A high-efficiency furnace vents out the wall and leaves the water heater alone on the chimney. What that line item on your quote is really paying for.
You get a quote to replace the furnace and there is a line on it for a chimney liner. The furnace you were expecting. The liner reads like something that got added on to pad the number.
It usually is not. In a lot of houses around the Wyoming Valley, that liner is the difference between an installation that vents properly and one that lets exhaust spill back into the basement. This is the time of year we have the conversation most, because October is when people are getting quotes and deciding what to do before the cold settles in. So here is the plain version of what that line is for, and when it honestly is not needed.
Why Replacing a Furnace Changes the Chimney
The furnace you are taking out probably vents into the masonry chimney. Older mid-efficiency gas furnaces run exhaust hot enough to rise up a brick flue on its own, and in most houses here that chimney was carrying both the furnace and the water heater.
A modern condensing furnace does not work that way. It pulls so much heat out of the exhaust that what comes off the end is barely warm and full of moisture, which is why it vents through plastic pipe out a sidewall instead of up the chimney. That is the efficiency gain working as designed.
The problem is what you leave behind. Pull the furnace off that chimney and the water heater is suddenly the only thing connected to it. One small appliance, trying to push exhaust up a flue sized for two.
Does a New Furnace Need a New Chimney Liner?
If the new furnace is high efficiency and the water heater stays on the chimney, then yes, almost always. The gas code sizes a vent for the appliances actually connected to it, and the sizing is not a formality. A flue that is too large for the load lets exhaust cool on the way up. Cool exhaust slows down. Slow exhaust condenses on the masonry, and the water in it is acidic enough to chew through mortar joints and spall the brick from the inside over a few seasons. Worse, a flue that cold may not establish draft at all, which means combustion gases look for an easier exit. That exit is your basement.
The fix is a correctly sized liner, usually stainless, run the height of the chimney and connected to the water heater, sometimes with insulation around it to keep the flue gases warm enough to rise. It is not an upsell. It is the part of the job that makes the rest of the job legal and safe.
There is a second option, and a good contractor should put it in front of you: take the water heater off the chimney too. A power-vented or replacement water heater that vents out the wall, a tankless unit, or an electric one leaves the chimney with nothing connected to it at all. If the water heater is already old, doing both appliances at once often costs less than a liner plus a water heater replacement two winters later. Our notes on when to replace a water heater go through how to tell where yours stands.
What Is an Orphaned Water Heater?
That is the trade term for the situation above: a water heater left by itself on a flue that was sized for a furnace and a water heater together. The name sounds harmless. The failure mode is not.
An orphaned water heater on an oversized chimney is the single most common way carbon monoxide ends up inside a house after a heating upgrade. It does not happen the day of the install. It happens on a cold, windy night in January when the draft reverses and nobody is awake to notice. The appliance is working fine. The venting is not.
This is also why we will not quote a furnace swap without looking at what else is on the chimney. Anyone who quotes a high-efficiency furnace installation over the phone, sight unseen, has not accounted for the thing that matters most.
Signs the Venting Is Not Working Right
Worth knowing whether or not you are replacing anything this year. Go down to the basement while the water heater is running and look for:
- Rust or white crusty deposits on the top of the water heater, around the draft hood, or on the vent connector. That is condensation that should not be there.
- Soot anywhere on or near the appliance.
- A vent pipe that is loose, rusted through at a joint, or sloped downward instead of rising toward the chimney.
- Moisture or staining on the basement wall around the chimney base, or mortar crumbs collecting at the cleanout.
- A persistent smell when the heat or hot water runs that does not fade after the first few days of the season. Dust burning off a furnace in October is normal, and we wrote about what that first-run smell means. An exhaust smell is different and it does not go away.
If a carbon monoxide alarm goes off, treat it as real every time. Get everyone outside, then call. You want working CO alarms on every level of the house and near the bedrooms, and if yours are more than seven to ten years old, the sensors are past their service life even though the test button still chirps.
Old Chimneys, Oil Conversions and Boilers
Much of the housing stock around Wilkes-Barre predates 1940, and the chimneys are a mixed bag. Some were never lined with clay tile at all. Plenty that were have tile that has cracked or dropped sections into the flue. We have opened cleanouts and found nests, bricks and eighty years of debris sitting where exhaust is supposed to pass.
Houses converted from coal to oil and later to gas carry another wrinkle. Each fuel wants something different from a flue, and a chimney that drafted an oil burner acceptably can be a poor match for gas, which produces far more water vapor. If your house has had two or three heating systems over its life, the chimney has probably never been sized for any of them properly.
Boilers are a different conversation again. A standard cast iron boiler still vents up the chimney, so replacing one does not orphan the water heater the way a furnace swap does. But the same deterioration applies, and a liner is often the right call anyway. Our boiler work starts with looking at the venting, not just the unit.
When You Genuinely Do Not Need One
Not every quote needs that line, and you should be able to ask about it without a fight.
Skip it when the replacement furnace is the same mid-efficiency type that vents up the chimney alongside the water heater, the flue is sound, and the sizing has not changed. Skip it when the water heater is coming off the chimney in the same job, leaving nothing connected. And skip it when a recent inspection shows the existing liner is intact and correctly sized for what is staying.
What should never happen is a liner priced into the job with no explanation, or a high-efficiency furnace installed with the water heater left orphaned on a bare masonry flue because nobody brought it up.
Ask Before the Install, Not After
Get this settled while you are still comparing quotes. Ask each contractor what happens to the water heater venting, ask whether the flue was actually inspected, and ask what the liner is sized for. The answers will tell you a lot about who you are hiring. If you are still deciding whether to replace at all, our walkthrough on repairing or replacing an aging furnace covers that side of it, and routine maintenance is where venting problems usually get caught first.
We have been putting heating systems into houses in Wilkes-Barre and across the valley since 1921, four generations of the same family, and a good share of the chimneys we work on were built before anyone here was born. Master licensed, insured, PA License #001864. The office runs Monday through Friday, 8am to 5pm, and we answer the phone 24/7 when heat quits at a bad hour. Call (570) 822-8158.