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Recommendation · Hire a Professional

Why Is My Furnace Blowing Cold Air, or Not Heating at All?

A brief cold blast at startup is normal. Cold air that never warms up isn't, and because gas furnaces involve combustion and carbon monoxide safety, the safe DIY checks here are genuinely limited.

DIY time
5–10 min
Cost range
$0–3,000+
Difficulty
Easy
Risk
High Risk

1

Before you start

Smell gas, or is a carbon monoxide alarm sounding? Leave now

Get everyone and the pets outside right away, leave doors open if you can, and don't use light switches, phones or anything electrical on the way out. Call your gas utility's emergency line or 911 from outside. Have a working carbon monoxide detector on every floor, and never use the oven or burners to heat the house.

Diagnose first, then act.

Two checks are safe, free, and worth doing before anything else. Beyond those, this is a category where knowing when to stop matters as much as knowing what to check. Gas furnaces involve combustion, ignition systems, and a real carbon monoxide risk if something's genuinely wrong.

2

Decision matrix

Every likely cause, with its own real cost and time, not one blended guess for the whole repair.

Decision matrix: cause, approach, cost, and time for Furnace Blowing Cold Air or Not Heating
Cause Approach Cost Time
Thermostat Set to “Fan On” Instead of “Auto” DIY $0–5 5 min
Dirty Air Filter Restricting Airflow DIY $10–20 5–10 min
Pilot Light Out or Ignition Failure Professional $75–300 30–90 min
Faulty Flame Sensor or Gas Valve Professional $50–300 30–90 min
Cracked Heat Exchanger Professional $500–3,000+ 30–90 min

3

Causes

Tap a cause to open it. Only one stays open at a time, so it's easy to compare.

4 · FAQ

Frequently asked questions

  • Is a short blast of cold air at startup normal?

    Yes. The blower can start before the burners fully ignite, pushing out air that was sitting in the ducts. If it never warms up, that's the actual problem.

  • Can I relight the pilot light myself?

    Many manufacturers post relighting instructions on a label inside the furnace panel, and some homeowners do this themselves. If you smell gas, you're not fully comfortable with it, or the pilot won't stay lit, it's a reasonable and safe choice to call a technician instead.

  • How serious is a cracked heat exchanger?

    Serious enough that most technicians recommend not running the furnace until it's inspected. It's a carbon monoxide risk, not just a comfort issue.

  • What should I do if my carbon monoxide detector goes off?

    Leave the house immediately and call your gas utility's emergency line or 911 before calling anyone else, including a furnace technician.

  • Why does my furnace run constantly without reaching temperature?

    This can point to several of the causes above, or a system that's undersized or struggling with airflow. A technician's diagnostic visit is the fastest way to narrow it down rather than guessing.

  • Is it worth replacing an old furnace instead of repairing a major failure?

    Often yes, on furnaces over about 15 years old. A major component failure like a heat exchanger frequently costs enough relative to a new unit that replacement becomes the more sensible option, and most technicians will present that comparison directly.

  • Why does annual furnace inspection matter?

    A furnace inspection before heating season typically costs far less than an emergency repair, and it's specifically designed to catch problems like a cracking heat exchanger or a failing ignition component before they become a no-heat or safety emergency in the middle of winter.

References

Still deciding

Two other questions worth answering

This guide covers whether you can do the job yourself. These cover the two questions that often come before and after it.

Repair it or replace it?

Whether this is worth fixing at all, based on age, repair cost, and how much life it has left.

What should this cost?

Parts pricing and labor rates for your state, so you can check a quote line by line.

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