Here is a scenario we run into constantly in Evanston and Des Plaines. A homeowner replaces a 1990s furnace with a modern high-efficiency model. The new furnace vents out the side wall through PVC. Great. But the old furnace shared the masonry chimney with the water heater — and now that water heater is alone in a big, cold, oversized flue. That is called an orphaned water heater, and it is one of the most commonly missed safety issues in our housing stock.
Why Orphaning Happens
In an older Chicagoland home, the furnace and the water heater were both natural-draft appliances vented into a single clay-lined masonry chimney. The furnace was the big heat producer: its hot exhaust warmed the flue and created the draft that carried both appliances’ combustion gases up and out.
When a 95% AFUE condensing furnace goes in, that furnace stops using the chimney entirely — it vents through PVC out the sidewall. The water heater, typically a 40,000 BTU atmospheric unit, is left trying to establish draft in a flue sized for 150,000+ BTU of combined input. Cold masonry, oversized cross-section, low exhaust volume: the recipe for a flue that will not draft.
What Goes Wrong
- Backdrafting. Exhaust — including carbon monoxide — spills into the basement instead of going up. This is the serious one.
- Condensation inside the chimney. Flue gases cool below dew point and condense. The acidic moisture attacks mortar joints and clay tile from the inside.
- Deterioration you can see. Rust on the draft hood, white efflorescence on the chimney exterior, crumbling mortar, tile shards in the cleanout.
- Failed inspections. Orphaned water heaters routinely get written up during real estate transactions, and the buyer’s inspector will price the fix for you.
The Four Legitimate Fixes
Any reputable contractor replacing a furnace in a shared-flue home should present one of these. If a proposal is silent on the water heater’s venting, that is a red flag.
1. Install a Properly Sized Chimney Liner
A flexible aluminum or stainless liner, typically 3″ or 4″, is run down the flue and connected to the water heater. This gives the appliance a right-sized, insulated path so it drafts properly and condensation is contained in a corrosion-resistant channel. This is the most common fix and usually the cheapest.
2. Replace the Water Heater With a Power-Vent Model
A power-vented gas water heater uses a blower and PVC piping to exhaust out a sidewall, abandoning the chimney entirely. Good choice when the water heater is already 8+ years old, or when the chimney is in poor enough shape that you want out of it altogether. Requires an electrical receptacle nearby.
3. Move to a Tankless or Heat Pump Water Heater
A sidewall-vented tankless unit solves the venting issue and adds endless hot water. A heat pump water heater eliminates combustion venting completely — no flue, no gas, and it dehumidifies the basement as a side effect. Either path also positions you for future electrification if you are considering a heat pump for heating and cooling down the road.
4. Common-Vent With the New Furnace
Only applies if the new furnace is a mid-efficiency (80% AFUE) unit that still uses the chimney. In that case the shared flue may be fine, but it must be re-sized per the venting tables for the new combined input. This is not an option with condensing furnaces.
2026 Cost Ranges
| Solution | Installed cost | Notes |
|---|---|---|
| Aluminum liner, 4″, to water heater | $950–$1,900 | Most common fix; price depends on chimney height and access |
| Stainless liner, insulated | $1,600–$3,200 | Longer life; required in some deteriorated flues |
| Power-vent water heater, 40–50 gal | $2,600–$4,200 | Includes new PVC venting and electrical |
| Tankless conversion | $4,200–$7,500 | Gas line upsizing often needed |
| Heat pump water heater, 50–65 gal | $3,400–$5,800 | Needs a 240V circuit and condensate drain |
| Chimney masonry repair / rebuild top | $1,200–$6,500 | Only if structure is already failing |
Add $250–$600 if the chimney needs cleaning and a camera inspection first, which we recommend on any flue that has not been looked at in a decade.
How to Tell If Your Water Heater Is Orphaned
Walk down to your mechanical room and check three things:
- Does the furnace vent through white PVC out a sidewall? If yes, it is a condensing furnace and it is not using the chimney.
- Does the water heater vent through a metal pipe into a brick chimney? If yes, it is alone in there.
- Is there a liner visible at the top of the chimney? Look for a round metal cap and pipe rather than an open clay tile.
If the answer is PVC furnace + metal-to-brick water heater + no visible liner, you likely have an orphaned appliance. A simple field test confirms it: with the basement door closed and the bath fans and dryer running, hold a lit match near the water heater’s draft hood while it fires. If the smoke pushes out toward you instead of being drawn up, it is backdrafting.
Carbon Monoxide Protection Is Not the Fix
You should absolutely have working CO alarms within 15 feet of every sleeping area and one in the mechanical room — Illinois requires them. But an alarm is a warning device, not a repair. If your water heater is spilling exhaust, fix the venting.
What a Correct Furnace Changeout Looks Like
When MMDS replaces a furnace in an Evanston or Des Plaines home with a shared masonry flue, the scope includes:
- Combustion-air and venting evaluation for the remaining appliances
- Chimney interior inspection before the old furnace comes out
- A liner or vent-conversion line item in the proposal, priced up front — not discovered mid-job
- Draft and CO testing on the water heater after the new furnace is commissioned
- Condensate routing for the new furnace to an approved drain, with a neutralizer where required
You can see our full scope on the furnace installation and replacement page, and our plumbing installation and repair team handles the water heater side so the whole job is one coordinated visit.
Frequently Asked Questions
My furnace was replaced five years ago and nothing bad has happened. Am I fine?
Not necessarily. Marginal draft is intermittent — it depends on outdoor temperature, wind, and whether exhaust fans are running. Many orphaned heaters draft acceptably in mild weather and spill on the coldest, windiest days. Meanwhile the chimney is quietly deteriorating from condensation.
Is a liner required by code?
Illinois mechanical code requires vents to be sized per the manufacturer’s and code venting tables. An oversized flue serving a single small appliance generally fails those tables, which is why a liner or vent conversion is the compliant fix.
How long does the work take?
A liner installation is a half-day job in most homes. A power-vent or tankless conversion is typically one full day.
Should I just replace the water heater instead of lining the chimney?
If the tank is under 6 years old, line the chimney. If it is 8+ years old, putting the money toward a power-vent, tankless, or heat pump water heater is usually the smarter spend, since you would be paying for the venting fix twice otherwise.
Will my homeowners insurance care?
Insurers do not typically inspect for this, but a CO incident traced to improper venting is the kind of claim that gets scrutinized. Buyers’ inspectors, on the other hand, catch it regularly.
Can I finance this?
Yes — Sunbit and PowerPay financing are available, including when the venting fix is bundled with a furnace or water heater replacement.
Get Your Venting Checked
If your furnace vents out the wall and your water heater still goes up the chimney, it is worth 30 minutes of a technician’s time to test the draft and look at the flue. We serve Evanston, Des Plaines, and the surrounding suburbs.
Call (847) 221-6280 or schedule an inspection. If the venting is fine, we will tell you that plainly. If it is not, you will get a fixed price on the correct repair before any work starts.