8 critical home repairs you can't ignore

Cracked heat exchanger

You know you've got a problem when: (In order of ascending seriousness) You have your furnace cleaned and inspected. Your carbon monoxide detector goes off. You get sick.

What causes it: The heat exchanger in your furnace is what separates the warm air that circulates through the house from the exhaust gases that are supposed to go up the flue. Cracks caused by metal fatigue can allow poisonous gases such as carbon monoxide to seep into the air you breathe. 

The worst that can happen: Carbon monoxide poisoning.

Fix it yourself? Not unless you're a trained HVAC technician.

Who should you call? A heating and cooling company to help you decide whether it's cheaper to install a new heat exchanger or replace the entire furnace. (Heat exchangers should never be patched.)