Does a Gas Water Heater Need a Chimney Liner?

Why a small appliance struggles in a big flue

A chimney works because warm exhaust is lighter than outside air and rises, pulling more exhaust behind it. A typical atmospheric gas water heater makes far less heat than a boiler, and it fires in short bursts. Its exhaust starts out cool, mixes with room air at the draft hood, and enters the chimney at a modest temperature. In a wide brick flue it spreads out, cools further and slows down. Some of it condenses into water on the walls, and on a cold start some of it may not rise at all.

The flue in many older houses in New Jersey and Pennsylvania was built for a coal furnace and later served a large oil boiler. That is a lot of chimney for an appliance the size of a water heater.

When the problem starts

It often begins with a heating upgrade. The old boiler and the water heater shared the chimney, and the boiler's hot exhaust kept the flue warm enough for both. The new high-efficiency boiler vents out the side wall, and now the water heater is alone in a flue built for something much bigger. The same thing happens when a house converts to a heat pump and keeps its gas water heater.

A water heater replacement can trigger it too. A newer, more efficient water heater sends cooler exhaust up the same flue, and a chimney that coped with the old one starts to condense with the new one.

Chimneys on outside walls make it worse. A flue running up an exterior wall, common on colonials and on the rear wall of rowhouses, loses heat to the cold air all winter, so the exhaust cools faster and condenses more. Water heaters in those houses often show venting trouble first in the coldest weeks of January and February.

Signs of a water heater venting problem

You can spot most of these from the basement floor, near the water heater and at the base of the chimney.

• Rust or white crusty streaks on the vent pipe or the top of the water heater
• Water dripping from the vent pipe joints or pooling at the chimney base
• Moisture or a damp, sour smell around the chimney in the basement
• Scorch marks or melted plastic near the draft hood
• Crumbling mortar or flakes of tile in the cleanout
• A carbon monoxide alarm that sounds when the dryer or exhaust fans run

A simple spillage check

With the water heater burner running for a few minutes, hold a lit match or incense stick just below the edge of the draft hood, the open collar on top of the heater where the vent pipe starts. The smoke should be pulled up into the hood. If it drifts out into the room, exhaust is spilling. Try it again with the dryer and kitchen exhaust running, since those pull air out of the house and make spillage more likely.

Spillage is a sign to call for service, not something to live with. Make sure a working CO alarm is on each floor; if one sounds, everyone leaves and someone phones 911 or the gas company from outside.

Sizing the water heater chimney vent

The liner size comes from the water heater maker's venting instructions and the venting tables in the fuel gas code, using the heater's input, the chimney height and the connector length. It does not come from the size of the old flue. Too big and the draft stays weak and wet. Too small and the exhaust cannot get out fast enough and spills at the hood.

If the water heater shares the flue with a furnace or boiler, the liner is sized for both together, and the connectors are joined as the instructions allow. The liner material has to be listed for gas appliances. Stainless steel and some aluminum liners are made for this use, and the liner maker's instructions say which appliances each one can serve.

• Water heater alone in a large masonry flue: the flue is too big, so a liner sized to the water heater goes in
• Water heater and boiler sharing a flue: the liner is sized for their combined load
• Sound clay flue already in the right size range: it is cleaned, inspected and kept in use
• Switching to a power-vent or electric water heater: the chimney is no longer needed, so the flue is capped and sealed

Water heater types and the chimney

Not every water heater uses the chimney. An atmospheric gas or oil water heater with a draft hood does, and it is the type that most often needs a liner. A power-vent water heater uses a fan to push its exhaust, usually through the side wall in pipe the maker specifies. A direct-vent model draws air from outside and vents through the wall. Electric and heat pump water heaters burn nothing and need no vent.

If your water heater is near the end of its life and the chimney needs a liner, it is worth comparing the cost of relining with the cost of a water heater that does not use the chimney. Either one solves the venting problem.

About water heater liner kits

Liner kits for water heaters are sold with a set length, top cap and connector. The parts are only half the job. The kit has to be the right diameter, the flue usually has to be cleaned first, and the liner goes in from the roof and has to be sealed at the top and connected properly at the bottom with the thimble made tight. Roof work on a tall chimney is not a homeowner job, and gas venting work often needs a licensed installer and sometimes a permit, so check local rules.

What the visit includes

A technician measures the flue and the chimney height, checks the water heater's rating plate and instructions, cleans the flue, and looks for debris, fallen tile and water at the base. If a liner is needed, the estimate names the diameter and material and covers the top plate, cap and connection. The finished liner is inspected yearly like any flue.

If the flue turns out to be sound and correctly sized, the visit ends with a cleaning and a clear answer that no liner is needed. That is a common outcome on chimneys that were relined during an earlier boiler change.