Wood Heater Maintenance – Internals


Firebrick is used in many wood heaters to protect steel or cast iron while increasing firebox temperatures for better combustion. Modern EPA certified wood heaters often use a lighter, lower density brick for higher performance. When replacing such brick it’s important to replace with the same brick type to maintain your heater’s efficiency. Cracked firebricks, which remain in position, do not have to be replaced immediately. The bricks in most heaters and furnaces are a standard size, which is half the size of a normal house brick. They measure about 4 1/2″ x 9″ x 1 1/4″ inches, and are referred to as firebrick ‘splits’. Standard splits can be purchased at some building supply stores, but the special low density bricks found in some EPA certified heaters must be purchased from a wood heater dealer.


Baffles in wood heaters reflect heat towards the fire, increase the length of the flame path and create a chamber for secondary combustion, all of which are essential for clean burning and high efficiency. They may be steel, cast iron, firebrick, ceramic fiber board or a combination of these materials. Since they are exposed to flame on both sides, baffles get very hot and may fail over time. Removal and replacement is usually detailed in the owner’s manual. Replacement parts can be ordered from heater retailers or directly from the manufacturer. Some horizontal baffles include a ceramic fiber blanket, which usually lies on top of the baffle. During maintenance and cleaning, this blanket must be pressed down flat so that it doesn’t block the area above the baffle where the exhaust flows. Ceramic fibers should be treated like asbestos; airborne particles should not be inhaled.

Special Refractory

The term refractory means a material that can tolerate high temperatures and is usually in the form of firebrick or ceramic fiber. Some heaters use custom-cast refractory components for a secondary combustion chamber. These are usually white or off-white material and may be very soft board like material or a hard masonry material. In either case they should be handled gently. Avoid breathing any dust created by handling. Repair may be possible in cases of breaking in some cases. Replace when necessary with factory-supplied components.

Air Tubes

Stainless steel air tubes are used at the top of the fire below the horizontal baffle in many modern EPA certified heaters. The intense heat in this location can cause them to sag or disintegrate in time. The tubes are removable by undoing the fastener or turning to unlock the keyed ends. Replace with factory parts and new fasteners.

Steel Parts

Interior steel parts in a heater may warp over time. In some cases this distortion is acceptable because it does not affect performance. In other cases, warping may allow exhaust to bypass the combustion system, producing a drop in efficiency. Warped parts should be replaced with components supplied by the appliance manufacturer.

Structural welded steel plates, such as sides, back and top, that warp may be unsightly, but provided that there’s no leakage, the appliance can continue to be used. These parts of welded steel heaters are not replaceable so if they crack or badly distort it means the body is shot and should be recycled.

Cracked castings

Cast iron may warp or crack through time, but it’s usually a sign of severe stress caused by overfiring, often due to leaks in joints between castings. Interior parts may be replaced with manufacturer-supplied parts. Exterior parts may be replaced by a complete teardown and rebuilding.