Common venting masterclass. Two appliances, one chimney — and what happens on the day one of them leaves. Episode seventeen of the Gas Service Animated series.

Two appliances, one vent. The furnace and the water heater each get their own connector, and each connector is sized on its own from the tables. But from the first tee upward, every shared segment must carry the combined load of both appliances. Three measurements drive everything. Total height H from the appliance outlet to the vent top. Lateral length L in the horizontal runs. And connector rise R. Same sizing bridge you crossed in the venting episode — now with two riders.

The two appliance tables come in two halves. The connector half keys on rise. Rise is measured from the draft hood outlet or flue collar up to the centerline where the vent gas streams come together — more rise means more capacity for that connector. The common vent half keys on total height, and taller vents pull harder and carry more. Find your height row, then confirm the shared vent can handle the combined input of both appliances together.

Where the connectors meet the vent, geometry matters. Each connector enters at its own level, and the smaller appliance's connector enters at the highest level the headroom allows. If the connectors join into a manifold before the vertical vent, apply a ten percent reduction to the common vent capacity, and keep the manifold within eighteen inches of length per inch of vent diameter. And when the vertical vent has to offset around framing, capacity drops again and the horizontal run is limited the same way. Straight and vertical is the strongest vent.

There is an upper limit too. The common vent area may not be more than seven times the smallest draft hood outlet area. The reason is the mildest day. Sometimes the smallest appliance runs alone, and its little stream of flue gas must warm the whole passage and establish draft all by itself. Check a three inch outlet. The area is about seven point zero seven square inches. Multiply by seven, and the common vent can be at most about forty nine and a half square inches. A large masonry flue can blow past that limit easily — remember this for the next scene.

Now the signature scenario. A high efficiency furnace replaces the old unit and vents out the sidewall in plastic pipe. The water heater — say forty thousand Btu per hour — is left orphaned in a masonry chimney that was sized for two appliances. That big cold flue may never warm up. Flue gas condenses on the masonry and attacks it, draft never establishes, and combustion products spill from the draft hood into the living space. This is the classic post furnace swap carbon monoxide risk — and it is exactly why the venting for whatever remains must be re-evaluated after every appliance change, including a reline assessment of that chimney.

The fix starts by treating what remains as a brand new single appliance system, sized from the single appliance tables. Usually the answer is a liner. A listed corrugated metallic liner is sized from the vent tables with its capacity reduced by twenty percent. If the chimney rides an exterior wall in a cold climate, verify it against the tables keyed to the local winter design temperature — in the coldest regions those tables may simply say no, and the vent needs a different route. Then prove the fix. Fire the water heater and spillage test the draft hood.

Common vents can climb a building, picking up an appliance at every floor. Each junction changes the load, so every segment gets sized on its own for everything feeding it from below. Segment height is measured from the highest appliance outlet entering that segment up to the centerline of the next tee above. The lowest connector and the vent below the first interconnection come from the single appliance tables. Multistory common vents must be Type B double wall with a listed cap, and only one limited offset is allowed.

So here is the discipline on any shared vent. Verify every connector and every common segment against the tables, using the combined load downstream of each tee. Measure rise to the interconnection centerline, not to the ceiling. Run the seven times check on the common vent area. And after any appliance change — an upgrade, a removal, a swap — re-evaluate the whole system that remains, then spillage test every appliance still on the vent. The chimney does not know the furnace left. You have to check.

Common venting in four moves. Size each connector alone, then size every shared segment for the combined load. Keep the common area within seven times the smallest outlet. Treat an orphaned water heater as a brand new system — usually that means a liner — and spillage test everything after any change. The printable Chapter thirteen Atlas sheets carry the full tables for your kit. Next episode: condensing appliances and plastic venting.