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In article , (Greg) writes:

| I'm in the process of remodeling my kitchen. I just got the new
| dishwasher and the plumbing and electrical are done. I was going to
| hook the dishwasher up but the sink and sink drain are not in yet.
| This area used to be a bathroom and under the floor right where the
| dishwasher will sit is a 1.5 inch drain for a tub that used to be
| there. The tub drain was taken below the floor and capped. I was
| thinking I could run the dishwasher drain to that old tub drain but
| the instructions say that the drain for the dishwasher must be at
| least 20 inches above the floor. Does anybody know why this is?

In the good old days, most dishwashers included a positive-closing solenoid
drain valve that would open only during the drain cycle. The original
dishwasher in my ~1959 house worked this way and was plumbed directly into
a drain trap below the floor. Unfortunately, the drain valve was eventually
cost-reduced away in the eternal quest to maximize profit margin. With no
valve, the water can drain out at any time if the end of the drain hose is
too low.

| Will I
| flood my kitchen if I use the drain under the floor?

Probably not.

|Will the
| dishwasher not drain properly?

It will drain just fine--even when you don't want it to.

|What if I ran the drain hose up 20
| inches and then down in to the floor, would that work?

Perhaps. Some manufacturers even offer that as a legitimate installation
option. Without a vacuum breaker at the top of the loop this arrangement
can still allow siphoning. But bewa unless the manufacturer specifically
mentions the high-loop option you might have problems. (I've had problems
even when they did.) Some models are so cost-reduced that they don't even
have a single "real" valve to select between drain and circulate. Instead
they have a little flapper that the controller tries to manipulate by pulsing
the pump just so. This operation appears to depend critically on the drain's
back pressure. If you don't have what is currently thought of as a standard
installation (open drain) it can fail...

Dan Lanciani
ddl@danlan.*com