It may sound far fetched, but fire investigators say if a fireplace is not installed correctly, the floorboards underneath it can spontaneous combust. And it's happening in old and new homes alike.

Just last week flames tore through a Provo home. Unlike most fires, this one started in the fireplace--the only safe place to build a fire, right? Guess again.

Firefighters say like too many fireplaces they see, this one was installed too closely to the wood joints.

"Over the last 14 years, the wood has decomposed to a point where it just ignited when they had a fire in the fireplace the night before."

Fire investigators say it's because of a dangerous process called pyrolosis, where heat is released too closely to the wood structure around the fireplace. Floorboards or joints then turn to charcoal and combust.

Firefighters say pyrolosis nearly destroyed a Salt Lake home last year when the occupants lit a fire in the fireplace and the floorboards ignited.

Ron Eggbert/Salt Lake Co. Fire Investigator: "It's like having a time bomb next to your furnace or in the wall of your home and you don't even know it."

Fire investigator Ron Eggbert says when it comes to fireplace combustion, there are two things you need to watch: Remodeled living rooms where the contractor or home owner hasn't followed code. And lightweight fireplaces in many new homes, mostly for decoration, can't stand up to constant use.

"The clock is ticking and every time you build a fire you advance the clock a little more and a little more."

Experts admit once the walls have been enclosed around a fireplace, it's almost impossible to see what kind of danger you may be in.

But there are some things you can look for. Look underneath your fireplace for signs of exposed wood. Check the underside of your mantle. The biggest danger sign is the smell of wood burning inside your home.