I get a version of this call almost every week. Somebody in Pittsburgh just had a new gas furnace put in, or they're about to, and partway through the conversation the heating company mentioned the chimney needing a liner. Now they're calling me a little unsure, wondering if it's a real requirement or somebody padding the bill. It's real. When you go from oil heat to gas, the furnace is honestly the simple part of the job. The chimney is the part that gets overlooked. Your old flue was built around an oil burner, and gas does not play by the same rules. So before you write the check, let me walk you through what actually changes up there.
Why Oil and Gas Furnaces Use the Flue Differently
Start with the fuel, because that is the root of it. Oil burns hot, and the exhaust leaves the house hot, so it shoots up the flue and out before it has much chance to cool down and cause trouble. Gas is a different animal. It burns cooler, the exhaust carries a lot more water vapor, and that vapor is slightly acidic on top of it.
Here is why that matters. When cool, damp gas exhaust hits the walls of a big cold masonry flue, it slows down and condenses, the same way your bathroom mirror fogs over during a hot shower. Now you have acidic moisture sitting on the inside of your chimney instead of leaving the building. Give it a few Pittsburgh winters and that moisture chews through mortar, brick, and tile from the inside out.
There is a sizing problem stacked on top of the chemistry. The flue in an older Pittsburgh house was sized for an oil furnace, which usually means it is bigger than a modern gas furnace wants. Gas needs a snug, correctly sized path to draft properly. Drop a small, efficient gas furnace into a wide oversized flue and the exhaust just loafs around in there instead of rising and getting out. Weak draft plus condensation is exactly the pairing you do not want.
What Happens to a Clay Tile Liner After Years of Oil Use
A big share of the homes I work in around here were built mid-century or earlier, and most still have their original clay tile liner. Clay tile was fine in its day, but decades of service add up, and oil heat is hard on it in its own way.
When I send a camera down one of these flues, the story is usually the same. The tiles are cracked, sometimes straight across, sometimes spider-webbed. The mortar joints between the tiles have washed out, leaving gaps where the sections meet. The faces are flaking off, what we call spalling. Often there is a soft, sooty buildup from the oil years coating the whole thing.
Now picture that tired clay liner, already cracked and full of gaps, and you have just hooked a gas furnace to it. The flue is the wrong size for gas, the liner is full of openings for acidic moisture to soak into, and the whole setup is primed to fall apart faster and vent worse than it did before. That is the spot a lot of Pittsburgh homeowners are sitting in without knowing it.
What the Fix Actually Is
The fix is straightforward, and it is not a tear-the-chimney-apart job. I run a stainless steel flexible liner down the inside of your existing flue, top to bottom, and connect it to the furnace. Stainless stands up to the acidic gas exhaust that eats clay and masonry, and because it is one continuous pipe, there are no cracked joints or gaps for moisture to find.
The part that matters most is the sizing. I match the liner to your specific furnace's BTU output, not to a round number off the truck. Too big and you are right back to the weak-draft problem. Sized right, the exhaust moves up and out the way it is supposed to, and the furnace runs the way the manufacturer built it to.
Day of, most single-flue jobs wrap up in a day. I work from the roof and at the furnace, the liner goes down the flue, gets connected and sealed at the top with a cap and storm collar, insulated where the situation calls for it, and I check the draft before I pack up. On cost, I will be straight with you. A liner is not a hundred-dollar fix, but it is a one-time job, and it is a lot cheaper than rebuilding a chimney the acid ate through or chasing a venting problem later. You get the number from me up front, no games.
What If You're Also Replacing the Water Heater?
This one catches a lot of people, so read it closely if it fits you. If your new furnace is a high-efficiency model, the kind that vents out the side of the house through a white plastic pipe, then that furnace is not using your chimney at all anymore. Good for your gas bill. But if your water heater is gas and still vents into the old chimney, it is now in that flue all by itself.
That flue was sized to carry the furnace and the water heater together. With the furnace out of the picture, it is far too big for the little bit of exhaust a water heater puts out. A water heater cannot push its fumes up a big cold flue on its own, so the draft stalls, and in a bad case the exhaust, carbon monoxide and all, drifts back down into the house. There is a name for this, the orphaned water heater, and people miss it because the furnace install itself looked perfectly clean.
If that is your setup, the water heater usually needs its own properly sized liner so it can vent on its own. If nobody raised it when your furnace went in, it is worth having a set of eyes on it.
Questions to Ask Before Anyone Touches Your Chimney
You do not need to be an expert to hire the right one. Ask these four and you will know pretty quickly whether someone knows the work:
- Are you sizing the liner to my furnace's BTU output, or just dropping in a standard size?
- What kind of shape is my existing clay tile actually in? Can you show me photos or video from inside the flue?
- Does this work need a permit in my municipality?
- Will the finished job pass a Level 2 chimney inspection?
Get the Chimney Looked At Before the Furnace Goes In
Going from oil to gas is a smart move for most Pittsburgh homes. Just do not let the chimney be the afterthought, because it is the part keeping the exhaust out of your living room. If you have a quote for a gas furnace, or one just went in, get the chimney checked before or right when the work happens. That is the easy time to deal with it. I will tell you straight what your flue needs, size the liner right, and get it venting safely. Here is the full rundown on chimney liner installation for gas furnaces and water heaters, or you can just call me and we will talk it through.