dishwasher drain

I have a similar situation to another post- an old dishwasher installation rigged in across from the sink in a small kitchen. Old DW leaked, so now replacing with nice Bosch model. Old DW had flexible plastic drain hose coming from its connection to a 20" loop behind it, then connected to a rubber drain hose that ran down, through floor, and then horizontal in joist space in tall crawlspace, then up to disposal. No "air gap" at the sink. We often had back flow into the dishwasher if we forgot to run the disposal for awhile and did a lot of dishes in the sink before running the DW later. Got pretty gross, although if we quickly turned the dial to wash it would drain off (then we would stop it- beauty of turn dial controls)... it definitely exceeds the total drain length reccommended by Bosch: 92" or 102" depending on the model.. that includes the loop they say... One: regardless of code, will installing an air gap at the sink (it has the hole) do the trick; Two: from the other post it sounds like the manufacturer's max drain length is more advisory, but will two or three feet extra, including all the verticals and loops, mean it will not work right?; Three: if there is an air gap device at the sink do we still need the loop at the DW; and Four: could we just route the drain to a ABS waste line in the crawlspace coming from the clothes washer, right near where the DW drain line currently penetrates the floor.. and if so, then if so, will keeping the loop at the DW provide the needed air gap? The ABS is 1-1/2" and only has the clothes washer on it until it gets to the main sewer stack. No way to reroute drain through cabinets, and it would only add length. Thanks for any advice you may have.

The reason for an air gap on a dishwasher is to prevent siphoning of dirty water from the drain line and back into the tub.

Check out this thread:

https://terrylove.com/forums/archive/index.php/t-108.html

Hj's post sums up both the looped hose and the air gap pretty nicely.

Also I don't know if this applies to all new dishwashers but mine had the drain line pre-looped on the back of the unit from the factory.

Wrex said:

The reason for an air gap on a dishwasher is to prevent siphoning of dirty water from the drain line and back into the tub.

Check out this thread:

https://terrylove.com/forums/archive/index.php/t-108.html

Hj's post sums up both the looped hose and the air gap pretty nicely.

Airgaps work on the principle that when you introduce a pocket of air between the dishwasher output and the drain line siphoning cannot occur.

Also I don't know if this applies to all new dishwashers but mine had the drain line pre-looped on the back of the unit from the factory.

Click to expand...

It operates on the principle of gravity and the fact that water will not flow up through air to enter the dishwasher drain backwards.

The principle is more like a faucet over a sink.

Ah I guess I got too hung up on the word siphon I stand corrected.

I always like to learn something new.

AKRBT said:

No way to reroute drain through cabinets, and it would only add length.

Click to expand...

Wonder why these DW setups were even conceived in the first place¿
Definitely an after the fact kind of situation. I would say that it would be best to move your DW next to the sink or atleast alot closer where it is possible to run some drain hose through some cabinets. Yes, it entails some cabinet work. This goes back to the "why is everyone always a plumber" thread. Time for someone to be a cabinetmaker....

Looping the discharge hose is not the same as the air gap. The loop acts as a trap while the air gap breaks vaccuum should there be a backflow condition.

The Bosch dishwashers don't work very well with air gaps for some reason.
The air gap is code here in Washington State, but for some reason, the Bosch DW does not get with the program. It pumps a little too hard.

I've noticed the same thing with Miele's also.