High-efficiency condensing furnace relining for Pelham Road homes — Category IV appliances require AL29-4C super-ferritic stainless, not standard 316L. Appliance efficiency rating determines the liner specification before anything is ordered. Written scope before work begins.
The NFPA 211 vent category system classifies gas appliances by flue gas temperature and whether the flue operates under positive or negative pressure. The category — not the fuel type alone — determines what liner alloy is permitted. Pelham Road homes with newer high-efficiency furnaces fall into Category IV, which has the most demanding liner requirements of the four categories.
| Category | Flue Gas Temp | Pressure | Condensing? | Typical Appliance | Liner Required |
|---|---|---|---|---|---|
| Category I | Above dewpoint — no condensate | Negative (draft) | No | Standard gas furnace, boiler (80% AFUE range) | 316L stainless or Type B vent |
| Category II | Below dewpoint — condensate forms | Negative (draft) | Yes | Rare in residential — some older boilers | 316L or AL29-4C depending on spec |
| Category III | Above dewpoint — no condensate | Positive (fan-assisted) | No | Some fan-assisted furnaces | AL29-4C or manufacturer-specific |
| Category IV | Below dewpoint — heavy condensate | Positive (fan-forced) | Yes | High-efficiency condensing furnace 90%+ AFUE | AL29-4C required — 316L not rated |
Both 316L and AL29-4C are stainless steel alloys used in gas appliance venting — but they are not interchangeable for condensing furnace applications.
316L is appropriate for Category I gas appliances where flue gas stays above the dewpoint and condensate does not form in significant volume. The flue operates under negative draft pressure.
AL29-4C is a super-ferritic stainless containing approximately 29% chromium and 4% molybdenum — a significantly higher molybdenum content than 316L. This composition is specifically designed to resist the highly acidic, high-chloride condensate of Category IV appliances operating under positive pressure.
Furnace AFUE rating and vent category confirmed from the appliance data plate or documentation before any liner is specified. A 90%+ AFUE rating confirms Category IV service — but the furnace documentation is checked rather than assumed from the AFUE alone.
Camera inspection documents existing flue condition and confirms there are no obstructions. Liner diameter calculated from furnace Btu input and vent connector size per Category IV sizing requirements — which differ from Category I sizing tables.
AL29-4C super-ferritic stainless liner in the confirmed diameter installed from furnace vent connector to chimney top. Top plate and liner cap installed at the chimney crown. Condensate drain fitting installed at the liner base where required by appliance installation instructions.