Replacing an old gas furnace or boiler with a newer standard-efficiency model raises important questions about chimney compatibility. Even standard-efficiency appliances (80 to 84 percent. Replacing an old gas furnace or boiler with…, AFUE) produce cooler flue gases than their predecessors, and this temperature difference can create condensation problems in masonry chimneys that previously performed without issue.
The Changing Temperature Profile
Older gas appliances from the 1970s had efficiency ratings of 60 to 70 percent. Their flue gases exited at 400F to 600F, providing strong natural draft. Modern standard-efficiency appliances operate. Modern standard-efficiency appliances operate at 80 to 84…, at 80 to 84 percent with flue gas temperatures of 250F to 350F. While not as low as condensing boilers (100F to 130F), it is low enough to cause condensation under certain conditions.
| Parameter | Old Low-Efficiency | Standard (80-84%) | Condensing |
|---|---|---|---|
| AFUE Rating | 60-70% | 80-84% | 90-98% |
| Flue Gas Temperature | 400-600F | 250-350F | 100-130F |
| Condensation in Masonry Chimney | Rare | Possible in unfavorable conditions | Guaranteed |
| Liner Required | No | Yes, in most cases per current codes | Yes, acid-resistant |
When Condensation Occurs
- Exterior chimney location: Cold outdoor temperatures cool the masonry mass.
- Oversized flue: Gases expand, slow down, and lose heat rapidly.
- Excessive chimney height: More surface area for he
For appliances with efficiency of 84 percent or less, flexible aluminum liners are a common cost-effective. Building Energy Efficiency, space. Approved for flue gas temperatures up to 480F.
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taining adequate draft.
inum liners are a common cost-effective solution. They are flexible enough to navigate flue offsets while maintaining adequate draft.
Type B Vent
Double-wall galvanized steel pipe with an insulating air space. Approved for flue gas temperatures up to 480F.
Stainless Steel Liners
Appliances approaching 84 percent efficiency may require AL29-4C stainless steel for corrosion resistance.
Liner Type Material Max Temp Best For Flexible Aluminum Corrugated aluminum alloy 400F Standard-efficiency gas, offset flues Type B Vent Galvanized steel double-wall 480F Straight chimneys, new installations Flexible Stainless (AL29-4C) High-alloy stainless steel 550F+ Any gas appliance, long service life Installation Requirements
- Full height: Liner must extend full chimney length.
- Approved cap: Type B vent cap required at top.
- Single appliance: Each liner serves only one appliance unless specifically designed for multiple.
- Manufacturer approval: Liner must be listed for the appliance type and fuel.
- Combustion air: Mechanical room must have adequate outdoor air supply.
Gas Fire Logs
Gas fire logs burn rich for visual effect, producing carbon buildup and low efficiency. The residual heat is usually sufficient to maintain draft and avoid condensation. However, if the flue is oversized or the chimney is exposed on multiple sides, a liner is recommended.
Conclusion
Standard-efficiency gas appliances produce cooler flue gases than their predecessors, creating condensation risks in masonry chimneys. Flexible aluminum liners and Type B vent are appropriate for most standard-efficiency installations. Proper sizing and full-height installation are essential.
For more on chimney systems, see our guide to chimney caps and venting.