We just had a new furnace installed in February. It's a 2-stage and there was no smell when the heat was on and the air was blowing through the vents. Once we turned the AC on, now there's a sulphur or rotten-egg smell. I've already poured bleach into any and all traps and followed it with water. But could this problem be from an improper installation of the furnace and blower too? If so, what could it be?

No improper install just the ((dirty sock syndrome)). Many people have it. You need to clean the A/C coil inside your furnace assembly. If you do not know how to get to the coil I recommend you call a pro because you can break off the small cap tubes ETC. If you do get to the coil use a indoor safe cleaner only.

Is the condensate drain tied into the sewer?

Hvac1000, I would have to say that the system is a little new to need the coil cleaned already, I bet it's a sewer problem, I bet it's either tied into the sewer without a trap, or the traps dry. My advice is to get the company out that installed the system to check things over, I'm sure it's still under warranty.

Hvacservicetech 07

Think about this. The furnace has been working all winter without a smell problem. It stands to reason that the blower on the furnace would have sucked in the sewer gas during the heating season when the furnace was working because the drain off the condensate pan was hooked up to the same place it is now. If the drain trap was hooked to the sewer it would have smelled worse in the winter because I can guarantee that the traps would be dry in the winter/heat season.

I kind of have a hard time rolling with the sewer idea.
BTW Dirty Sock Syndrome can happen to new equiptment. It is a major complaint on some new installs due to the fact of closer coil fin spacing on the higher efficiency units,the use of different oils in the coil manufacturing process,(( did you know you are supposed to clean the evaporator coil when installed new from the factory? )), and if you do not clean the evaporator it takes up to 30 days for the oil to wash off the coil.

A new uncleaned evaporator will allow the condensate to literally blow off the coil into the duct work and the mold/mildew starts. While this would seem hard to believe it is true.

Air conditioning uses higher blower speed for cooling than heat. The static pressure could be high enough to pull sewer gas, if the drain is not correctly run. Most codes don't allow you to tie into a plumbing vent or drain with out an air gap or trap, if at all.

I have similar problem with new HVAC unit. Dirty Sock Syndrome in new unit. Never in old one. Searching for answers. Thanks to all trying to help.