A lot of the brick chimneys standing in Somerville and Rocky Hill were built with two flues side by side, one for a fireplace that rarely sees a fire anymore and one for the boiler or water heater that runs through the heating season. When a sweep finds a cracked tile or a missing section in that second flue, somebody in the house usually asks if the boiler can just vent up the fireplace flue instead, since it is sitting there unused. The short answer is yes, but only by giving up the fireplace for good, not by sharing one flue between two appliances.
Two flues built for two different jobs
A fireplace flue is sized for the large volume of air a wood fire pulls through an open damper, a chimney that moves a lot of air fast while the fire burns and cools between uses. A boiler or water heater flue is sized much narrower, matched to the steady, lower-volume draft that appliance's manufacturer specifies. Dropping a boiler's vent connector into a flue built for a fireplace means the gas moves slowly through a passage far wider than it needs, cools faster than it should, and condenses against the masonry instead of carrying straight up and out. That condensation is where soot staining, mortar damage and eventually carbon monoxide spillage back into the house start.
What the rules require before that flue gets reused
New Jersey's fuel gas and mechanical rules do not let a vent connector enter a flue that also serves a fireplace unless the fireplace opening is permanently sealed off from that flue. Where an oil-fired appliance connects into a flue that once served a fireplace, the rules call for a noncombustible seal below the connection point and an access opening left in place for cleaning, since lint, soot and debris still have to be reachable. The masonry lining itself also has to suit whatever appliance ends up connected to it, which rules out routing a boiler vent connector into an unlined or oversized fireplace flue and calling it done.
Route one, relining the original heating flue
The first option, and often the one that keeps the most of the house intact, is relining the flue the boiler already uses. Chimney Repair covers installing a liner sized to the appliance's draft requirements, pulling the old cracked tile liner or bypassing it entirely with a new metal liner run top to bottom. This route keeps the fireplace flue untouched and the fireplace itself usable, which matters on a house in Somerville or Rocky Hill where the fireplace is original to the building and worth keeping functional even if it rarely gets lit.
Route two, retiring the fireplace for the boiler's sake
When the original heating flue is too far gone to reline, crumbled tile that cannot be bypassed cleanly or a passage too tight for a liner to fit through, the fireplace flue becomes the option left. That only works if the fireplace opening is permanently sealed, a solid noncombustible closure rather than a damper left shut, with the new liner installed to match the boiler's vent size and an access panel built in for cleaning. Once that work is done, the fireplace is retired. It is not a seasonal swap between fireplace use and boiler venting, it is a one-way decision, and a homeowner weighing this route should treat it that way before committing.
Route three, venting the boiler through a sidewall
Some newer boilers are built to vent through a sidewall rather than up a masonry chimney at all, bypassing both flues. Where the appliance supports it and the wall location works for clearance and termination height, this route sidesteps the question of which flue to use entirely. It still leaves the old heating flue needing attention, either capped off and left unused or relined if the fireplace flue stays dedicated to the fireplace and nothing else uses the second passage.
Why both flues get inspected before anything is decided
None of these routes gets chosen without a look at both flues first. Chimney Cleaning clears soot and debris so a camera can see the tile condition, cracks, gaps at the joints, and whether the passage is a consistent size top to bottom or narrows partway up. The fireplace flue gets the same look even if the plan is to seal it off, since a flue with a collapsed section partway up changes what sealing and relining that passage involves. Skipping this step and guessing at flue condition from the basement or the roofline alone is how a reline job gets started before anyone knows the liner will not fit through a narrowed section higher up.
What decides the cost of the fix
What this work costs depends on which of the three routes fits the house, how long a liner run is needed, whether the fireplace opening requires new masonry to seal properly or just a metal closure plate, and how much of the existing flue tile has to come out before a liner goes in. A written estimate after both flues are inspected lays out which route applies and what Chimney Repair covers on that chimney, rather than pricing a reline or a seal-and-relocate job before anyone has looked at what condition either flue is in.