A friend asked me about a problem with a dishwasher. In a nutshell, drain water from the sink is getting into the dishwasher. Let me try to describe the hookup, then ask for opinions about how to fix it. The discharge hose from the dishwasher comes from the bottom of the dishwasher, makes a high loop to the underside of the countertop, then into a seperate trap. It doesn’t connect into the tailpiece like most dishwasher discharges would, but instead has it’s own trap, seperate from the trap for the sink. (see attached photo) There is not an air gap. Ordinarily, I would automatically blame that, but there are a couple of things I can’t figure out. The dishwasher doesn’t get used, so there’s no way water can be siphoning back, right? That can only mean that discharge water from the sink is pushing up high enough to make it past the high loop in the discharge hose. I can’t understand this since the discharge hose is within an inch of the underside of the countertop and it’s an undermount sink. I think there must be some blockage downstream, but it still seems to me that unless the sink were full to the brim that water would have trouble making it over the high loop in the discharge hose. So, any help is greatly appreciated. He can’t easily add an airgap since that would involve drilling a hole in a granite counter top. I’ve seen vented loops that might work in place of an airgap, but again, siphoning back doesn’t seem to be the problem. I thought about a spring loaded check valve, but wondered if that would put too much back pressure on the discharge pump in the dishwasher.
Any help is greatly appreciated.
Replies
I am not a plumber! However, it is time for some mainline work and possibly proper venting. Was this a diy plumbing job or contracter?
It was connected up by the counter guy after realizing that the new sink and strainer didn't leave enough room for a dishwasher style tailpiece. Which category does he fall into?
I can't see your picture, but I'm gonna guess that the dw trap is downstream of the sink, and there is no vent upstream. When the sink is full and the drain is opened, the water pressure of the sink water pushes the trap water back up thru the dw hose, creating a siphon, and as long as it takes the sink to drain, water is running into the dishwasher. The correct fix would be a vent upstream of the trap. There are code approved vents for just this situation.
http://www.taunton.com/finehomebuilding/pages/fh_174_056.asp
Does the water get into the DW only when you have say 1/2 a basin of water and you unplug the strainer?
I think there must be some blockage downstream,
That the most likely cause of the problem.
but it still seems to me that unless the sink were full to the brim that water would have trouble making it over the high loop in the discharge hose.
It's not a siphon in this case it's because of the large amount of water discharged compared to what can be drained away through the common drain.
Quickstep,
You don't have room for a dishwasher tailpiece, but you do have room for a Y with a hose barbed arm in the run between the two sinks. That would raise the dishwasher connection and move it away from the water falling out of the sink into the trap.
Ron
Given that this is a Taunton site... check out Rex Cauldwell's book on Remodel Plumbing.
He notes that the dish washer outlet should be tied into the SINK'S trap and NOT have its own trap.
As Ronbudgell points out - there's plenty of room to tie into the horizontal line from the other sink.
I actually went and made one of those crazy Rex Cauldwell traps for my sink. Probaly far more expensive than it was worth, but it was a fun project. Cleared up a ton of room under the sink for more cleaning stuff.
needs a vent, to break the sipon. 2+3=7
Do you think tying it into the horizontal line will still prevent water from backing up into the dishwasher if the line downstream is blocked? (assuming of course that the downstream line is properly vented) Also, what's the Rex Cauldwell trap? Sounds interesting....
If the dishwasher drain is connected in the horixontal run and the line downstream is blocked water will back up to the height of the raised loop under the countertop and then fall over it into the diswasher. At that point, the sinks will be nearly full, too.
What's happening now is more a dynamic process and can be stopped by getting rid of the second trap. I'll bet you could test this by draining the sinks slowly instead of just pulling the plugs normally. If they drained slowly there would not likely be enough pressure to force water around the dishwasher trap and up over the raised part of the drain line, and that would identify the problem, wouldn't it?
Ron
Brownbagg hit the nail on the head.
The line is constant full--- so any back pressure from the other drain will easily push water into the other trap.
Just a quick note--- it is against code to have two traps on a single trap arm, just to let ya know.