What is an orphaned water heater, and why can a new furnace mean a chimney liner?
An orphaned water heater is a gas water heater left venting alone into a chimney it shared with an 80% furnace. In older Minneapolis and Saint Paul houses, a new 90%+ furnace moves to PVC, the flue runs cold, and the usual fix is a liner, about $1,500–$4,000 nationally.
Key points
- It happens most in older Minneapolis and Saint Paul houses where the furnace and water heater shared one brick chimney.
- A condensing furnace vents through PVC, so the old flue is now too big and too cold for the water heater's small, cooler exhaust.
- The results: weak draft, condensation that eats mortar and tile, exhaust spilling at the draft hood and a carbon monoxide risk.
- The code Minnesota adopts says a chimney must be resized when an appliance is permanently disconnected from it.
- Fixes: a properly sized metal liner, a water heater that doesn't use the chimney, or keeping an 80% furnace on a lined flue.
An orphaned water heater is a gas water heater left venting alone into a chimney that used to carry an 80% furnace's exhaust too; once the furnace moves to PVC, the oversized flue runs cold and drafts poorly, and the usual fix is a metal liner sized for the water heater. A furnace quote in an older Twin Cities house sometimes includes that liner for an appliance nobody is replacing. It isn't an upsell: it repairs the vent the furnace just walked away from.
How does a water heater end up orphaned?
In a lot of Minneapolis and Saint Paul houses built before the 1960s, the furnace and the water heater both connect to one brick chimney. The furnace's hot exhaust kept that chimney warm, and the water heater rode along on the draft.
A condensing furnace extracts most of the heat from its exhaust, so what leaves it is cool and carries a lot of water. That gas can't be sent up a masonry chimney, and it exits through PVC pipe in a wall or the roof instead. Our 96% vs 98% AFUE guide covers why condensing furnaces vent that way.
What's left is a small water heater burner, sending modest heat into a big masonry flue. On an outside wall in January, that flue is cold.
Why does an oversized, cold chimney flue cause trouble?
Chimney draft comes from hot gas rising. The larger and colder the flue, the longer the exhaust takes to warm it, and the weaker the draft. Two problems follow:
- Condensation. Water vapor is one of the main products of burning natural gas. When it hits a flue wall colder than its dew point, it condenses. That water is slightly acidic, and over time it softens mortar, cracks clay tile, rusts the vent connector and drips back onto the water heater. The white crust and rust you sometimes see at the base of an old chimney is that water at work.
- Poor draft and spillage. A water heater with a draft hood relies on the chimney pulling exhaust up and away. When the draft is weak, some of the exhaust spills out of the draft hood into the basement instead. That exhaust carries moisture and can carry carbon monoxide.
Tight houses make it worse. A kitchen range hood, a bath fan and a clothes dryer all pull air out of the house. If the chimney's draft is already weak, that suction can reverse it.
What does Minnesota code say about an orphaned water heater?
Minnesota adopts the 2018 International Fuel Gas Code by reference in Minnesota Rules part 1346.5050. The model text of that code, section 501.15, says that where an appliance is permanently disconnected from an existing chimney or vent, the chimney must meet the requirements that follow, and the first of them is plain: the chimney or vent "shall be resized as necessary to control flue gas condensation" and to give the remaining appliances the draft they need.
The City of Minneapolis spelled out how it applies this in a 2011 inspections policy sheet on existing chimneys and vents. When a gas furnace or boiler is removed from a chimney, the gas installer is responsible for verifying that any appliance still connected is sized to control condensation and get proper draft, and the city treats that as part of the new installation. The same sheet says an approved metallic liner must be installed in a masonry chimney used to vent gas appliances, and that unlined chimneys need a separate pre-inspection for approval to keep using them. Our Minneapolis and Saint Paul permit guide covers how permits work in both cities.
Water heater makers say the same thing in their manuals. Bradford White's installation manual for its atmospheric gas water heaters says the chimney must be lined and in good condition, and warns that when another appliance on the vent is removed, the venting system "may now be too large." It tells the installer to size the vent from the National Fuel Gas Code tables and to check for spillage at the draft hood once the heater has run for several minutes.
A Department of Energy Building America guide on high-efficiency furnaces makes the practical point: when the furnace leaves a shared vent, the remaining appliance's vent has to be checked against the vent tables, and exterior masonry chimneys will need relining. It adds that skipping this step underestimates the cost of the replacement.
