Damaged chimney flues fitted with new liners in Parramore

Gas appliance flues follow their own rules

Section R1003.11.3 sends flue lining systems for gas appliances to the code's fuel gas chapter, and liner makers refer gas sizing to NFPA 54 and the appliance manufacturer. A liner sized by fireplace rules can be wrong for a furnace or water heater sharing the chimney.

When a gas appliance vents into your chimney, we size and specify that flue by its own rules and cite the reference in the estimate, so anyone who reads it can check the sizing. If the gas appliance is due for replacement, we suggest settling that first, since the new unit's manual may call for something different.

Three situations where insulation helps

One manufacturer lists three situations where an approved insulation wrap enhances the performance and life of the chimney. We explain which, if any, apply to your chimney before you decide, and whether insulation in your case is a requirement or a recommendation.

Insulation also changes the space the liner needs, so the decision comes before the route is proven, not after.

• An installation that must meet UL 1777 at zero clearance to combustibles
• A chimney located on the outside of the house
• Appliances expected to produce low flue gas temperatures

Fireblocking around the chimney

Spaces between a chimney and the floors and ceilings it passes through must be fireblocked, under the residential code, with noncombustible material, securely fastened in place.

When the attic or a ceiling opening lets us see that area, we note what is there. Missing or combustible fireblocking becomes a separate finding with its own recommendation, not part of the liner price. It matters to relining because the same area is where clearance to framing is usually checked, and the two findings are easier to understand side by side.

Planning around every connected appliance

Liner makers note that the arrangement depends on the size and configuration of the chimney and on the type, number and location of the appliances vented into it.

We list every appliance connected to the chimney and every flue it contains before designing the relining. Nothing gets sized in isolation. A basement or garage appliance that shares the chimney is easy to forget, so we ask about it and look for its connection.