Switched to a Heat Pump? What to Do With the Chimney

What changes when the boiler goes

For decades, the chimney in many older houses carried a large, hot stream of exhaust from an oil or gas boiler. That heat did more than vent combustion gases. It warmed the masonry from inside through every winter, drying out rain that soaked in and keeping the flue above the temperature where exhaust condenses.

Take the boiler away and the chimney turns into a cold column of brick. If anything still vents into it, that exhaust now meets colder walls. If nothing does, the stack still collects rain, freezes and thaws like any masonry on the roof, only without the heat that used to help it dry out.

Hybrid systems still use the flue

Many heat pump installations in the Northeast keep the old boiler or furnace as backup for the coldest days. That flue is still live. The difference is that the backup may run only a few days a winter, starting from a cold chimney each time, so condensation forms on the walls before the flue warms up.

A backup system needs the same yearly chimney inspection as a primary one, and arguably more care, because a flue that sits idle for months is where birds nest and debris collects unnoticed. The first cold snap of the year is the worst time to discover the passage is blocked.

Short runs make it worse. A boiler that fires briefly, shuts down and fires again rarely brings the masonry up to temperature, so most of its exhaust meets cold brick. Ask your installer how the backup is set to switch on, and tell the sweep, because it changes how closely the flue base and liner need watching.

The water heater left behind

In a great many houses the boiler and a gas water heater shared one flue. With the boiler gone, the water heater is left on its own in a passage meant for two, which the trade calls an orphaned water heater. Its small, cool exhaust can struggle to rise, condense in the cold masonry and spill back at the draft hood instead of leaving the house.

There are several ways to fix it, and the right one depends on the water heater's age and the flue's condition:

• Line the flue with a liner sized for the water heater alone
• Replace the water heater with a model that vents through the side wall
• Switch to an electric or heat pump water heater and retire the flue
• Make sure each floor has a working CO alarm while the question is open

Closing a flue nothing uses

Once every fuel-burning appliance is disconnected, the chimney should be closed off on purpose, not just left. In the basement, the thimble openings where pipes once entered are sealed with noncombustible material so basement air, odors and pests cannot move through them. Rags, foil or a plastic cap pushed into the hole do not count.

At the top, the flue gets a cover that keeps rain, snow and animals out while letting a little air move through, so moisture inside the stack can escape. Sealing a masonry flue airtight at both ends can trap dampness in the brick. If a fireplace on the same chimney has its own sound flue, it can stay in service.

An unused chimney still takes the weather

Without heat inside, a chimney dries slowly after every storm. Water that soaks through a cracked crown or open joints stays in the masonry longer, and freeze-thaw cycles do more damage to brick that is wet when the temperature drops. Leaks that the warm flue used to hide can start to show.

Signs often appear indoors first. A damp patch or brown stain on the chimney breast, white salty deposits on basement brick, peeling paint around the chimney or a musty smell in a top-floor closet all suggest water moving through a stack that no longer warms up. Those are worth a look before the plaster or framing around the chimney gets involved. In attached houses, a neighbor's chimney sharing the party wall can also be the source, so note whether the stain sits on a shared wall before anyone starts opening plaster.

Keep it, repurpose it or take it down

Keeping the chimney makes sense when it is in reasonable shape or still serves a fireplace, since a capped and maintained stack also leaves room for a future stove or appliance. Some owners use an unused flue as a chase for new piping, but only where the equipment instructions allow it and no appliance vents into that flue.

Removal makes sense when the chimney serves nothing, needs major work and nobody wants a fireplace. The common version takes the stack down below the roof and closes the roof over it; full removal through the house is a bigger project involving floors and walls. In rowhouses a chimney is often shared with the neighbor, and permit rules differ from town to town, so confirm both before planning demolition.

Questions to settle with your installer

Heat pump installers focus on the new equipment, and the chimney can fall between trades. A few questions asked before or right after the installation save a second round of surprises. Bring the answers to the chimney visit, since they decide what the flue still has to do.

• Is the old boiler or furnace being removed, kept as backup or left disconnected?
• Does the water heater still burn gas or oil, and where does it vent?
• Were any pipes removed from the chimney, leaving open holes in the basement wall?
• Will any new lines or ducts run through the chimney or the old flue?
• Who is responsible for closing off or relining the flue once the work is done?

What the visit covers

A post-conversion visit starts in the basement with a list of what is still connected and what has been removed. Each flue is checked for size and condition, any orphaned appliance is measured against its flue, and open thimbles are noted. On the roof, the crown, cap, flashing and mortar joints are looked over, since the stack is now on its own against the weather.

From there you get the options side by side, whether a liner for a remaining water heater, a proper closure and cover, or partial removal, written up in a free written estimate, so you can decide calmly rather than after a leak.