Furnace guide
Furnaces fail in predictable ways: igniters and sensors quit, blowers wear out, and eventually heat exchangers crack. This guide explains what causes failures, how to read your symptoms, which ones are safety issues, and what replacement actually involves.
The causes
Most foundation damage traces back to one of four sources. Identifying the cause determines which repair approach applies and whether other work, like drainage correction, needs to happen first.
The heat exchanger expands and contracts through every heating cycle for decades. Cracks eventually form, which can let combustion gases reach household air. A confirmed crack is the most common reason an older furnace is condemned.
Hot surface igniters, flame sensors, and pressure switches are consumable parts. They cause most no-heat calls and are usually economical repairs on a furnace with remaining life.
The blower runs for both heating and cooling, so it ages faster than the burner side. Failing bearings and capacitors announce themselves with noise before they quit entirely.
Furnaces last 15 to 20 years on average. As they age, efficiency drops below the nameplate rating, repairs cluster, and replacement parts for older models get harder to source.
Urgency
Foundation problems exist on a spectrum. Most homeowners either underreact to serious movement or overreact to cosmetic cracks. Here is how to read the difference.
The furnace heats reliably, is under 12 years old, and shows no combustion warning signs. Change filters, keep annual service, and note the age so replacement is planned rather than forced.
Symptoms that point to declining performance or an aging unit. Get quotes while the furnace still runs, because emergency replacements in a cold snap cost more and offer fewer choices.
Combustion and safety symptoms need same-day attention, and a dead furnace in freezing weather is its own emergency for pipes and occupants.
Identify your problem
Choose the closest match. The goal is not to diagnose from a screen, it is to figure out what information to collect before pricing or calling a contractor.
The furnace will not start, or it starts and shuts down before producing heat. The thermostat calls but nothing happens.
Often an igniter, flame sensor, or control board repair on a younger furnace. On a unit past 15 years, get the repair quote and a replacement quote side by side.Air moves but never warms up, or the furnace starts and stops every few minutes without holding temperature.
Could be a sensor, a clogged filter, or an oversized or failing furnace. Recurring short cycling on an old unit accelerates wear on the parts that cost the most.The burner flame is yellow or flickering instead of steady blue, soot appears around the furnace, or a carbon monoxide alarm has gone off.
Treat this as a safety stop. Shut the furnace down and get same-day professional service. Combustion problems are the one furnace symptom that should never wait.A technician has condemned the furnace for a cracked heat exchanger, often during a service or tune-up visit.
A cracked exchanger on an older furnace usually means replacement, since the repair approaches replacement cost. It is also the diagnosis most worth a second opinion before spending five figures.The furnace is 15 or more years old and the air conditioner is on a similar clock. You are facing two replacements within a few years.
Replacing both together saves $500 to $2,000 in combined labor and gets matched equipment. It is also the natural moment to price a heat pump against the pair.The furnace still runs but is past 15 years old, repairs are becoming annual, and heating bills keep climbing.
Planned replacement beats emergency replacement on price and choice. This is the right time to compare standard versus high-efficiency models on your actual fuel usage.What gets fixed
Foundation repair is not one thing. The right method depends on what is causing the problem and how far it has progressed. Each approach has a different scope, cost range, and set of exclusions.
Igniters, flame sensors, capacitors, pressure switches, and control boards are repairable failures. On a furnace under 12 years old, these repairs usually make clear financial sense.
Best when the diagnosis is a single failed part and the heat exchanger is sound.Pricing detail in the cost guideSwapping an aging gas furnace for a new standard-efficiency (80 percent AFUE) model using existing venting and ductwork. The most predictable replacement path in mild and moderate climates.
The default when existing venting meets code and fuel costs are moderate. Equipment for standard models runs $2,300 to $3,700 before labor.Pricing detail in the cost guideCondensing furnaces at 95 percent AFUE and above wring more heat from the same fuel but require PVC venting and condensate drainage, which adds installation scope on top of the pricier equipment.
Strongest in cold climates with high fuel costs. Expect the venting change to be itemized in an honest quote.Pricing detail in the cost guideReplacing both sides of the system at once with matched equipment. Bundling saves $500 to $2,000 in combined labor and avoids pairing a new furnace with a dying coil and condenser.
Makes sense when both units are past 12 to 15 years. This is also the natural moment to price a heat pump against the pair.Pricing detail in the cost guideReplacing an oil or electric furnace with gas, or replacing the furnace concept entirely with a heat pump. Conversions add gas line, electrical, or removal scope beyond a standard swap.
Driven by local fuel prices and incentives. Oil-to-gas and electric-resistance-to-heat-pump switches usually have the strongest operating cost case.Pricing detail in the cost guideNext steps
Use the resource that matches where you are in the process.