HVACopediaAllied · Furnace troubleshooting
Protecting walls from acidic flue gas
What is going on
Flue gas from a condensing furnace can be slightly acidic and can stain or eat into some building materials at the termination. A protective shield is required when the optional tee is used. Check what the exhaust will actually hit before you finish the termination.
Likely causes
- Flue gas plume hitting siding, brick or other building material
- Optional tee used without a shield
- Gaps and cracks at the wall around the termination letting moisture in
Check in this order
- If flue gas will hit building material at the termination, install a corrosion-resistant shield at least 24 in. square.
- Build the shield from wood, plastic, sheet metal or other suitable material.
- Seal all seams, joints and cracks in the affected area with an appropriate sealant.
- Re-inspect the wall after the first season for staining or damage.
Applies to: Sidewall terminations
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Source: Allied 92G1UH installation instructions manual, p. 30. Written in our own words; check the manual for your exact model.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.