I have a 7 year old house located in Texas. The upstairs air handling unit has a drain for condensate that empties into my wife's sink between the sink drain and the P-Trap. Over time some nasty stuff has grown in the line and the smell comes up through the drain. I'm at the point where I can no longer ignore my wife's complaining and would like to know what my options are. A local HVAC rep told me I could not move the drain line to the other side of the P-Trap (Trapped Side) but did not tell me why. I would like to know why and would also like to know what my best option is to clean out the line. I'm thinking of isolating the drain and using draino, then flushing. I tired dumping a chlorox and water mixture down the pipe and at first I thought it worked but my wife's more acute sense of smell tells me it did not. Thanks for the help.

For starters, is the condensate line at the upstairs unit trapped? It should be. If your ductwork is good and tight you can have a vacumn in the drain line. If its trapped there is no reason you can't connect that condensate line after the p-trap downstairs.

The condensate line is not trapped and that may be the answer to the problem. The line comes straight out of the wall and "T's" into the sink drain pipe. At the air handler the condnesate line is attached to a air handling unit with a 6 inch or so open stub-up inside the attic.