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Chimney · Safety

Gas appliance venting into an old flue

A flue sized for a wood fire is frequently oversized for a modern gas appliance, which prevents proper draft. That is a venting assessment, not a sweeping job.

Quick answerOld masonry chimneys were often sized for wood or coal fires. A modern gas furnace, boiler or water heater makes much less, and cooler, exhaust. In a flue that’s too big, the exhaust can cool before it leaves, so it may not draft properly and its moisture condenses inside the chimney. That’s a venting assessment by a qualified professional, not a sweeping job. The usual fix is a properly sized liner.

This often comes up when an old furnace or water heater is replaced with a more efficient one, and the new appliance is connected to the same chimney without anyone checking whether the flue still suits it.

If a CO alarm sounds

Get everyone outside immediately and call 911 or the fire department from outside. Don’t go back in to investigate. See carbon monoxide alarm sounding.

1Why an old flue can be wrong for gas

  • Too big. Exhaust in an oversized flue rises slowly and cools, which weakens the draft.
  • Condensation. Gas exhaust carries water vapor. In a cold, oversized flue it condenses, and the acidic moisture eats away at mortar and tile.
  • No liner, or the wrong one. An unlined or deteriorated flue can leak exhaust into the house.
  • Shared flues. Connecting several appliances to one flue needs careful sizing and is not a guess.

2Signs to watch for

  1. Recent appliance changeA new furnace or water heater on an old chimney is the most common trigger. Ask whether the venting was assessed.
  2. Moisture and stainingWhite staining on the outside of the chimney, crumbling mortar, or water at the base of the flue can mean condensation.
  3. Debris at the cleanoutBits of tile or mortar at the bottom of the chimney suggest the flue is breaking down.
  4. Alarms and symptomsA CO alarm, or headaches and nausea at home that ease when you leave, are an emergency. Get out and call 911.

3Fixing it

What you can do

  • Install and test carbon monoxide alarms
  • Ask whether the venting was checked when the appliance was installed
  • Book an inspection and ask for written findings

Hire a qualified pro

  • A venting assessment of the appliance and flue together
  • Installing a liner sized for the gas appliance
  • Correcting connectors and shared-flue setups
  • Any permit or inspection your area requires
Not a DIY job

Never modify gas appliance venting, connectors or flue liners yourself. Errors can let carbon monoxide into the house. Check local code; this work often needs a permit and a licensed installer.

4What drives the cost

The assessment itself is the small part. If the flue is unsuitable, the cost depends on the chimney’s height, the liner type and size, how many appliances share the flue, and roof access. Ask for the liner size and why it matches your appliance, in writing.

Chimney cost

New gas appliance on an old chimney?

Talk to a qualified chimney pro about a venting assessment before the next heating season.

877-360-4657

5Questions

Will sweeping the chimney fix it?

No. Sweeping removes deposits; it doesn’t change the size or condition of the flue. This needs a venting assessment.

Didn’t the installer check this?

They should have. Ask them, and ask to see it in writing. If not, have it assessed.

How often should it be inspected?

NFPA 211 recommends an annual chimney inspection, including chimneys that serve gas appliances.

Written by Jaylan Jones. I Like To Exterior Services earns a referral fee when a call connects to a contractor. See our business-model disclosure.

877-360-4657