HVACopedia

Trane · Furnace troubleshooting

Resize remaining vent when a furnace is removed from a shared vent

UH1 / DH1

What is going on

Pulling a furnace out of a shared vent leaves the other gas appliances on a vent that was sized for more input than they put out. I have found plenty of water heaters sitting on a leftover oversized flue, spilling and rusting at the draft hood. The fix is to size the vent for what is actually left, using the code tables.

Likely causes

  • Vent system sized for the removed furnace plus other appliances
  • Weak draft or spillage on the remaining appliances
  • Condensation in an oversized vent after the furnace heat is gone

Check in this order

  1. Find the Btu/hr input of all remaining appliances on the vent.
  2. Measure the diameter, rise and lateral of the existing vent, and count the number and type of bends.
  3. Use the tables in the current National Fuel Gas Code (ANSI Z223.1 / CAN/CGA B149.1) or Exhibit J of ANSI Z21.47 / CAN/CGA-2.3 to check or resize the vent.
  4. Check draft at the remaining appliance with it running.
  5. Let a licensed gas fitter correct the vent if it is oversized.

Applies to: Furnace replaced in a common vent system

More furnace guides

Related

Trane error codes · Trane parts · Trane tips · Calculators and reference tables

Take it to the job

All guides, error codes and tips work offline in the HVACopedia app.

Open the app

Source: Trane AUH1B080A9421B installer's manual, p. 18. 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.