Symptom
Furnace keeps turning on and off?
A furnace that runs for a few minutes, shuts off and restarts over and over is short cycling. Common causes are a clogged filter making it overheat, a dirty flame sensor, a thermostat problem, or a furnace that is too large for the house. Replace the filter first; if it continues, it needs diagnosis, because repeated overheating wears the furnace.
Worth booking. Usually not an emergency, but repeated overheating stresses the heat exchanger. Book a visit rather than living with it.
What usually causes this.
These are the common possibilities, not a diagnosis. A page cannot tell which one it is — that takes a technician testing the system.
- Thermostat
- The house itself
- Filter & return
- Heat section
- Blower
- Venting
- Overheating from restricted airflow
- A clogged filter, closed vents or a weak blower lets heat build up until the limit switch shuts the burners off. It cools, restarts, and repeats.
- Dirty flame sensor
- Burners light and shut off within seconds, several times in a row.
- Thermostat placement or settings
- A thermostat near a supply vent, in direct sun, or with an incorrect cycle setting can shut the furnace off early.
- Oversized furnace
- A furnace much larger than the house needs heats it in short bursts. It's hard on the equipment and leaves rooms uneven.
- Venting or pressure-switch problem
- If the furnace can't prove its venting is clear, it shuts down for safety.
What you can safely check.
- Replace the filter and make sure all vents and returns are open.
- Check that the thermostat isn't next to a supply vent, a lamp or in direct sunlight.
- Note how long each cycle lasts — seconds or minutes. It tells the technician a lot.
Leave these alone
- Adjusting limit switches, gas pressure or burner settings.
- Blocking off supply vents to push heat elsewhere — it makes overheating worse.
Call rather than book online if:
- A carbon monoxide alarm sounds
- You see soot, scorch marks or a yellow, flickering flame
- You smell gas
What we check when we get there.
In order, and every time — the point of a diagnostic is to find out which cause it is before anything is replaced.
- 01The technician times the cycles, measures temperature rise and static pressure, and checks the limit, flame sensor, inducer and venting.
- 02The heat exchanger is inspected, since repeated overheating is exactly what cracks one.
What it is likely to cost.
A filter or thermostat fix costs little. Flame sensor, pressure switch and blower repairs are priced before work. If the furnace is badly oversized, the long-term answer is at replacement time, sized with a load calculation.
- Heating diagnostic
- $89
- Flame sensor
- From $149
- Pressure switch
- From $199
- Blower motor
- From $499
Prices are starting points for standard work in the Savannah area. Final price depends on the system and site, and is confirmed before work begins. Permits and code-required upgrades may be additional.
Questions people ask about this.
- How long should a furnace run each cycle?
- It varies with the weather and the house, but repeated cycles of only two or three minutes are a sign something's wrong.
- Can short cycling damage my furnace?
- Yes. Frequent starts wear the igniter and other parts, and repeated overheating stresses the heat exchanger.
Work out the decision:System Sizing Guide
Further reading:Which air filter, and how often, in a humid house?Getting a furnace ready for its first cold night
The service that fixes it.
Heating & Furnace Repair is the work behind this symptom. Its page covers what is involved, how it is quoted and what we stand behind.
- 01Heating & Furnace RepairFurnace not lighting, blowing cold, or cutting out early? We get heat back and check it is safe.
- 02Maintenance PlansTwice-yearly service that catches small faults before they become July breakdowns.
- 03AC Installation & ReplacementNew and replacement cooling systems, sized to the house rather than to whatever was there before.
Symptoms that sit next to it.
Systems rarely fail in one way only. If one of these is also true, say so when you book — it narrows the diagnosis before anyone arrives.
Furnace or heat pump not heating, blowing cold air, cycling or running up the bill.
- Furnace won't turn on or lightA gas furnace that won't light is most often a worn igniter, a dirty flame sensor, a clogged filter tripping a safety limit, or a thermostat or power problem.
- Furnace blowing cold airA furnace blowing cold air is often a thermostat set to fan On rather than Auto, a furnace that has shut its burners off on a safety limit, or an ignition problem.
- Heat pump not heatingA heat pump that runs but doesn't heat well may have a thermostat set incorrectly, a dirty filter, an iced-up outdoor coil, a low refrigerant charge, or a reversing valve or defrost problem.
- Heat pump iced up or stuck in defrostIn cold, damp weather a heat pump's outdoor coil collects frost, and the system runs a defrost cycle to melt it — steam and a few minutes of cooler indoor air are normal.
- Aux heat always on / high winter billAuxiliary heat is the backup — usually electric resistance — that helps a heat pump in very cold weather or during defrost.
Not sure which it is?Describe it to the assistant
Everything else we are called about.
The whole index, in three groups.
HUMIDITY, AIRFLOW & ENERGY BILLS
House feels humid or dampSome rooms too hot or coldPower bill suddenly higher
Get it looked at.
We cover Savannah, the Coastal Empire and the South Carolina Lowcountry. Tell us what the system is doing — you do not need to know the cause. The diagnostic price is given to you before the visit, and the repair price before any work starts.
No heat, or no cooling at all?Call rather than send a request — (912) 858-3737 is answered around the clock.