Why is my furnace not heating?

A furnace that won't heat is most often a dirty filter choking airflow, a failed igniter, or a thermostat problem — in that order of cheapness. A furnace that starts, runs a minute, then stops is usually a dirty flame sensor. True no-heat in deep cold justifies the after-hours call; a mild-weather failure usually doesn't.

Likely causes

• Clogged filter — Restricted airflow overheats the furnace until the safety switch shuts it down.
• Failed igniter — Modern furnaces light with a hot-surface igniter that cracks every 5–7 years. Clicking with no flame points here.
• Dirty flame sensor — The furnace lights, runs a minute, and quits — the sensor can't prove flame, so it shuts down safely.
• Thermostat or power issue — Dead thermostat batteries, a tripped breaker, or an open furnace door switch — check before you pay for a visit.

What you can check yourself

• Replace the filter, and confirm the thermostat is on 'heat' with fresh batteries if it uses them.
• Check the breaker and the furnace's own power switch (it looks like a light switch, usually near the unit).
• Make sure the furnace access door is fully closed — many units won't run with the door switch open.

Typical cost

Typical repairs in Washington, DC: service call $100–$250; igniter $150–$350; flame sensor cleaning/replacement $150–$300; blower motor $400–$800.

When to call a pro

Gas smell, a carbon monoxide alarm, or a furnace that short-cycles with a burning smell — leave and call from outside. Otherwise, no heat below ~40°F with kids or elderly at home counts as urgent.

HVAC pros in Washington, DC

• Ubuntu Air Duct Cleaning & Service · 1101 14th St NW Ste 210, Washington, DC · +12407905501
• Nhr Ac & Heat A Ductless Mini Split Boutique · 1300 I St NW, Washington, DC · +12026702779
• Cooling Tower · 1300 Pennsylvania Ave NW, Washington, DC · +18448884468
• A1a Complete Ac Services · 1432 U St NW, Washington, DC · +12026524506
• AC Repair · 2015 14th St NW, Washington, DC · 2029858885
• Riley Heat And Air · 906 I St NW, Washington, DC · 12028880737
• DC HVAC · 1126 11th St NW #203, Washington, DC · 2028043697
• Fresh Air Duct Services · 1310 Wisconsin Ave NW, Washington, DC · +18772516344

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Data by Servik — verified local pros from official license records and public sources. Cost figures are typical ranges, not quotes; a pro's written estimate is the real number.