waste run, and I can see it in from the basement.

The trap itself is tucked in-between the floor joists, so it's clearly lower than the lowest drain on the DW.

Is that a "good" way to drain a DW? I assume there's no high loop, unless it's directly behind the DW. I cannot possibly see any siphon effect happening, due to the fact the drain is lower than the DW, and with the P-trap in place, no sewer gas.....I guess, of course, if I don't run the DW for 6 months the water may evaporate....

I have no problem running the/a high loop to the disposer drain....but if it isn't broke and the drain is there now, I'd be just as happy to keep on using it.

Thanks

I'd tie it in to the disposal and cut that trap off in the basement (cap it or put a clean-out on the end of that drain line).

Most new dishwashers come with the "high loop" already installed (clipped to the side of the machine, so that's not really an issue. The problem I have with using the dedicated trap is that velocity can also cause the trap to lose it's seal. The water is pumped out of the dishwasher with some force, so it's conceivable that all the water could shoot right past the trap and down the drain - leaving the trap empty.

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AllanJ

· #3 · Dec 28, 2012

A properly sized P trap and riser is a perfectly good way to drain a dishwasher. What is important is that the P trap is large enough (2 inches is usually enough) to accept the outflow without overflowing.

The dishwasher drain hose must fit loosely into a P trap riser, although it may be clamped tightly onto a disposer drain. In the latter case, the disposer opening up into the sink acts as the air gap to prevent overloading the dishwasher drain pump if the drain pipe should become clogged.

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Chokingdogs

Discussion Starter · #4 · Dec 28, 2012

Thanks for the replies....the DW in question is an ancient ( 1969 - 1971 era ) Hobart KitchenAid model, so odds are no built-in air gap loop for drain hose on the side of the case. The waste pipe it drains into is, I believe, 2" copper ( entire house is plumbed with copper, supply and waste lines, no PVC anywhere ). It might be 1.5", but I doubt it, and there didn't appear to be any kind of riser in the P-trap, just some fitting in which the drain hose was hose clamped to.

Aside from the DW being old enough to remember the Watergate break in, it still works like a champ.....noisy as all get out though....LOL

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techpappy

· #5 · Dec 28, 2012

IMHO ...if it ain't broke ...don't fix it!!

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