Furnace repair cost

Most furnace repairs cost $150 to $700. The failures that stop a furnace are usually cheap and predictable: an igniter, a flame sensor, a pressure switch. The one that ends the conversation is a cracked heat exchanger, and that is a replacement, not a repair.

A diagnostic visit is $80 to $200 and most contractors credit it against the work. On the first genuinely cold night of the year, expect the high end and expect to wait.

What the common repairs cost

Evening, weekend and holiday rates typically add 50% to 100%.

The two failures worth understanding

The flame sensor is a thin metal rod that sits in the burner flame and proves to the control board that the gas actually lit. It fouls with a film of oxide over a season or two, stops conducting, and the board shuts the furnace down after three or four attempts. This is the single most common no-heat call, and the fix is often cleaning a $25 part rather than replacing it. If you are quoted $600 for a flame sensor, you are being quoted for the visit and the story, not the part.

The heat exchanger is the wall between the burning gas and the air you breathe. A crack in it lets combustion products into the supply air, and carbon monoxide is the reason no technician will leave a cracked exchanger running. Out of warranty on a furnace over twelve years old, replacing the exchanger costs most of a new furnace and buys you an old furnace. Replace the furnace.

If a contractor condemns a heat exchanger, ask to see it: a camera down the burner compartment, a photograph, something. It is a serious diagnosis, and it is also the most common one used to sell equipment that did not need selling.

Repair or replace

Divide the repair by the years you realistically expect afterwards, and compare that to a replacement divided by eighteen.

A $4,500 furnace over eighteen years is $250 a year. A $350 igniter that buys five more years is $70 a year, obviously worth doing. A $1,100 draft inducer on a seventeen year old furnace that might give three more years is $367 a year, which is worse than replacing it.

Replace rather than repair when:

• The heat exchanger is cracked and out of warranty
• The furnace is past eighteen years and the repair is over about $1,000
• You have had two significant repairs in two seasons
• The furnace is a standing pilot model, which puts it well past twenty years
• You are replacing the air conditioner anyway, since the indoor coil sits on the furnace and doing both at once saves $800 to $1,200

What to check before calling

The thermostat. Batteries, and that it is set to heat rather than emergency heat or fan.

The breaker and the switch. A furnace has a wall switch that looks exactly like a light switch, usually at the top of the basement stairs. It gets turned off by accident more often than anyone admits.

The filter. A blocked filter overheats the exchanger and trips the limit switch, which produces a furnace that runs for a few minutes and shuts off, over and over.

The gas. If the stove lights, gas is on. If you have a propane tank, look at the gauge.

The condensate drain. A high-efficiency furnace makes water, and a blocked drain trips a safety switch that stops the furnace. A tray of water under the unit is the sign.

The flame, if you can see it. Blue and steady is right. Yellow, lazy or lifting away from the burner is a combustion problem, and that is a call, not a troubleshooting step.

Frequently asked questions

My furnace blows cold air. What is wrong?

Most often the burners are not lighting and the blower is running anyway, which points at the igniter or the flame sensor. Sometimes it is the thermostat set to "fan on" rather than "auto", which runs the blower continuously between heating cycles.

The furnace starts, runs a minute and shuts off, repeatedly.

That is short cycling on a safety, and the two usual causes are a dirty filter restricting airflow and a fouled flame sensor failing to prove the flame. Change the filter first; it is free and it is often the answer.

Is a cracked heat exchanger really dangerous?

Yes. It is the barrier between combustion gases and your supply air, and carbon monoxide is odorless. Do not run a furnace a technician has red-tagged, and put CO alarms on every floor regardless.

How long should a furnace last?

Fifteen to twenty-five years for a gas furnace, with the wide spread coming down to maintenance and how hard it has run. Past eighteen, weigh every repair against replacement.

Should I buy an annual service plan?

The maths usually works out roughly neutral, which means you are buying priority scheduling in a cold snap rather than savings. That can be worth it. Read whether it includes parts or only the visit.