Dishwasher drain confusion

My old house has cast iron drains. When a laundry washing machine was added, a 12' length of 1 1/2" plastic drain was inserted directly into the cast iron sewer line under the house. When a dishwasher was added, a T fitting was placed in the laundry drain about 2' before the sewer connection.
The dishwasher drains properly but makes a loud noise prior to draining. Can the drain be harming the dishwasher?

I don't think that that is either a safe or a legal way to connect the DW, and the washing machine would not meet today's codes, either. The washing machine should be a 2" pipe, the DW is not designed for a direct connection - it needs an air gap, and you didn't mention if this branch was vented, which if not present, is another code problem.

If there was a backup on the drain, you could end up with sewage in the DW, and without a proper air gap into a trapped drain, you have a direct link to the sewer for gas, insects, vermon to get into the DW...not nice or sanitary.

jadnashua said:

I don't think that that is either a safe or a legal way to connect the DW, and the washing machine would not meet today's codes, either. The washing machine should be a 2" pipe, the DW is not designed for a direct connection - it needs an air gap, and you didn't mention if this branch was vented, which if not present, is another code problem.

If there was a backup on the drain, you could end up with sewage in the DW, and without a proper air gap into a trapped drain, you have a direct link to the sewer for gas, insects, vermon to get into the DW...not nice or sanitary.

Click to expand...

An air gap is not required every where if you connect to the sink drain, why? The loop is not high enough to stop sewer from entering the dish washer.

To the orignial poster, are you sure there is even a trap on the drain at the dish washer?

The drain hose behind the dishwasher is looped and attached to the underside of the counter top, then decends through the floor and attaches to the laundry drain.

2020 said:

The drain hose behind the dishwasher is looped and attached to the underside of the counter top, then decends through the floor and attaches to the laundry drain.

Click to expand...

There is so much wrong with this, but the loop does not make a trap. It should have a trap and stand pipe.

The air gap is a must in a case like this, but you need to plumb proper drains and vents with proper traps and pipe sizing. You are in a dangerous situation for people living in the house.

If there is a trap on the clothes washer the DW will suck it dry trying to vent itself and you seem to not even have a trap on the Dishwasher.

2020 said:

My old house has cast iron drains. When a laundry washing machine was added, a 12' length of 1 1/2" plastic drain was inserted directly into the cast iron sewer line under the house. When a dishwasher was added, a T fitting was placed in the laundry drain about 2' before the sewer connection.
The dishwasher drains properly but makes a loud noise prior to draining. Can the drain be harming the dishwasher?

Click to expand...

That sounds like more than a few serious health hazards, if we're understanding you correctly.

Can you post some pics?

The dishwasher should be draining into the drainpipe of your kitchen sink, on the first piece of pipe under the sink above the trap. I found this picture on a google search... in some places, you don't need the air gap, you can just loop the hose high.

If the kitchen sink is too far away, you can install a standpipe nearby, connect it like a washing machine.

Either way, you ALWAYS need a trap (that's the downward loop in the drain pipe, see 2nd pic). It stays full of water, which stops sewer gases .

You washing machine also needs a trap - there should be a vertical 2" pipe going into it, and then THAT assembly goes down to your sewer pipe. (see 3rd picture).


You need to learn how to do plumbing, or get a real plumber in there, ASAP before somebody gets sick. Aside from the backwash into your dishwasher risk (washing your dishes in diluted sewage, without knowing it), sewer gases can be pretty dangerous.


(edit - forgot to give credits for pics - don't want to get sued for copywright infrigement...

http://www.fascias.com/en-gb/dept_46.html
http://www.brettmartin.com/building/soilwaste/wastetraps/index.aspx
http://www.appliance411.com/faq/washer-drain.shtml)

Thanks. I will have a profesional make the repairs rather than attempt it myself.