From the Exclusive Right to Sell Listing Contract (effective July 1, 2009):

I suspect the legislation was created because of several high profile carbon monoxide related deaths over the Winter. Even without the law, installing one is a smart idea.

As it happens, I once had my own brush with CO poisoning.

Years ago, I was house sitting on Nantucket during the winter. One cold night, the exhaust pipe on the furnance shook itself loose. The HVAC guy probably hadn’t fastened it together properly. I might not have even discovered it, except something didn’t smell right about the air in the house, so I went down to the basement to check things out. I found a dark, dirty smear of exhaust on the ceiling, directly over the furnance. Fuzzy headed, I recall staring blankly at the stain and the disconnected pipes. Suddenly it clicked. We evacuated the house and immediately had it repaired. Sometimes, I wonder how close we came to not waking up at all.

You’ll find a variety of CO detectors at your local hardware store. When I bought a new one over the winter, the stores had been running low. Call ahead or order online. Amazon.com has them in prices range from $20 to $35.