HVACopediaAllied · Furnace troubleshooting
Replacing a furnace that shared a common vent
What is going on
When the new Allied furnace is vented on its own and the old furnace used to share a common vent, the remaining appliance is left on a vent that is now too large. Without the heat of the old furnace, the flue gas cools and may not draft properly. Verify draft on the remaining appliance before you leave, and do not assume the old vent is fine.
Likely causes
- Vent was sized for both appliances together
- Cooled flue gases in an oversized vent giving weak draft
- Condensation or spillage at the remaining appliance's draft hood
Check in this order
- Check the size of the existing vent for the remaining appliance.
- Check draft with the remaining appliance running on the existing vent.
- Correct the vent size if the draft is not proper.
- Look for signs of spillage, rust or staining at the draft hood.
- If you are unsure of the vent sizing, have a licensed technician check it, since this is a carbon monoxide matter.
Applies to: Common vent replacement
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Source: Allied 92G1UH installation manual, p. 20 (PDF p. 20). 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.