HVACopediaAllied · Furnace troubleshooting
Return air connection and sealing
What is going on
Improper return connections can allow combustion products into the living space.
Likely causes
- Return ducts connected to the back of the furnace
- Gaps or leaks in the base or platform
- Return air shared with a fireplace or stove
Check in this order
- Do not connect return air ducts to the back of the furnace.
- Seal the return air system to the furnace with sheet metal screws and joint tape.
- Seal the furnace airtight to the return plenum in platform installs.
- Do not use a door as part of the return air duct system.
- Make sure the base gives stable support with no sagging, cracks or gaps.
- Do not connect return or supply ducts to other heating devices such as a fireplace or stove.
- For side return, cut only within the cabinet markings and keep the opening within the stated dimensions.
Applies to: Upflow and horizontal installations with return air
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Source: Allied 92G1UH installation instructions manual, p. 10, 14. 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.