Key takeaways
- Short answer: A furnace that turns on and off every few minutes is short cycling.
- Start with a new air filter
- Dirty flame sensors are a top cause
- Oversized furnaces short cycle by design
- Typical hvac cost: $7,000–$12,000 for a combined AC and furnace replacement.
Common causes of furnace short cycling
- Clogged filter: restricted airflow overheats the heat exchanger and trips the high-limit switch.
- Dirty flame sensor: the furnace lights, then shuts off because it can't confirm a flame.
- Thermostat placement: a thermostat near a supply vent or heat source reaches temperature too quickly.
- Blocked vents or flue: closed registers or a blocked exhaust trip safety controls.
- Oversized furnace: heats the house too fast, wasting fuel and wearing parts.
What it costs to fix
| Fix | Typical cost |
|---|---|
| New air filter | DIY |
| Thermostat replacement | $150–$500 |
| Blower motor replacement | $300–$2,000 |
| Furnace replacement | $2,000–$6,900 |
Flame sensor cleaning is a routine part of a tune-up. If the heat exchanger is cracked or the furnace is badly oversized, replacement may be the better value.
What you can safely check
- Replace the air filter.
- Open all supply and return vents.
- Check the outdoor exhaust and intake pipes for snow, leaves or nests.
- Make sure the thermostat isn't in sun or above a vent.
Short cycling with a burning smell or a carbon monoxide alarm is an emergency. Shut the furnace off and call a technician.
Related questions
Is furnace short cycling dangerous?
It's usually a safety shutdown doing its job, but repeated overheating can crack a heat exchanger, which can leak carbon monoxide. Have it checked.
How often should a furnace cycle?
Roughly 3–8 cycles per hour is common in cold weather. Cycles of just a minute or two are a sign of a problem.
Can a dirty filter really cause short cycling?
Yes. It's the most common cause, and the cheapest fix.
Other hvac questions homeowners ask
Should I replace my AC and furnace at the same time?
Often yes when both are over 12–15 years old. A matched system runs more efficiently, and you pay for one installation visit and one permit instead of two.
How much does a new AC unit cost for a 2,000 sq ft house?
Most 2,000 sq ft homes need a 3–4 ton system. A central AC replacement alone typically runs $2,500–$9,000 nationally, and replacing both the AC and furnace costs about $7,000–$12,000.
Is it worth repairing a 15-year-old AC?
Usually not. Use the $5,000 rule: multiply the unit's age by the repair cost. If the result is over $5,000, replacing is usually the better value. A $400 repair on a 15-year-old unit scores $6,000.
HVAC at a glance
Timeline
Most AC or furnace swaps take one day. A full system with new ductwork takes 2–5 days.
Permits
Replacing an AC, heat pump or furnace usually needs a mechanical permit, and the installer normally pulls it.
DIY or pro?
Filters, clearing debris from the outdoor unit and flushing the condensate line are safe DIY jobs. Refrigerant, gas and electrical work need a licensed technician.
What HVAC repair and replacement costs
Replacing a central AC and furnace together typically costs $7,000–$12,000 installed. An AC alone runs about $2,500–$9,000 and a furnace $2,000–$6,900, depending on size, efficiency and brand. Most single repairs cost $250–$700.
Keep reading
Sources: PickHVAC — Furnace & AC replacement cost. Ranges are rounded estimates; see our editorial policy.