Every fall I see the same quote shape in Massachusetts: a clean line for a 95% furnace, a respectable total, and nothing about the chimney. Then the installer pulls the old 80% unit, caps the flue, and the 15-year-old water heater that shared that flue starts spilling exhaust into the basement. The liner that should have been on the quote shows up as a change order for $1,800 or more, in November, with no heat in the house.
This is a guide to that one line item: what happens to a chimney when a mid-efficiency furnace comes out, what the fix costs, and the questions that tell you whether a contractor has already thought about it.
Why an 80% furnace and a 95% furnace need different venting
An 80% AFUE furnace sends its exhaust up the flue hot, commonly several hundred degrees Fahrenheit. That heat is the whole design: it makes draft, and it keeps the water vapor in the exhaust as a gas all the way out of the roof. The chimney, usually a masonry flue with or without a clay tile liner, or a metal Type B vent, does the job with no help from a fan.
A 90%-plus condensing furnace, such as a Carrier Infinity 96, a Trane S9V2, or a Lennox EL296V, pulls so much heat out of the exhaust that the gas leaving the unit is only warm. It cannot rise through a tall chimney on its own and it would condense inside it. So these furnaces use a draft inducer fan and plastic pipe (PVC, CPVC or polypropylene depending on the manufacturer), usually 2 or 3 inches in diameter, run out a sidewall or up through the roof. They also produce condensate, typically somewhere between half a gallon and a gallon for every hour the furnace runs on a mid-size unit, and that condensate is acidic, roughly pH 3 to 4. It needs a drain, and often a neutralizer cartridge before it hits a floor drain or a condensate pump.
That means a swap is never "same spot, same pipe." You are adding a plastic vent route, a combustion air pipe if it is a two-pipe unit, and a condensate path. All of that should be priced as installation, not as a surprise.
The orphaned water heater problem
In a lot of New England houses, the furnace and a gas water heater share one chimney. The flue was sized for both appliances firing together. When the furnace leaves, the water heater is alone in a flue that is now far too large for its output. A 40-gallon atmospheric water heater at 30,000 to 40,000 BTU/hr makes much less hot exhaust than the flue was built for.
The results are predictable. Exhaust cools before it exits, water vapor condenses on the masonry, and the chimney starts to spall, stain, or drip rust-colored water. Draft weakens, and on a windy night or with the kitchen exhaust fan running, combustion products can spill back out of the draft hood into the basement. That is the carbon monoxide scenario. It is also the reason the Massachusetts fuel gas code, 248 CMR 4.00, which adopts NFPA 54 with state amendments, treats venting as a sizing calculation and not a guess.
The line to look for on the quote: "Water heater venting addressed." If the quote says nothing about the water heater, ask directly what happens to it. Either answer is fine as long as it is written down: a new liner sized for the water heater alone, or a replacement water heater that vents some other way.
Your three options, with realistic numbers
| Option | Typical added cost | What you get | Watch for |
|---|---|---|---|
| Stainless or aluminum flexible liner sized for the water heater | About $1,200 to $2,800 for a typical two-story chimney | Keeps the existing water heater, safe draft | Needs a chimney cap, a sized liner (often 3 inches), and an inspection of the flue first |
| Power-vent or direct-vent gas water heater out a sidewall | About $2,500 to $4,500 installed | Retires the chimney from service | Needs an outlet near the unit and a second sidewall penetration |
| Heat pump water heater (50-gallon) | About $2,800 to $5,000 installed before incentives | No flue, lower operating cost, dehumidifies the basement | Needs about 700 cubic feet of air around it, a condensate line, and 240V circuit |
Those ranges are what I see in typical 2026 Massachusetts quotes, not a guarantee, and a hard-to-reach chimney or a damaged flue tile moves them. If your water heater is past 12 years old, that tips the decision: it is cheaper to replace now than to line a chimney for.
Sidewall vent rules that people miss in Massachusetts
The sidewall termination for the new furnace is not a place you get to pick on looks. Manufacturer instructions set minimum clearances from windows, doors, gas meters and air intakes. In snow country the termination also has to stay above the expected snow line, and it should not discharge across a walkway where the plume freezes into ice. Massachusetts adds requirements for sidewall-vented gas equipment under 248 CMR 5.00, including carbon monoxide alarm and signage requirements, so ask the installer to show you where they will mount each. The gas fitting also needs a permit pulled by a licensed gas fitter and signed off by the local gas inspector.
