Only if the flue is sized for everything connected to it. New Jersey's residential code holds a chimney flue to a size that cannot run smaller than the connector feeding it, and a flue serving more than one appliance gets its own separate sizing rules rather than just adding the two connector sizes together. In a Collingswood two-family where each unit replaced its own furnace or water heater over the years, often with a different contractor doing each job, nobody along the way necessarily went back and checked whether the shared stack still matched the combined load. The chimney can look sound from the basement floor and still be wrong for what is tied into it.
How two systems end up sharing one stack
Attached housing in Collingswood and nearby parts of Camden County was often built with one shared basement chimney serving both units, sometimes as a single large flue and sometimes as two liners run side by side inside the same masonry stack. As decades pass, each owner or each unit's tenant gets a new furnace, a new water heater, or both, and whoever installs that equipment typically reconnects to whatever thimble or opening is already there rather than opening up the chimney to check the flue against the new appliance's input rating. That is not laziness on the installer's part. It is a reasonable assumption that an existing connection already met code when it was built, which is not always true once a second unit's load gets added to the same flue years apart from the first.
Tracing each connector before calling the chimney sound
A chimney that looks fine from the basement or the roofline still needs every connector traced back to where it enters the stack before anyone can say the system is sized right. That means following the pipe from each furnace and each water heater to its thimble, noting the connector diameter at that point, and comparing the total input of everything tied into a given flue against what that flue is rated to carry. Two water heaters and two furnaces venting into the same liner is a different calculation than one of each, and a flue built decades ago for a single large coal or oil appliance was never sized with today's smaller gas appliances in mind, let alone four of them. This tracing is part of what a Chimney Repair assessment covers on a two-family, and it is also the point where Chimney Cleaning turns up buildup specific to one appliance rather than the whole stack, which helps narrow down which connection is the problem.
One flue inside the stack, or two
Some of these Collingswood two-family chimneys were built with two separate clay tile liners run up through the same masonry shell, one assigned to each unit from the start. That arrangement keeps each unit's furnace and water heater venting into its own flue, sized for just that unit's load, and it tends to hold up better over decades of separate ownership changes to the equipment. Other stacks were built with a single liner meant to serve both units together, on the assumption that the combined load at construction time fit the one flue. A homeowner cannot tell which setup is in place by looking at the crown from the yard. Opening the cleanout and running a camera up the liner is the only way to confirm whether there are one or two flues inside that stack, and that confirmation has to happen before any sizing conversation makes sense.
What changes when one furnace moves off the chimney
A common turn in these two-family setups happens when one unit upgrades to a high-efficiency furnace that vents through a side wall with its own plastic pipe instead of using the masonry chimney at all. That takes one appliance's load off the shared flue, which sounds like it should make things easier on the stack, and sometimes it does. But a flue that was already sized close to the edge for the combined load can end up oversized for whatever is left venting into it, usually an older atmospheric water heater or the other unit's furnace. An oversized flue cools exhaust gas faster than it should, which invites condensation inside the liner and a slower draft that does not pull the way it is supposed to. This is one of the reasons a chimney evaluation after any equipment change on either side of a two-family needs to look at what is left on the flue, not just what was removed from it.
Signs at the draft hood worth noticing
Staining, rust streaking, or a chalky residue around a furnace or water heater's draft hood is usually the first visible sign that something on that connection is not venting the way it should, whether that is a sizing mismatch, a blockage further up the flue, or spillage from another appliance sharing the same liner. In a two-family, these signs sometimes show up on one unit's equipment while the other unit's connection looks clean, which is itself a clue about which flue is carrying more than it was built for. The U.S. Consumer Product Safety Commission recommends a yearly check of chimneys and flues connected to fuel-burning equipment for exactly this reason, since problems in a shared stack do not always announce themselves evenly across every appliance tied into it.
Booking the work when there is an owner and tenants
Scheduling gets more involved in a two-family than in a single house, since relining or resizing a shared flue usually means both units lose heat and hot water for the stretch of the job, not just the unit whose appliance prompted the call. An owner-occupied two-family in Collingswood with a tenant in the other unit needs that coordinated ahead of time, since the tenant's furnace and water heater are affected even if the original complaint came from the owner's side. Once the chimney work is done, whichever heating contractor installed each furnace or water heater needs to come back to confirm the reconnection at the thimble, check that the appliance still drafts correctly into the newly sized or relined flue, and sign off on that connection separately from the masonry work. A written estimate for this kind of job should spell out where the chimney scope ends and where the heating contractor's reconnection begins, since those are two different trades meeting at the same pipe.