New Furnace, Old Chimney: The Venting Problem in Mokena Homes
By Dan Joel, Elite Sweepers Inc.
Here is a situation we run into regularly and homeowners almost never know about. A furnace gets replaced with a modern high-efficiency unit, which vents through a plastic pipe out a side wall rather than up the chimney. The installation is done correctly and the furnace works beautifully. What nobody addresses is the water heater, which was sharing that chimney and is now venting into it alone.
What an orphaned water heater is
The short answer: an appliance left venting by itself into a flue sized for it plus something much larger.
When a furnace and water heater shared a chimney, the flue was sized for their combined output. The furnace was the larger contributor, and it produced most of the heat that drove the draft and kept the flue warm. Remove it and the water heater is a small appliance trying to establish draft in a flue far too large for it, in a chimney that now stays cold between short burner cycles.
Why an oversized flue is a problem
The short answer: the exhaust cools, slows, and condenses inside the chimney instead of leaving it.
Draft depends on a warm column of gas rising. A small volume of exhaust in a large cold flue loses its heat to the masonry quickly, and cooled exhaust does not rise well. Two things follow. Gases may not clear the chimney reliably, which is a venting concern for an appliance producing combustion products. And the moisture in the exhaust condenses on the flue walls, where it is acidic, attacking mortar joints, deteriorating clay tile and rusting metal components from the inside.
How to tell it applies to you
The short answer: a newer furnace venting out a side wall, and a water heater vent still going into the chimney.
Look at your furnace. If it vents through white plastic pipe running out through a wall, it is a high-efficiency unit and it is no longer using the chimney. Now look at the water heater. If its vent pipe still runs up into the chimney, you have the situation described here. Signs it is already causing trouble include rust or white staining on the water heater’s vent connector, moisture or staining on the chimney in the basement, and a persistently damp smell near it.
What the fix looks like
The short answer: a correctly sized liner for the water heater alone.
The established solution is to line the chimney with a liner sized for the remaining appliance. That gives the exhaust a passage matched to its volume, so it keeps its heat, rises properly and clears the chimney without condensing along the way. It also protects the masonry from the condensate that was attacking it. Our chimney relining page covers what the work involves.
Why it gets missed
The short answer: it falls between two trades.
The heating contractor installed the furnace correctly and the furnace is not the issue. The chimney is not their scope, and nobody was asked to look at it. It is a genuine gap rather than anyone doing poor work, which is why it is worth knowing about as a homeowner. If your furnace was replaced in recent years and nobody discussed the chimney with you at the time, it is worth having the venting checked.
Getting it assessed
Appliance connections and flue sizing are part of a full chimney inspection, and a change of appliance is one of the recognized reasons for a level 2 with a camera scan. Related masonry damage is covered on our chimney repair page.
For local scheduling, see our chimney service in Mokena, with costs on our pricing page. Elite Sweepers Inc is insured and bonded, works seven days a week from 7 AM to 7 PM, and gives you the price up front. Call (708) 432-3977.
Appliance Venting Questions
What is an orphaned water heater?
A water heater left venting alone into a chimney after the furnace that shared it was replaced with a high-efficiency unit venting elsewhere. The flue is now far larger than the remaining appliance needs.
How do I know if my furnace still vents through the chimney?
Look at the vent pipe. A high-efficiency furnace vents through white plastic pipe running out through a side wall. If your furnace vents that way but the water heater pipe still enters the chimney, the situation applies to you.
Why is an oversized flue a problem?
A small volume of exhaust in a large cold flue loses heat quickly, so it does not rise well and its moisture condenses on the flue walls. That condensate is acidic and attacks mortar, clay tile and metal components.
What is the fix for an orphaned appliance?
Lining the chimney with a liner sized for the remaining appliance. That matches the passage to the exhaust volume so gases keep their heat, rise properly and clear the chimney without condensing on the way.
What are the signs this is already causing damage?
Rust or white staining on the water heater vent connector, moisture or staining on the chimney where it passes through the basement, deteriorating mortar joints inside, and a persistently damp smell near the chimney base.
Should my heating contractor have caught this?
It commonly falls between trades rather than being anyone doing poor work. The furnace installation itself is usually correct, and the chimney is not part of that scope, which is exactly why it goes unnoticed.
Does this matter if I never use my fireplace?
Yes, and it matters more. This is about the water heater venting, which runs year-round regardless of whether anyone lights a fire. The fireplace being unused has no bearing on it.
Do I need an inspection after replacing a furnace?
It is worth doing. A change of appliance is one of the recognized triggers for a level 2 inspection, precisely because the venting requirements of the chimney change when what it serves changes.
Published October 1, 2026