How much does it cost to replace a flue pipe?

How much does it cost to replace a flue pipe?

A backdrafting water heater or furnace flue can let carbon monoxide into the house, and rust on a metal flue can mean it isn't drafting. Get a licensed plumber or heating, ventilation and air conditioning (HVAC) technician to find the cause and price the fix in writing before your deadline. A water heater flue is a small job: HomeLintel puts it at $100 to $350, and Modernize puts a furnace flue at $300 to $800. If the flue backdrafts, the cause sets the price, and that can run into the thousands.

Key takeaways

• A rusted water heater vent on a flue that drafts is the cheap case: a diagnostic visit ($75 to $150, Modernize's furnace figure) plus the pipe ($100 to $350, HomeLintel).
• The International Association of Certified Home Inspectors (InterNACHI) doesn't require inspectors to look inside flues, so negotiate from the technician's written finding.
• A water heater left alone on an old furnace flue may be most cost-effective to replace.

What do backdrafting and a rusted flue mean?

A flue carries exhaust from a furnace or water heater out of the house. Backdrafting is when that exhaust comes back inside instead. InterNACHI names the causes: exhaust fans, fireplaces and dryers pulling indoor air pressure down, or a flue that is badly configured or blocked.

Tim Rausch Plumbing, an Appleton, Wisconsin plumber, adds two more. One is a vent pipe with too long a horizontal run. The other is a house short on combustion or makeup air, where a single bathroom fan can be enough to make a water heater backdraft.

A common version is the "orphaned" water heater. It used to share a flue with an older furnace. When the furnace was replaced with a high-efficiency model that vents on its own, the water heater stayed on a flue sized for the bigger appliance. Alone, its exhaust isn't hot enough to draft, per InterNACHI.

That's where rust comes in. Without a proper draft, water vapor in the exhaust can condense on the flue walls, and in a metal flue that can cause corrosion, InterNACHI says. A dark residue on top of the water heater, or scorched or melted material around its draft diverter, are other signs InterNACHI links to backdrafting. No soot doesn't rule it out.

Is backdrafting dangerous?

Yes. The Environmental Protection Agency (EPA) lists "back-drafting from furnaces, gas water heaters, wood stoves and fireplaces" as a source of carbon monoxide. It calls carbon monoxide "an odorless, colorless and toxic gas" and warns that "CO can kill you before you are aware it is in your home." InterNACHI says an orphaned water heater showing signs of backdraft "should be corrected by a professional immediately."

If you suspect carbon monoxide poisoning, the Consumer Product Safety Commission says to "get outside to fresh air immediately, and then call 911." After that, call a licensed professional. The house isn't yours yet, so send the finding to your agent in writing today.

How is it fixed, and what does it cost?

Each cause has its own fix.

• Exhaust fans pulling air out of the house: a makeup air duct, per Tim Rausch Plumbing.
• A long horizontal vent run: move the water heater closer to the chimney, or switch to a Type B vent connector, which heats up faster and helps the draft, per the same plumber.
• An orphaned water heater: a properly sized flue, a metal liner in a masonry chimney, or a different water heater. Many orphaned water heaters are near the end of their life, and InterNACHI says a new electric or high-efficiency one may be the easiest and most cost-effective fix.
• A rusted pipe with no draft problem: replace the pipe, once the technician confirms the vent drafts.

Kept prices every line of an inspection report for $79, usually under 4 hours, and marks the ones that need a professional quote.

Here are the two ends for a water heater flue. If the technician finds a rusted pipe and a sound draft, you pay for a diagnostic visit, at Modernize's furnace figure, and HomeLintel's pipe: $75 + $100 = $175 at the low end, $150 + $350 = $500 at the high end. If a new water heater's cooler exhaust can't rise up an old masonry chimney, one San Jose client of that Bay Area installer paid $4,800 for an emergency liner, on top of the tank.

Who checks it before your deadline?

Call a licensed plumber or HVAC technician for a gas water heater or furnace flue. Call a chimney sweep for a wood stove pipe or a masonry chimney. Your inspector's note starts the process but isn't the diagnosis. InterNACHI's Standards of Practice say an inspector is "not required to: inspect, measure, or evaluate the interior of flues or chimneys." InterNACHI also says sizing a flue is beyond the inspector's job and belongs to a specialist.

Ask for a written finding that covers four things:

• Whether the appliance backdrafts with the exhaust fans and dryer running, the worst-case test InterNACHI describes, using smoke or a flame at the draft diverter
• The cause, and whether the rust comes from it
• The fix, with a price
• If the water heater is orphaned, a price for a new water heater too

The worst-case test matters. Tim Rausch Plumbing says a water heater can draft fine for a technician on a quiet visit and still backdraft once the fans are on. For how fast you can get a bid, see how to get repair estimates during the inspection period.

What should you ask the seller for?

Ask for the fix your technician recommends, at the technician's written price. Name the scope, the trade and the bid in the request. If the seller has the work done, ask for the invoice showing what was replaced.

What to ask for after a home inspection covers how this line fits your list, and the buyer repair request letter template gives the wording around it.

What if the seller says no?

Check your inspection contingency. Your contract sets whether you can cancel and by when, so confirm the dates with your agent. Before you decide, set the seller's answer against the technician's written price. When to walk away after a home inspection covers how to weigh it.

Does a rusted flue or stove pipe need to be replaced?

Don't just swap it. HomeLintel says rust at the draft hood or on the connector is a common sign of backdrafting or condensation, so the venting should be assessed first.

A stove pipe is the sweep's call. The Chimney Safety Institute of America follows the fire safety standard NFPA 211, which calls for chimneys, fireplaces and vents to be inspected at least once a year for soundness, deposits and correct clearances. Ask the sweep to say in writing whether the pipe is sound, and to price the repair if it isn't.

What if my report has other heating findings?

Work through them in heating and cooling findings on an inspection report. Put a price on every line before you send the request.

If the flue is one line on a long report, send the whole report to Kept and spend your remaining days getting the technician's bid.

Sources

• InterNACHI, "Backdrafting"
• InterNACHI, "Orphaned Water Heaters"
• InterNACHI, Home Inspection Standards of Practice
• Environmental Protection Agency, "Carbon Monoxide's Impact on Indoor Air Quality"
• Consumer Product Safety Commission, Carbon Monoxide Information Center
• Chimney Safety Institute of America, homeowner resources
• HomeLintel, "Water Heater Flue Pipe Replacement Cost"
• Modernize, "How Much Does it Cost to Repair a Furnace?"
• Better Water Heaters, "Water Heater Backdrafting"
• Tim Rausch Plumbing, "Why Water Heaters Backdraft, How to Fix"