I am swapping out our old Dishwasher (which was in the house when we
bought it) for a new one. I found that the existing drain tube goes
directly down to the basement and joins with the plumbing there,
however, the installation instructions say it is important that the
drain tube be terminated 20" above the floor that the dishwater sits
on (as in under the kitchen sink), but they dont explain why. Is this
something I need to worry about?, and is there a way to install the
drain properly and still use the existing drain that goes down to the
basement? i.e. can I run the drain tube 20" up before going down to
the basement or is there something else to this I dont understand?-
The mfr may spec that so that water can't self-siphon from the M/C.
Yes, loop it up the req'd distance and send it down.
You may wish to confirm that there is a trap down below
to prevent sewer gas coming back up.
I'll let some professional plumber give you the technical and practical
answer, but my instinct tells me that the 20" rise is to prevent a
siphoning of all the water in the washer. I was once told (by the
serviceman) that a certain amount of water has to remain in the washer
pump housing, so that seals don't dry out.
The connection to the sink drain at 20" above the floor provides a means
of venting. Once the water has finished pumping into the sink drain, air
from the sink drain enters the DW drain hose and prevents siphoning out
the DW drain hose and pump. If you put a loop in the DW hose, the long
drop to the basement might siphon the water from the looped DW hose and
the pump housing, or maybe it might not, or it could be hit or miss each
time the DW drains.
Now, I'll let the professionals weigh in.
From the Bosch Appliance web page:
"When the dishwasher has completed its cycle, there will be a small
amount of water left in the sump area under the filters. This water is
left behind to keep the gasket wet and prevent it from drying out and
Although this is on a Bosch site, it may apply to other brands. My
dishwasher was a Kenmore when the serviceman told me that the water left
behind was necessary.
Many manufacturers and some plumbing codes require an "air gap" fitting.
For sanitary reasons, the dishwasher should drain through one of these
fittings which provides a 1" air separation between the end of the drain
hose and the rim of the drain it's going in to to prevent back syphonage
into the DW if the house drain backs up.
Dana the Inspector (and plumbing contractor.)
I did the same chore a few months ago (replaced the DW). I think the reason
for routing the drain hose up, then down to a drain connection is to provide
a "trap" function, which keeps some water in the hose. This prevents some
sewer gases from entering the DW and eventually the kitchen.
Applying this logic to your situation, it would seem that even if you loop
the drain hose up to the 20" level, then down to the basement you may have a
problem. The reason is this. Imagine the DW is pumping drain water out.
The hose is full of water all the way to the basement drain. Now the DW tub
empties, then the flex hose empties and the only water left is in the
vertical pipe leading to the basement. As that water falls down the pipe it
creates a vacuum above it that draws all of the remain water out of the
hose. Any sewer odors / gases may then gently waft up through the hose, to
your nose (excuse the prose). This problem is avoided in most home plumbing
systems by use of a vent pipe that prevents the vacuum from forming.
Have I explained this clearly or should I give it another shot?
A lot of good advice has already been posted on this,
but let me add just a bit more.
Older DW's used a drain valve that stayed closed during
wash, then opened electrically to the drain the wash water
out. That meant you could just run the drain hose straight
down through the floor with no problems. Newer machines
no longer use this valve, but now depend on a drain 'high
loop' to keep the water in the tank during wash - and to
prevent greywater from siphoning over into the DW.
As Dana said, be sure to check on your local codes (ask
your local plumber or plumbing & heating supplier, they'll
know), bc in some areas an air gap is required.
Here in my area, air gaps aren't req'd by code, but I will say
that a 20" loop will NOT be enough to prevent greywater
migration from your sink. You'll want to tie the drain hose
up as high as possible. I always cable tie a loop of it to one
of the sink mounting straps, to get it right up in under the
Another reason for a loop in new high-end models is for
soil-sensing. Some of the new machines, Whirlpool TT's, for
example, are actually sensing the water's *weight* during
wash, a heavier solution telling the computer the amt of soil
in the water. The loop on these is usually attached securely
to the DW's side so it's difficult for installers to remove, and
you'll see labels warning against messing with it. Removing it
to get more length - a pretty common mistake - will cause
these machines to misbehave.
That's probably more information than you ever wanted to
know <grin>, but I hope some of it's of help.
Dave's Repair Service
New Albany, PA
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the best way to perform this installation would be to use a vacuum
breaker fitting at the 20" level as per directions. Then you can go
down through the floor. To meet code this will probably require the
use of an indirect, air break, trap. I've installed commercial
dishwashers this way with no questions.
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