why is there exhaust when the heat isn't on

[dannbrow]

Noticed this morning that there is exhaust coming out of the vent, but the heat is not on in the house. It is in the 40's outside, thought maybe it was just the sun on the cold early morning roof but it is definitely coming out of the vent that usually only has smoke/vapor/exhaust (whatever the proper term is) coming out when the heat is on. Should I be *

[juster]

If you have a gas hot water tank, it may be that the tank is on and heating your water. Uses the same chimney.