How to Vent a Water Heater to a Chimney

Guard against carbon monoxide poisoning with these step-by-step instructions.

When venting a gas water heater to a chimney, be aware that holes in the chimney can prevent a good draw on the water heater, causing downdrafts and presenting the risk of carbon monoxide poisoning. To avert this problem, you can vent the water heater to the largest hole in the chimney and cap the remaining holes.

Be sure to check your gas water heater annually for cracks or leaks in the flue. Most building codes require 4-inch flue pipes on water heaters.

Here's how to vent a water heater to a chimney:

Materials and Tools:

pointing trowel

drill with sheet-metal screw bit

sheet-metal shears

crimping tool

flue piping

90-degree rotating elbow joints

appropriate-sized increasers/reducers

sheet-metal caps for holes in chimney

sheet-metal screws

furnace tape

furnace cement

safety glasses

Steps:

Remove the existing flue pipe

  1. Set the water-heater thermostat to its lowest setting, typically the "vacation" setting. This will prevent the water heater from turning on and burning you with exhaust gases.
  2. Remove the anchoring screw from the base of the flue pipe.
  3. Remove the flue pipe.

Install the new flue pipe

  1. Install an increaser in the largest hole in the chimney. Fill any voids with pieces of gravel or brick, and then seal the increaser in place with furnace cement.
  2. Install smoke caps in the smaller holes and seal them with furnace cement.
  3. Install an increaser on the flue base on the water heater. Always increase the size of the flue pipe as you go to the chimney, and make sure that the flue always pitches upward to avoid trapping exhaust gases.
  4. Install the 90-degree elbows on the two increasers, and then rotate the elbows so that they're aligned with one another.
  5. Measure the distance between the two elbows, add 2 inches for overlap and transfer the measurements to the flue pipe. Cut the pipe to the appropriate length with sheet-metal shears.
  6. Fasten the flue pipe and fittings together with sheet-metal screws. Use three screws per joint.
  7. Place furnace tape around the joints to ensure tight seals.
  8. Turn the water heater thermostat back to the appropriate setting.
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