Last month, we replaced our dishwasher, garbage disposal, and kitchen sink/faucet as part of our kitchen update. Our new sink is a 'double equal' basin like our old one. The only difference is that the new sink is one inch deeper (9" instead of 8") on each side. The sink is a Kohler Hartland, dishwasher is Kitchenaid, and disposal is Insinkerator.

From the day that the new sink/garbage disposal were installed, I have noticed that water comes up through the drain about 1" into the garbage disposal side when I drain a sinkful of dishwater from the other side. Also, when I turn on the garbage disposal, the water/debris backs up into the opposite side. If the opposite side sink is full of water and the drain basket is closed, the force of the water from the garbage disposal pushes it open and forces the dirty disposal water into it.

I had none of these issues before the new sink/disposal were installed. (But when I think about it - the dishwasher was installed by the appliance dealer a couple of weeks before the sink/garbage disposal. And I don't think I had the issues with the new dishwasher alone - it was just after the sink/disposal were hooked up.) I called the plumber who installed the sink/disposal and told him what was happening. His response was that I must have a blockage and he doesn't do that kind of work. I told him that the problems started immediately after he installed the new items, and I didn't have any other signs of a blockage. He couldn't imagine what might be causing the problem but said he would stop by one day this week to take a look.

Can anyone suggest what the problem might be so that I can 'enlighten' the plumber? I would call a different plumber, but since we already paid this one to do the work correctly, I would like to have him fix it at no charge.

Thanks for any insights.