I have a Lennox Brentwood (Bis Ultima) fireplace that I use for supplement heating. In the owners manual it specifies that "IHP wood stoves are designed for use as supplement heating and not to be used a continuous heat source". Is this for all fireplaces? I don't plan on using it as my main source of heat, but what is the reasoning? Also, what fails on these types of fireplaces? Is there a max life to these?