If a contractor says a permit is not needed for a furnace swap, that is the end of that conversation.
Cost: what the 95% swap actually pays for
I will not give a single price, because a Worcester triple-decker, a Springfield ranch and a Newton colonial have very different ductwork. But here is how a typical 80,000 BTU, 95% single-stage-or-two-stage replacement breaks down when I reverse-engineer the quotes:
- Equipment (furnace, coil if cooling is included, thermostat): roughly 40 to 50 percent of the total.
- Venting, condensate and combustion air: $500 to $1,500 for a clean sidewall route, more with a roof penetration or a long run.
- Chimney or water heater correction: $0 if the furnace had its own flue, $1,200 to $4,500 if it shared one.
- Gas permit, disposal, startup and combustion test: a few hundred dollars. Ask for the CO and combustion readings printed on the invoice.
- Duct fixes (return sizing, a cracked plenum, a missing filter rack): where the quotes differ most.
If you are weighing a furnace against a cold-climate heat pump, it changes the math, because a heat pump has no flue at all. Mass Save has run whole-home heat pump rebates and a zero-interest HEAT Loan, though terms change, so check current numbers at masssave.com before you sign. I cover the rebate side in the Mass Save heat pump guide, and the oil-fired version of this decision in oil to heat pump in Massachusetts.
How to spot a quote that skipped the chimney
Before you pick a contractor, go to the basement with a flashlight and look at where the furnace flue goes. If a single metal pipe leaves the furnace and joins another pipe from the water heater before entering the chimney, you have a shared flue. Photograph it and send the picture with your request for quotes. A contractor who answers with a specific liner size is thinking. A contractor who says "we'll cap it off and you're fine" has not looked.
Also ask whether they will do a combustion air check. Condensing furnaces that draw indoor air in a tight, newly air-sealed house can starve the water heater. Two-pipe sealed-combustion furnaces avoid that, which is one reason I lean toward them in houses that have been through a Mass Save weatherization visit.
When I would not replace the furnace at all
If the 80% furnace is under 12 years old, passes a combustion and heat exchanger inspection, and the house has no cooling plans, replacing it for efficiency alone often takes 15 or more years to pay back at Massachusetts gas rates, and the chimney work can double that. In that case a tune-up and a duct-sealing job usually beat a swap. If the heat exchanger has failed, then it is a different story, and I wrote about getting a second opinion on a cracked heat exchanger for exactly that situation.
Compare Massachusetts furnace quotes
Get quotes from local contractors who price venting and chimney work as part of the job.
Massachusetts furnace replacementMore local detail: Massachusetts HVAC contractors, Worcester HVAC, and the national HVAC service hub.
Frequently asked questions
Can I just cap the old chimney and leave the water heater?
No. Once the furnace is gone, the water heater is alone in a flue sized for much more exhaust. It can cool the gases, condense inside the masonry and backdraft carbon monoxide into the basement. The flue needs a properly sized liner, or the water heater needs to vent another way.
Does a 95% furnace always need PVC pipe?
Almost always. The exhaust is too cool to rise through a chimney, so manufacturers specify PVC, CPVC or polypropylene vent pipe, and many allow a two-pipe setup that brings combustion air in from outside. Follow the installation manual for the exact model.
Where does the condensate go?
Into a drain line to a floor drain, laundry standpipe or a condensate pump. The liquid is mildly acidic, so many installers add a neutralizer cartridge, and some local plumbing rules require one before it enters older drain lines.
Do I need a permit to replace a furnace in Massachusetts?
Yes. Gas work requires a permit pulled by a licensed gas fitter and an inspection by the local gas inspector. Ask the contractor for the permit number before work starts.
How long does the chimney liner last?
A stainless steel flexible liner for gas appliances is commonly warrantied for decades by the manufacturer, while thinner aluminum liners are rated for lower temperatures and shorter service lives. Ask which product and gauge is on the quote.
Is a heat pump cheaper than a furnace plus chimney work?
Often closer than people expect, because a heat pump needs no flue and may qualify for Mass Save incentives. Compare the full installed cost after rebates, and check your winter heating load and electric rate before deciding.
Should the sidewall vent worry me in winter?
It should be placed above the expected snow line and away from walkways, doors and windows. A termination that ices over can shut the furnace down, so ask where the plume will go before the hole is cut.