You mean, besides the fact that NFPA211 clearly states “Masonry chimneys shall be lined” (22.214.171.124)? That’s the bottom line. Any professional who tells you a liner is unnecessary is inviting a serious liability issue. But let’s put the legalities and technicalities aside for a moment and focus on what’s important to the homeowner. A liner is, ultimately, a physical barrier between a catastrophic event and your house. It’s there to stop something very bad happening to your property or yourself. What do we mean by a “catastrophic event”?
In the case of a fireplace or other wood burning appliance, we’re generally talking about actual fire. Flames, sparks, or embers spreading to a combustible part of the house. There are also issues of heat transference igniting nearby combustibles without actually being touched by flame. In the case of oil and gas heating units, we’re thinking more about carbon monoxide problems. CO entering your house along with other toxic by-products of combustion.
A chimney liner, properly installed, is a critical component to the safety of your home. Don’t ignore a professional when he says you need a liner. Chances are he’s seen enough avoidable catastrophes to know what he’s talking about.