Dishwashers always have some water left in the bottom after use, which you want! Why? If the pump seal ever runs dry, the seal can overheat and fail. The last water being pumped out is usually fairly clean rinse water, so the little that runs back into the unit is typically clean and not an issue.

I like not having an air gap (not code requirement here). A hump in the hose works fine.

Don't double hump the drain line.

Thank you for the reply!

Problem is, sometimes the water looks dirty.

So, still don't double hump it? Is that because it would actually help prevent backflow, but the pump seal might run dry?

Thank you!
Francy

The remaining water in the dishwasher is water that remains after the pump has removed all that it can do. Different type pumps could remove more of the water. But a "double hump" might easily result in more residual water left in the dishwasher.

After thinking about it (my sink and dishwasher have been unhooked for a while), I remember when this problem happens:

It must be backflow from the sink, b/c it happens during cleanup. I'll be running the sink and disposal, and the dirty water will start to burble up in the dishwasher. Then it sits there until I run the dishwasher. If I don't unload the DW first thing in the morning, the dirty water will back up--even though it doesn't actually touch the clean dishes.

Soooooooo....

Add a backflow preventer (and have it back up into the sink??)?
2 high loops?
Or just keep on living with it, as I have been for the past couple years.

Thank you!!

Francy

Well, I just talked to our plumber. He's convinced the problem is a clog somewher in our drain line (we have an 80 year old house, and we had some snaking done on the upstairs shower when we moved in, but not much lower down). He said the single high loop should handle any backflow problems if the drain line is clean.

So, a-snaking we shall go....

Francy

Is the high loop all the way to the underside of the counter?
It is pretty rare for a kitchen sink to ever be filled high enough to cause a high loop to fail.
The DW already has an electric solenoid operated drain valve.
You might want to check if debris has become lodged in the valve.
Most use a plastic ball on the solenoid shaft that is held closed by a spring, and then pulled open by the solenoid to drain the unit while the pump reverses direction so the arms are no longer driven.
If the valve cannot close fully all the water after the valve in the drain line will run back into the DW, air gap of high loop.

There are a number of issues here so let us examine them one at a time.

1.If you will take the time to read your owners manual you will see that there is supposed to be some water left in the sump. The purpose of that water level is to protect the motor seals by absorbing heat when the dishwasher heating element is on during the drying cycle. If it did not have that water layer the seals would quickly dry out, crack and leak.

2.Now in regards to installing a backflow preventer on the line. The answer here is absolutely NO, the codes prohibit installing any valves on a drain line.

3. Quote "I'm having trouble visualizing the physics involved here, and determining whether the water would have sufficient force to backflow up a second loop"

When the pump is running the line is full of water and the discharge end of that line is higher than the input end at the pump. This means that the moment the pump stops the water will begin falling back towards the dishwasher by gravity flow and in so doing it creates a syphon. Not only could it syphon the water over a second high loop, it could syphon the water over a loop that is 32' high (Less 1' for each 1000ft above sea level at the installation location and the friction head of the line.)

No doubt your high loop is immediately behind the dishwasher and you have an excessively long line. This means that it will continue siphoning the water back to the dishwasher until air reaches the top of the high loop and breaks the siphon. The solution would be to run the line as flat as possible from the dishwasher to the sink location, then make the high loop as close to the sink connection as possible. In this configuration as the siphon develops it would get air to the top of the high loop quickly and break the siphon. Adding a second high loop would do nothing but increase the volume of standing water in the line and would ultimately make the problem worse.

It must also be understood that the high loop is not intended to prevent the discharge water from the dishwasher from backflowing to the machine. It is intended to prevent cross contamination by dirty discharge water from the sink flowing to the machine when the sink is drained.

4.Quote"our DW hose is very long",
Dishwasher pumps are high volume, extremely low pressure pumps and are only intended to push water through a maximum of 10' + 3' of vertical head. When it is necessary to increase the length of the line beyond 10' the line diameter should be increased by one nominal trade size to compensate for the additional friction head.

Thank you for that detailed reply lazypup!

The loop is built in on the Bosch, so it is at the back corner of the dishwasher. We could move it to the sink though...

Our issue pretty much seems to be cross contamination (which the loop should prevent). We get dirty backflow into the dishwasher when I am using the sink and disposal. The dirty water sits in the DW until I run it. I always run it before the backflow gets too deep! But it does get deep!

My husband snaked 25 feet into the drain the other night, but don't know if that made any difference. Not sure yet if we should call in the pros with the "real snake." We did have this line snaked by pros almost 2 years ago, but our plumber said that if our 80 year old line were clogged with grease, etc. it could collapse back on itself after the pros snaked it, and thus still be clogged.

Hopefully we'll get the sink and DW hooked back up this weekend (kitchen construction going on) and we can see if the snaking helped at all.

Thank you again for taking the time to reply!

:-)

Francy

Have the drain valve of the DW checked.
Something is preventing it from closing all the way.