There’s a romance attached to wood stoves that folks don’t feel for their gas or oil furnaces. The reasons include economics, aesthetics, efficiency and environmental concerns. Today, wood stoves offer homeowners the promise of a heating system that’s independent of local utilities, plus the lure of cozy evenings cheered by gently flickering flames.
An Exit for the Smoke
If there’s a wood stove in your life (or in your future) and you already have a masonry fireplace in your home, you may want to use your fireplace’s chimney to vent your wood stove or wood burning fireplace insert. Such a choice would seem both sensible and economical.
However, any heating system works best when all its parts are designed – at the outset – to work together. A furnace operates best when the flue size is carefully matched to the furnace capacity. It is the same with a wood stove; it is the safest and most efficient when attached to a chimney whose flue size most closely matches the size of the vent pipe which connects the stove to the chimney.
Today’s Wood Stoves & Inserts
There are two types of wood stoves that can be connected to fireplace flues: freestanding stoves and fireplace inserts. These stoves can be vented into chimneys constructed of masonry or a factory-built metal system that’s been designed, tested and listed for use with wood burning appliances; they can also connect to an existing fireplace chimney, if the height and position of the stove’s flue collar permits it.
Since 1984, national codes and standards have dictated that a connector pipe extend from the outlet of the stove or insert, up through the fireplace damper, and to the first flue tile of the masonry chimney. Arguably the best installation option is installing a stainless steel liner from the top of the stove to the top of the chimney. This method provides the most efficiency and is the easiest to sweep and inspect.
Stoves & Inserts Connected to Fireplaces: A Different Equation
When installing an insert into a fireplace the size of an insert’s firebox is smaller than that of the masonry firebox, so the existing masonry flue may now be disproportionately large. The chimney draft is created like a tornado, when a warm front meets a cold front. Venting into an oversized flue is like pouring a cup of hot water into a sink full of cold water. The cooler temperature causes the smoke to linger and even drop below the dew point and deposit large quantities of creosote on the chimney walls.
Creosote is a brown or black combustible deposit produced when smoke condenses. Creosote needs to be monitored and removed to help reduce the risk of chimney fire. Smoke condenses inside both the firebox and smoke chamber [see Figure 2] and may produce a ceramic-like hard glaze of condensed creosote – which is hazardous, difficult and potentially expensive to clean and which damages masonry materials through the corrosive action of acids it contains.
By today’s standards the installation of a hearth stove or insert where the connector pipe does not extend to at least the first flue tile is not acceptable even if the manufacturer’s instructions don’t require it. The best installation option is the installation a stainless steel liner from the top of the stove to the top of the chimney. This method provides the most efficiency and is the easiest to sweep and inspect.
This new chimney liner when sized correctly for the wood stove or insert is less likely to be a problem. In addition, it is an easy and economical way to extend the life of your chimney, since the new liner protects the existing structure from heat deterioration and acid-based smoke condensation.
Operation & Maintenance
An approved wood stove-to-fireplace installation will help assure your safety. Annual inspections and sweeping of these systems by a CSIA Certified Chimney Sweep® will enhance their safety and efficiency.