Thread: Why isn't a check valve required immediately as the drain leaves the Dish Washer?

Because the pump for the DW has to pump to the height of the air gap, shouldn't the drain have a check valve to prevent back flow into the DW tub. I know it is not a requirement but it seems this would be cleaner for the dishwasher and eliminate any oder from food particles that may still be in the waste water. Perhaps the friction loss caused by the check is the reason causing the pump to work harder than if no check were installed. Perhaps the rinse water cycles are supposed to make this water sanitary enough and not stink.... as Johnny Bench says. The other day I took apart the hose from the Disposal to the Air gap because it was restricting flow and causing water to come out of the air gap into the sink above. Apparently the family that lived in the now vacant home used a lot of grease in their cooking, that caused a buildup of grease making the hose diameter smaller, causing the restriction. When I flushed it out at the hose bib, a tube of grease blew out onto the ground. It was about 1/8" thick. Who'd have thunk....

Originally Posted by Loren Sanders Sr.

Because the pump for the DW has to pump to the height of the air gap, shouldn't the drain have a check valve to prevent back flow into the DW tub. I know it is not a requirement but it seems this would be cleaner for the dishwasher and eliminate any oder from food particles that may still be in the waste water. Perhaps the friction loss caused by the check is the reason causing the pump to work harder than if no check were installed. Perhaps the rinse water cycles are supposed to make this water sanitary enough and not stink.... as Johnny Bench says. The other day I took apart the hose from the Disposal to the Air gap because it was restricting flow and causing water to come out of the air gap into the sink above. Apparently the family that lived in the now vacant home used a lot of grease in their cooking, that caused a buildup of grease making the hose diameter smaller, causing the restriction. When I flushed it out at the hose bib, a tube of grease blew out onto the ground. It was about 1/8" thick. Who'd have thunk....

Check valves don't work well or reliably with solids such as is in the waste water from a dishwasher.

Originally Posted by Jim Luttrall

Check valves don't work well or reliably with solids such as is in the waste water from a dishwasher.

Should have thought of that... makes sense to me. Thanks Jim

Originally Posted by Loren Sanders Sr.

Because the pump for the DW has to pump to the height of the air gap, shouldn't the drain have a check valve to prevent back flow into the DW tub.....

Not needed

Originally Posted by Loren Sanders Sr.

I know it is not a requirement but it seems this would be cleaner for the dishwasher and eliminate any oder from food particles that may still be in the waste water. ....

Basically, when the DW completes the final rinse cycle only a small amount of clean water is not discharged to waste. Because there is an airgap, water in the discharge line drains into the tub. Dirty dishes are loaded into the DW and the cleaning cycle begins. A water valve opens and fresh water is added. The fresh water valve is above the level of any water remaining in the DW. This prevents backflow into the potable water system. At the end of this cleaning cycle, the pump discharges the now dirty water. After the final rinse cycle is completed, the water is discharged again, and the DW is ready for a new load.

1st Since there is an airgap on the potable water line, there is no possibility of contamination of the potable water from backflow.
2nd Also, since any water that drains back into the tub is discharged at the end of the 1st cleaning cycle, this water poses no risk of contamination.
3rd The airgap on the discharge side prevents siphonage of waste water into the DW.
All 3 work together to insure your dishes are clean, and risk of contamination from waste water has been eliminated.

Originally Posted by Loren Sanders Sr.

Perhaps the friction loss caused by the check is the reason causing the pump to work harder than if no check were installed. ....

Yes

Originally Posted by Rick Cantrell

Not needed



Basically, when the DW completes the final rinse cycle only a small amount of clean water is not discharged to waste. Because there is an airgap, water in the discharge line drains into the tub. Dirty dishes are loaded into the DW and the cleaning cycle begins. A water valve opens and fresh water is added. The fresh water valve is above the level of any water remaining in the DW. This prevents backflow into the potable water system. At the end of this cleaning cycle, the pump discharges the now dirty water. After the final rinse cycle is completed, the water is discharged again, and the DW is ready for a new load.

1st Since there is an airgap on the potable water line, there is no possibility of contamination of the potable water from backflow.
2nd Also, since any water that drains back into the tub is discharged at the end of the 1st cleaning cycle, this water poses no risk of contamination.
3rd The airgap on the discharge side prevents siphonage of waste water into the DW.
All 3 work together to insure your dishes are clean, and risk of contamination from waste water has been eliminated.



Yes

Good explanation of the cycle and why there should be no contamination. Thanks for posting.

Is a "high loop" on the discharge line important when it discharges into the disposal?

Originally Posted by Lon Henderson

Is a "high loop" on the discharge line important when it discharges into the disposal?

Yes
Without a high loop, waste water from the sinkwould easily flow back into the DW by gravity.
The high loop eliminates backflow by gravity. However backsinphonage is still possible. For this reason, an air gap installed on the sink is recommended, and in some areas it is required.