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 Boston, MA: 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 Boston, MA
- Premier Mechanical · 220 Commercial St, Boston, MA · 6175809115
- Smart Click Energy · 175 Portland St Fl 4, Boston, MA · +16179734130
- Boston Heating & Air Conditioning · 276 Washington St #104, Boston, MA · 8008195184
- Colt Home Services · 110 Stuart St, Boston, MA · 6174463690
- Farina Corporation · 24 Spice St, Charlestown, MA · +16172420365
- Hunter & Sons AC & Cooling · 227 Cambridge St, Cambridge, MA · +13392205000
- Arlex Energy · 275 Massachusetts Ave, Boston, MA · +17818623400
- S Boston Plumbing Specialities · 15 Elkins St, Boston, MA · 6173159036
Get free quotes from Boston, MA pros →
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.