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Drain for dishwasher that's not next to sink

We're putting in a new dishwasher that will not be located directly next to the sink. It's on the other side of the room under what will be a new countertop. The plan is to drain the dishwasher through the floor into the crawlspace and tie it directly into the 2" horizontal drain line that serves the kitchen sink.

The current thought is to drop a 1.5" line vertically through the floor into the crawlspace, install a p-trap, then run it horizonatlly over to the 2" line. My questions are as follows and any clarification would be appreciated.

1) My instructions say I need at least 18" loop or an air gap in order for the dishwasher to operate properly. Is this only if I'm draining "upwards" to the sink, or do I still need that when draining straight down to the drain in the crawlspace? Since the DW drain will be tied to the kitchen sink drain far downstream of the sink p-trap, do I really need to be concerned about a clog backing up water into the DW? If not, then why do I need an air gap?

As I understand it, the air gap prevents reverse siphoning so that a clog in the drain will not back up dirty water into the DW. But since I probably won't have to worry about a clog on a 2" line from the DW waste (the DW drain hose is only 5/8" dia, so if stuff can make it through there...)?

2) And now for the drain vent. To simplify the drain vent runs, my plan was to put an air admittance valve (Studor valve) in the crawlspace tied in with the horizontal portion of the DW drain line, but others have said I need to vent the drain to the roof. Is there an issue with putting an AAV in a crawlspace? Any suggestions?


Re: Drain for dishwasher that's not next to sink

lets back up.... Do you really want to place the dishwasher across the room? I'm not sure you'll be very happy with that. Typically you rinse dishes off before you place them in the dishwasher. Having to bring them across the kitchen doesn't make a lot of sense.

Re: Drain for dishwasher that's not next to sink

I am not a plumber, but I would look into putting a full S trap at the back of the dishwasher cabinet hole, keeping the top of the drain pipe 24-30" high. Then secure the dishwasher drain hose 4-6" into the pipe but do not seal it tight. Forget any venting apparatus as it would be open vented. Tying into the drain line under the floor as you mentioned sounds reasonable. I personally do not care to have a p-trap in the crawlspace.

Re: Drain for dishwasher that's not next to sink

My house did not originally have a dishwasher, so it is installed well away from the sink at the end of a row of cabinets. I have water and drains coming through the floor from my unfinished basement. I don't have an air gap at all, nor do I have a separate vent for the dishwasher connection. The dishwasher drain hose is set in the same drain pipe as my clothes washing machine drain house. About 4 feet from these (downstream) there is a P-trap and then directly into a 4" main to my septic tank. No vent. No air gap, never had a problem at all.

If there was a backup, for me, it would be no worse than with the clothes washer. However, you should consult a plumber I think because a backup in your crawlspace would be more difficult to manage.

Good luck

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