How can you tell if your water heater is orphaned?
You can look for these without touching anything:
- The furnace was replaced with a condensing model and the water heater still connects to the brick chimney.
- Rust flakes on top of the water heater, or rust and scale collecting under it.
- Discolored or melted plastic near the draft hood, or scorch marks on the top of the heater.
- Water stains, damp spots or white deposits on the chimney in the basement, or loose mortar and spalling brick on the chimney outside.
- A damp, sour smell near the water heater when it runs.
- A carbon monoxide alarm sounding.
If a CO alarm goes off, everyone goes outdoors first and the call to 911 or your gas utility is made from there. Minnesota law requires working CO alarms within 10 feet of every sleeping room, and our CO detector placement guide covers where else they belong. For the furnace side of the same risk, see can a furnace leak carbon monoxide.
Don't try a draft test by blocking vents, and don't disconnect or patch the vent connector yourself.
The fixes
| Fix | How it works | When it fits |
|---|---|---|
| Metal chimney liner sized for the water heater | A stainless or aluminum liner runs down the flue and connects to the water heater. The smaller passage warms fast and drafts properly | The most common fix when the chimney itself is sound |
| Power-vent or direct-vent water heater | A fan pushes exhaust out through a wall vent, so the chimney is no longer used | No good chimney route, or a chimney you'd rather abandon |
| A water heater that doesn't need the chimney | A tankless gas heater with its own vent, or a heat pump water heater with no combustion at all | When the tank is near the end of its life anyway |
| Keep an 80% furnace on the chimney | The water heater keeps sharing the flue, which still has to be lined and sound | Houses with no workable PVC route, or a short stay in the house |
HomeGuide's oil-to-gas conversion guide lists a chimney liner as one line of that job at a typical national range of $1,500–$4,000 or more. For replacing the water heater instead, HomeGuide puts a tankless gas water heater at a typical national range of $2,100–$5,600 installed and a heat pump water heater at $2,000–$4,600 installed. Our tankless vs tank guide compares those options.
What we do on a furnace replacement
When we quote furnace replacement, we look at everything else on the chimney before we price the job. If the water heater would be orphaned, the fix goes in the written quote, so the price you sign is the price of a safe installation. On our installed ranges, an 80% furnace runs roughly $4,500–$6,000 and a 96–98% furnace roughly $6,500–$9,500, and the furnace replacement cost guide shows where venting work lands in that range. Our page on furnace installation in Minneapolis covers the older city houses where this comes up most.
If you already had a furnace replaced and the water heater still sits on the old chimney, have it looked at before the heating season's coldest weeks. Call 612-429-3465. We're open 7 days, 7am–9pm, and a service call is a $79 trip charge, waived when you approve the repair.
Common questions about heating
Does every water heater need a liner when the furnace is replaced?
No. It depends on the chimney, the water heater's size and what the vent tables allow. A water heater on a Type B metal vent often needs little change. An unlined or clay-tile masonry chimney, especially one on an outside wall, is the case most likely to need a liner. The installer should check the remaining vent as part of the furnace job and put the answer in the quote.
What are the signs a water heater isn't drafting?
Rust flakes or scale on top of the water heater or at its base, melted or discolored plastic near the draft hood, water stains or white crusty deposits on the chimney in the basement, crumbling mortar or a damp smell near the flue, and a carbon monoxide alarm going off.
Can I avoid the liner by keeping an 80% furnace?
Sometimes. If the new furnace stays on the chimney, the water heater isn't orphaned, but the chimney still has to be sound and, in Minneapolis, lined with metal to vent gas appliances. You also give up the gas savings of a condensing furnace for its whole life, which adds up over a Minnesota winter. We price both options when a house is a close call.
Who is responsible for checking the water heater when my furnace is replaced?
The installer. Minneapolis's inspections policy puts it on the gas installer to verify that any appliance left on the chimney is sized to control condensation and draft, and the Minnesota-adopted fuel gas code requires the chimney to be resized when an appliance is permanently disconnected from it. Ask for the water heater venting to be listed on the written quote.
Sources
- Minnesota Statutes 299F.51 (carbon monoxide alarms) revisor.mn.gov
- HomeGuide: oil-to-gas conversion costs homeguide.com