In a gas furnace, the heat exchanger is the component that is responsible for heating up the air inside the unit. Age and constant use of a gas furnace can lead to the heat exchanger cracking, which can lead to serious problems. If the heat exchanger has a crack, dangerous chemicals, such as carbon monoxide, that are created while the air is being heated can seep out. Thus, a cracked gas furnace heat exchanger is an issue that should be taken very seriously from a safety standpoint.
If you're shocked at the sight of your water bill when it comes in the mail, that's a good sign you have a water leak somewhere. It may be puzzling if you don't hear water and can't see any wet spots in your house. Sometimes, plumbing pipes are buried under the ground or underneath your foundation. When a pipe is buried, it's very difficult to know you have a leak until you get a high water bill.