We have a bathroom across the hall from my infant son's room. When sitting in there by the register, I noticed a faint sewage smell when the forced air turned on. Then, when my 2 year old had an accident in the tub and it was cleaned out with bleach, I could smell the bleach when the forced air turned on. I concluded there must be a leak in the PVC drain pipe that is somehow getting into the HVAC. Have not seen any water stains. Cannot smell it in any other room. I am not sure where either the drain pipe or the HVAC runs and don't want to start indiscriminately putting holes in the walls and floors. Suggestions?

The smell is from gas that is able to come back up the drain instead of being blocked by 'traps' full of waste water (like the ones under all your sinks). This can happen when a trap full of water is sucked down too hard, not leaving enough in the bend to stop gas. It may not have been built with a trap or vented properly, or there could be a break in a waste or vent pipe right near a hot air duct. It can be in a drain not used very often. If you can't examine the basement or under floors and in walls etc, buy a device such as SureSeal to put in the drain you think is the culprit. You need to know the diameter of the drain. If that doesn't work, call a plumber.