How to Make a Side Flue

Ventilation is a crucial safety consideration with furnaces and chimneys.

If you have a wood- or coal-burning furnace or want to install one, you need somewhere for all the smoke and exhaust to go. It's not always practical or cost effective to run a chimney straight up from the furnace through the roof. There may be multiple stories in the structure, or perhaps a chimney would need to run through a critical living area. Instead, it may be more efficient to fabricate a side flue, which transports smoke and other exhaust chemicals laterally — first, outside the building, before they're released into the atmosphere.

Things You'll Need

• Ventilation pipe
• Screwdriver
• Duct mastic
• Solder
• Soldering iron
• In-line vent fan

Instructions

1. Locate the exhaust port on your furnace. Normally, this port is on the top of the furnace and clearly marked.

2. Measure the diameter of the exhaust port. You need an HVAC ventilation pipe of that diameter to attach to the exhaust port.

3. Attach the first leg of your ventilating pipe to the furnace exhaust port using a screwdriver and screws. Insulate the fitting with solder and reinforce with duct mastic or insulating tape.

4. Fasten a duct joint to the top of the first leg of of your vent pipe. Slip the duct joint over the exposed vertical vent pipe. Point the other opening of the duct joint at the wall portal through which you'll be running the flue. Seal the connection with duct mastic.

5. Install a duct fan near the joint, along the horizontal vent pipe, for installations that are longer than a few feet. Use a powered fan specially designed for HVAC purposes and to withstand heat and condensation. You can usually buy duct fans at hardware stores. To attach a duct fan, fasten the vent pipes to each opening and connect the fan to a power source. The fan helps prevent exhaust from backing up into the system, as well as propels exhaust away from the building.

6. Cut a length of vent pipe long enough to reach from the joint or, if applicable, from the vent fan to the exit portal of the building and 4 to 6 inches beyond. Run one end of the pipe to the wall portal, then fasten the other end to the pipe joint or fan. Seal and reinforce with duct mastic.

Tips & Warnings

• Furnace gasses are potentially toxic. If you're unsure of how to prevent exhaust fumes from backing up, consult a professional HVAC technician.

References

• EnergyDesignedHomes.com: Duct Sealing
• HVACPartners.com: CRFLUEHD001A01 Installation Instructions