How to Vent Two Appliances Into One Flue

Review local building and fire codes to determine any local limitations on venting two gas appliances into the same flue. The size of the liner and common vent they will share can differ from one area to another. If the two gas appliances are using different fuels such as wood and natural gas, check that the codes allow for the mix. Just as an example, in New Hampshire, solid fuels, commonly referred to as wood stoves, cannot be vented with any other appliance unless it's connected to specific codes.

Inspect the chimney for any signs of damage or obstruction. The liner or common vent can become damaged if pieces of an older chimney are collapsing on the common vent.

Clean out the chimney and make any necessary repairs. Replace the chimney liner with a liner large enough to meet the venting requirements of the two gas appliances if the current liner is not adequate.

Solicit a licensed contractor to install the correct size common vent to be used by both appliances if the current one is too small. Some jurisdictions require proof of the calculation used to determine appropriate size.

Turn off the gas using the manual shut-off located upstream of the gas appliances. Shut off power to the units with the appropriate switch. To ensure that power is cut off, it is advisable to cut off the power at the main breaker. Use a multimeter to verify the power is off at both appliances.

Disconnect the vent that attaches to the chimney liner.

Attach a wye (pronounced "y") connector to the chimney liner using three metal screws. Some jurisdictions require that the appliance with the lowest Btu input should attach to the wye in a position higher than the appliance with the greatest Btu input or have a separate higher access connection point. Attach the vent from the appliances to the wye connector using metal screws.