Installing Wood Stoves and Inserts Into Masonry Fireplaces

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, it's likely that you have used (or wish to use) your fireplace's chimney as the chimney for 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. Just as a furnace operates best when the flue size if the chimney is carefully matched to furnace capacity, so a wood stove is safest and most efficient when attached to a chimney whose flue size most closely matches the flue collar outlet of the stove.

While wood stoves can be successfully connected to fireplace flues (a flue is the inner section of a chimney and is designed to carry away smoke and other toxic products of combustion), certain standards must be met. Here's a look at what they are and why they are important…

Today's Wood Stoves & Inserts

There are two types of wood stoves that can be connected to fireplace flues: freestanding stoves and fireplace inserts.

Freestanding stoves can be connected to chimneys built especially for them. The chimneys may be of masonry construction or be a factory-built metal system that's been designed, tested and listed for use with wood burning appliances. Freestanding stoves also can connect to an existing fireplace chimney, if the height and position of the stove's flue collar permits it. When this type of installation is done, the stoves may be called hearth stoves.

Fireplace inserts are a special type of wood stove and are specifically designed to fit into the firebox (where logs normally go) of an existing fireplace and to use the fireplace flue to vent smoke and other by-products of combustion. Since the insert must be smaller than the fireplace opening, there is usually a surround panel attached to the stove which extends out around the fireplace opening to seal the firebox from room air.

Since 1984, national codes and standards – as well as many manufacturer's installation instructions – have dictated that a connector pipe extend from the flue collar outlet of the stove or insert…up through the fireplace damper…and into the first flue tile of the masonry chimney. (See Figure 1.)

How an Open Fireplace Works

Fireplaces aren't designed to vent (or carry away combustion by-products from) wood stoves or wood burning inserts. They are a uniquely designed solid fuel burning system in their own right. The fireplace system consists of the firebox, a damper (the mechanism that regulates air flowing up the chimney), a smoke chamber (the area between the damper and the flue) and a flue (a passage inside the chimney through which the smoke rises). Together, they draw off the smoke and gases produced by burning wood. How well they work depends in large part on whether the sizes of the firebox, smoke chamber and flue are in correct proportion to each other.

FIGURE 1.Wood Stove Insert installed into Fireplace according to 1984 NFPA 211 Standards

The applicable rule of thumb (which is included in many local codes) is that the area of the firebox opening should only be about 10 times larger than the area of the flue'' interior dimensions -–a 10:1 ratio. Fireplaces built according to this formula seldom smoke. Since fireplaces are largely decorative, rather than a serious heat source, that's an important feature. Figure 2 shows a typical smoke pattern as combustion gases exit an open fireplace, and how an insert disturbs that pattern.

FIGURE 2.  Correct smoke pattern in an open fireplace, right compared to disturbed pattern, left, in a fireplace with insert installed incompletely


Stoves & Inserts Connected to Fireplaces: A Different Equation

Whenever a wood stove or insert is vented through a masonry fireplace system – even if the code requirements laid down in 1984 are followed – the ratio on which efficient operation of the masonry system initially was based may be changed. For example, the size of an insert's firebox is smaller than that of the masonry firebox, so the existing masonry flue may now be proportionately too large. An over-sized flue causes a reduction in the speed at which air moves out of the chimney. This lets the smoke that exits the wood stove linger inside the chimney, cool down and deposit condensed creosote on the chimney interior.

Creosote is a brown or black flammable deposit – given off when smoke condenses – which must be monitored and swept out to keep your system safe.

Major creosote problems are created when wood stoves or inserts do not meet the 1984 "first flue tile" standard, and vent smoke directly into the fireplace. Such smoke condenses inside both the firebox and smoke chamber (See Figure 2) and may produce a ceramic-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.

Never permit continued use of this type of installation, even if your stove is old enough that manufacturer's instructions do not require the connector pipe to extend into the first flue tile. Insist on a safe installation for yourself and your family.


Exceeding Minimum Requirements

The "first flue tile" installation method for wood stoves and inserts, which are vented through masonry fireplaces, solved many problems. For those committed to the use of a wood stove or insert for heat, a more efficient, easier to maintain and ultimately safer method is available, which your chimney specialist may recommend for you. That method calls for the vent pathway to extend from the stove collar all the way to the top of the flue – creating a new chimney liner within the chimney. (See Figure 3 and 4.)

This new chimney liner is sized correctly for the wood stove or insert, so drafting the smoke out of the system 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.

FIGURE 3.  Insert installed into masonry fireplace with properly sized flue extended to top of chimney.


Types of Lining Systems

There are three types of tested and listed lining systems available: stainless steel; poured-in-place or pumped; and, ceramic. The one you select may be determined by the kind of wood stove or insert you have and the condition of your chimney's structure. Your chimney specialist will help you decide which system will serve you best.

Stainless steel chimney liners – tested and listed to Underwriters' Laboratories (U.L.) standards – run from your stove to the top of your chimney and should include an insulating system to assure stable temperature within the flue and help prevent heat transfer to combustible parts of the house.

Pumped liners create a newly-sized flue by pumping a special slurry mixture around a form, allowing it to harden and then removing the form. Poured-in-place systems use a vibrating "bell" to compress the mixture into place. The mixtures are tested and listed to U.L. standards as safe chimney liners. They result in a smooth, hard-surfaced flue that runs from the top of the chimney to the smoke chamber and sometimes continues to the damper area. A length of connector pipe makes the final connection from the new flue to the stove.

A ceramic liner is installed inside the chimney with a special setting tool, and a connector is extended to the wood stove. An insulator is poured around the liner. The liner and insulation together are tested and listed to U.L. standards.


Warning! Inserts & Factory-Built Fireplaces

Do not install a wood stove – whether freestanding or insert type – into a factory-built fireplace and chimney system, unless the insert you are using has been tested and listed for that system…and in the installation of that particular stove is approved by the manufacturer of the chimney system (not just by the stove manufacturer).

A house fire may result from installing an insert into a fireplace system that is not designed for either the weight or the intense heat the stove produces. You will also void the warranty issued by the manufacturer of the factory-built fireplace and chimney system.


When You Properly Size Masonry Fireplace Flues for Stoves & Inserts

If you want to use the fireplace in the future, a removable stainless steel liner is the appropriate choice. If the proper sizing of the flue is achieved in a more permanent manner in order to accommodate a wood stove or insert, future use of the fireplace may depend on some modification.

Operation & Maintenance

An approved wood stove-to-fireplace installation will help assure your safety. Annual inspections and cleaning of these systems by a CSIA Certified Chimney Sweep will enhance their safety and efficiency.

Information ęCopyright 1994 Chimney Safety Institute of America Inc.