I noticed sewer smell from A/C vents about three weeks ago when the heat is not on.
When I turn heat on, it smells less(but is still there)My husband got int the crawl space wondering if there is any leak from sewerge pipes, but he could not find any ovious leak.
When we opned the smll door to the crawl space, we noticed that was the exact smell coming form A/C vents. We do not have the sewer smell outside of the house or upstairs at all it comes only from downstaris' vents.
The smell is more intense from the vents closer to furnace somewhat, but if is coming from all the vents downstairs.
The furnace is about 10 years old and never had any issue before. Since we turn the hear off at night, the smell in the house gets worst in the morning.As soon as we turn the heat on in the morning, we get less smell.(we do still smell it,though.)
I will really appreciate any suggestion. Thank you in advance. We are so desperate especially because we will be selling the house soon.

Sorry for my spelling errors.(I am not a native English speaker,but still should have checked my spelling more carefully.)I have additional information for my issue. We checked all the drains and toilets and have no bad smell on them. The smell we are getting from A/C heathing vents is the same smell as it is in the crawl space, a little bit of sewer and dirt kind of smell.We thought it must be a loose connection of a vent , but would one looose connection affect all the vents?

Your furnace is probably not causing the smell but is drawing it in and spreading it around the home. You have a sewer or sewer vent stack leak.

Thank you for your reply. Could you please explain what "sewer vent stack leak " is ?

It is the vent pipe that usually goes out through the roof of a home. In some homes there could be more than one. The vent is there to allow the drains to be able to run free since it opens the sewer drain to the atmosphere and also allows the sewer gas a place to go instead of into the home. You have a problem and a plumber might be your best bet.

Drainage, Wastewater and Vent Systems (http://www.bobvila.com/HowTo_Library/Drainage_Wastewater_and_Vent_Systems-Pipes_Drains_and_Fittings-A1611.html)

Drain-waste-vent system - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Drain-waste-vent_system)