Can a Furnace and Water Heater Share a Chimney Flue?

How common venting works

Throughout the region's older housing, one chimney flue often serves both the heating appliance and the water heater. Each appliance has its own connector pipe, and the two meet at a tee or wye fitting before entering the chimney, or enter the chimney at separate points. The rising column of warm exhaust in the flue pulls both streams up and out.

This is called common venting, and codes allow it for appliances that vent the same way: non-condensing appliances whose vents operate under negative pressure. In the trade these are Category I appliances, which include traditional natural-draft furnaces, boilers and water heaters with a draft hood, and many fan-assisted furnaces and boilers.

Which appliances can share

The appliance's rating plate and installation manual state how it may be vented. These are the general patterns.

Sizing: the part that makes or breaks it

A shared flue has to handle two situations. When both appliances run at once, it must carry their combined exhaust without backing up. When only the smaller one runs, often the water heater on a summer day, it must still draft well enough that the small, cooler exhaust stream rises instead of lingering and condensing. A flue that is right for the pair can be far too big for the water heater alone.

Codes publish venting tables that set flue and connector sizes for single and multiple appliances by height, lateral run and input. Philadelphia's permit standard for vent work, for example, points to the residential code's tables for single Category I appliances and separate tables for vents serving two or more Category I appliances. Appliance manuals add their own requirements, and the installer works from both.

Connector details that matter

Beyond the flue, the connectors themselves decide a lot. Each connector should rise toward the chimney rather than run flat or sag, be as short as practical, and be sized for its appliance. Where the connectors join, the smaller appliance usually enters above the larger, or the two meet in a fitting designed for it, so one stream does not push into the other. Joints should be secure, and where a connector enters the chimney it should be sealed to the thimble.

Loose, sagging or rusted connectors are one of the most common findings in basement boiler rooms. They are also one of the easiest fixes.

When one appliance is replaced

Sharing works when the pair was designed together. Change one and the math changes. The most common case in older homes: the old furnace or boiler is replaced with a high-efficiency, direct-vent model that exits through the wall, and the water heater is left alone in a large masonry flue sized for both. That is called an orphaned water heater. Its small, cool exhaust struggles to warm the big flue, drafts poorly, and condenses, which eats at clay tile and mortar.

How big is too big? The same Philadelphia standard gives one concrete limit for gas vents serving a single draft-hood appliance: the vent's effective area may not be less than the draft hood outlet, nor more than seven times its area. An old masonry flue built for a coal or oil boiler can easily exceed that for a small water heater, which is why the leftover appliance so often needs its own liner.

The fix is usually a liner sized for the water heater alone. In New Jersey, the state's chimney verification form for appliance replacements asks the signer to confirm the existing chimney is appropriately lined and sized for any appliances still using it.

Warning signs on a shared flue

Problems on a shared flue tend to show at the water heater first, because its draft hood is open to the room.

• Warm, moist air or a sharp smell at the water heater's draft hood when it runs
• Rust, scorching or melted plastic near the draft hood
• Water or rust stains on the connector or at the chimney thimble
• White crust or dampness at the base of the chimney or on the cleanout door
• A CO alarm that goes off when the heat and hot water run together

A safe check from the basement

With the water heater running for a few minutes, a lit match or incense stick held near the edge of its draft hood should show the smoke being pulled in. Smoke pushed outward or drifting into the room means exhaust is spilling. Repeat with the furnace or boiler also running. Do not reach into or touch hot pipes, and if you notice anything abnormal, stop and call.

Exhaust contains carbon monoxide, which is dangerous. Any home with gas or oil appliances needs CO alarms. An alarm or anyone feeling unwell means leaving first and calling for help from outside, through 911 or the gas company.

What an inspection checks

A chimney inspection of a shared flue looks at the flue's size and condition, whether it is lined and with what, the connectors and how they join, the thimble, the cleanout and the termination at the top. The technician compares the arrangement with the appliances' ratings and the sizing the codes call for, and checks the draft with the appliances running. If the flue no longer suits the pair, the usual answers are a correctly sized liner, new connectors, or separate venting for one of the appliances.

Planning a replacement

If you are replacing either appliance, involve the chimney side before the new equipment is chosen. Deciding up front whether the remaining appliance will need a liner, or whether both should move to new venting, avoids a new boiler sitting idle while the chimney is sorted out. Any change of venting is quoted in writing, free, before work starts.

The same goes for adding equipment. A second water heater, a gas dryer or a generator should not simply be tapped into an existing chimney connector. Each addition changes the load on the flue and needs its own sizing check.