I have heard that a fireplace adds 12% to the value of house when selling. Can anyone confirm this, because it sounds like a lot?

A fireplace certainly adds value, but I doubt if anyone can put such an exact figure on it. Some of that depends on the area and whether the buyer wants a fireplace.

Gas will also earn you more than wood-burning, unless it's an antique-style wood-burning which fits with the architecture of the home. 12% seems high though, no matter what kind of fireplace it is.