Re: Does dishwasher need an arc in the drain?
As others have said, the arc is necessary to prevent backflow into the dishwasher (and an air gap above the counter is even better). It's required by code.
However, you've mentioned that you want to be able to fully drain the dishwasher to protect it from freezing, and it's nowhere near the sink. So you probably could get away with going straight down, but the inspector might not pass it if you did.
Some dishwashers have a drain solenoid valve (not sure about all), so I would think that the water shouldn't all drain out until it's supposed to.
If you do end up putting the loop in, when you want to winterize the dishwasher, just dump some RV antifreeze in and pump it out. The remaining antifreeze should protect the DW.
(This whole backflow thing makes me wonder why backflow preventers aren't required on the INLET of dishwashers. I mean, isn't there a possibility that a burst pipe could suck nasty washwater back into the supply lines creating a health hazard? I believe they are required on commercial dishwashers; not sure why not on residential.)
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.