Can a High-Efficiency Furnace Vent Into a Chimney?

Efficiency changes the exhaust

An older atmospheric furnace sends a lot of its heat up the chimney. That hot exhaust is what makes a masonry chimney work: warm gas is lighter than the outside air, so it rises on its own and pulls fresh combustion air in behind it. The chimney was built around that behavior.

A condensing furnace or boiler pulls so much heat out of its exhaust that the water vapor in it turns to liquid inside the appliance's second heat exchanger. What leaves is lukewarm, saturated and pushed by a fan under slight pressure. In a brick or clay flue that exhaust would keep condensing, soak the masonry with acidic water and could leak out through mortar joints into the house.

Vent categories in plain terms

Gas appliance installers sort venting by two questions: is the exhaust under negative or positive pressure in the vent, and is it hot enough to stay above the point where it condenses? The answers put an appliance into a category, and the category decides what kind of vent it can use. The two you meet most often in houses are these:

Categories II and III exist too, but they are uncommon in homes. Whatever the label, the furnace's data plate and manual state how it must be vented, and those instructions outrank any rule of thumb about efficiency numbers.

• Category I: non-condensing, negative pressure; can vent into a properly sized and lined chimney
• Category IV: condensing, positive pressure; needs its own sealed, listed vent pipe
• Typical Category I: older atmospheric and mid-efficiency fan-assisted furnaces
• Typical Category IV: high-efficiency condensing furnaces and boilers

What a condensing furnace's vent needs

The manufacturer lists the vent materials allowed for each model, often PVC, CPVC or polypropylene, and sets the maximum length and number of elbows. The pipe has to slope back toward the furnace so condensate drains to the appliance and out through its condensate line, rather than pooling in the pipe or dripping from the termination.

Many of these units are direct vent, meaning they draw combustion air from outdoors through a second pipe rather than from the basement. Both pipes end at a termination with set clearances from windows, doors, grade and the expected snow line, all spelled out in the manual. None of that involves the masonry flue, which is why a condensing furnace does not need a chimney liner.

Using the chimney as a chase

In rowhouses and attached homes, a side-wall vent can be hard to place: the side wall may face a neighbor a few feet away, or the front may be the one open outside wall. Some manufacturers allow their vent pipe, and sometimes the intake pipe, to run up through a chimney that no longer serves anything. The chimney becomes a shaft that hides the pipes; it is not the vent.

That route is only acceptable when the installation instructions for that exact model permit it, and generally only when nothing else vents into that flue. At the top, the pipes end in the termination the manufacturer specifies, and the flue opening around them is closed off. An ordinary chimney cap set over the pipes is not a substitute for that termination.

• The manual for that model lists chase installation as allowed
• No water heater, fireplace or other appliance shares the flue
• Pipes run continuous and sealed from furnace to termination
• The flue top is closed around the pipes and the listed termination installed

Mid-efficiency furnaces in a masonry chimney

A fan-assisted, non-condensing furnace still produces warm exhaust that can go up a chimney, but it sends less heat up the flue than the furnace it replaced. In an oversized or unlined masonry flue, that cooler exhaust can condense on the walls, especially where the stack runs up a cold exterior wall.

These furnaces usually need a liner sized for them from the venting tables and the manufacturer's instructions, in stainless or another material the furnace and liner listings allow. Skipping the liner because the old furnace worked fine in the bare flue is a common and costly mistake; the old furnace had far more heat to spare.

The appliance left on the chimney

When the old furnace shared its flue with a gas water heater, moving the furnace to a side-wall vent leaves the water heater orphaned in a flue now far too big for it. That needs its own fix: a liner sized for the water heater, or a water heater that vents differently. The time to plan for it is when the furnace is being quoted, not after the first winter of rusty water collecting under the old flue.

A fireplace in the same stack is a separate question. Its own flue can stay in use beside a flue that serves as a chase or holds a water heater liner, as long as each flue is sound and nothing crosses between them. The fireplace should not share a flue with any of the furnace or water heater venting.

Side-wall vents in a Northeast winter

A condensing furnace's vent puffs a white plume on cold days. That is water vapor and it is normal. What is not normal is a termination buried in drifted snow or crusted with ice, which can trip the furnace's pressure switch and shut off the heat. After a nor'easter, clear snow away from the intake and exhaust pipes from the ground, and keep shrubs, firewood and trash cans out of their clearance zone.

Ice can also build on the wall or walkway below the termination where condensate drips or the plume freezes. If that happens every winter, the termination location or kit may need a look from the installer.

What the chimney side of the job includes

Before a new furnace is installed, the chimney is checked to see what else uses it, the condition and size of each flue, and whether the top and crown are sound. If the plan is a chase installation, the flue is cleared and the top is prepared for the closure and termination. If a water heater or mid-efficiency furnace stays on the chimney, the liner is sized to it.

Those options go into a free written estimate, which lets the heating contractor and the chimney work can be scheduled together and the house is not left with an orphaned flue.