Why Does a Furnace Limit Switch Keep Tripping?
A furnace limit switch keeps tripping because the furnace is running too hot, and Carrier says it typically trips because of severely restricted airflow across the heat exchanger. Dirty filters, blocked return grilles, too many closed supply registers, a struggling blower motor and a dusty air conditioning coil above the furnace are the causes Carrier names. The switch is doing its job by shutting the burners off before heat damages the furnace.
What the furnace does after repeated trips is worth knowing too. Carrier's troubleshooting guide for one of its two-stage 80% furnaces shows that if the limit circuit stays open longer than 3 minutes, or trips 10 times in a row during high heat, the control locks the furnace out and waits 3 hours before resetting itself. That same circuit includes the flame rollout switch, which needs a manual reset. This guide explains what the limit switch does, what trips it, the warning signs of a failing switch, and what you can check before calling for service.
What Does a Furnace Limit Switch Do?
A furnace limit switch is a temperature-sensing safety switch that turns the blower on and off at the right times and shuts down the burners if the furnace overheats. Trane explains that it monitors the temperature inside the supply plenum, the chamber where heated air leaves the furnace. If that temperature climbs too high, the switch cuts gas to the burners and keeps the blower running to cool things down. The gas valve reopens only after the temperature falls back to a safe level.
Carrier describes how a common style of limit switch behaves, using a bimetallic probe that reacts to heat:
Around 130°F, the switch closes the blower circuit so only warm air reaches your rooms.
After the burners shut off, the blower keeps running until the temperature drops to around 100°F.
If the temperature passes the upper safety limit, often around 200°F, the switch opens the gas valve circuit and stops the flame.
Exact settings vary by furnace, and Carrier notes that a replacement must match the original's temperature ratings and probe length.
What Makes a Furnace Limit Switch Trip?
Carrier lists these causes, all tied to airflow:
Dirty or clogged air filters. A blocked filter starves the furnace of return air, and the heat exchanger overheats.
Blocked or closed vents. Closing too many supply registers or pushing furniture against return grilles upsets the airflow the system was designed around.
Blower motor problems. A dirty blower, a failing capacitor or a broken motor cannot move air across the heat exchanger.
A dirty evaporator coil. If you have central air, the indoor coil sits above the furnace, and a dusty coil acts as a barrier to airflow.
That matters for anyone who shuts vents in unused rooms to save energy. Carrier's point is that closing too many registers upsets the balanced airflow the furnace was designed around, which can push it past its limit. The guide to improving airflow in your home explains how supply and return air need to stay balanced.
Trane recommends replacing the furnace filter every 1 to 3 months depending on the filter, and keeping furniture and curtains away from registers and returns. The guide to how often to change HVAC filters covers the details.
What Are the Signs of a Bad Furnace Limit Switch?
A switch can fail in two directions, and the symptoms differ.
When it sticks closed, Carrier says the blower may run constantly because the switch thinks the furnace is still hot. When it sticks open, the switch tells the control the furnace is overheating, and the furnace will not ignite at all.
Trane and Carrier both list these warning signs:
The furnace stops abruptly in the middle of a cycle
The blower will not shut off
The furnace runs but does not blow hot air
Short cycling, where the furnace heats briefly and shuts down before warming the house
A furnace that blows cool air has several possible causes beyond the limit switch, and the guide to a furnace blowing cold air walks through them.
What Happens When the Limit Switch Trips Over and Over?
On the Carrier furnace covered in its troubleshooting guide, an open limit circuit shows as status code 33. The blower runs for at least 4 minutes or until the switch closes again. If the circuit stays open longer than 3 minutes, or if the furnace logs 10 straight limit trips in high heat, the code changes to 13, limit circuit lockout, and the furnace waits 3 hours before trying again.
Carrier's guide groups several switches in that circuit: the main limit, the draft safeguard, the flame rollout switch and a blocked vent switch on models that use one. Beyond dirty filters and restricted ducts, it lists restricted venting, a loose blower wheel, excessive wind and inadequate combustion air as causes to check. A tripped flame rollout switch requires a manual reset, and Carrier's guide ties it to inadequate combustion air.
The stakes go beyond comfort. Carrier explains that the limit switch is the failsafe that keeps the heat exchanger from overheating and cracking, and a cracked heat exchanger can leak carbon monoxide into the home.
What Can You Check Before Calling for Service?
These steps are safe and address the airflow causes Carrier lists:
Replace the furnace filter, even if it was changed recently.
Open every supply register in the house.
Move furniture, rugs and curtains away from registers and return grilles.
If your central air coil sits above the furnace and has not been cleaned in years, mention that when you schedule service.
Trane is clear on what not to do: do not try to bypass or reset the limit switch yourself. If the furnace still trips after the filter and vents are sorted, call an HVAC professional. Carrier adds that a switch should be replaced only if it fails a continuity test, shows damage or corrosion, or keeps tripping after the airflow problems are fixed. The guide to a furnace that won't stay on covers related shutdown causes.
How Much Does a Furnace Limit Switch Replacement Cost?
Trane estimates limit switch replacement at $100 to $375, depending on the furnace model, location, service call fee and whether it is an emergency repair. Carrier puts the part alone at about $15 to $75.
The bigger cost risk is replacing a good switch while the real problem remains. A new switch on a furnace with a clogged filter or a weak blower will trip just like the old one, which is why technicians check airflow before swapping parts.
FAQ
Can I reset a furnace limit switch myself?
Trane advises against it. Some rollout switches have a small reset button, but Carrier says repeated trips mean the overheating cause has to be found first.
Can a dirty filter make the limit switch trip?
Yes. Carrier and Trane both tie a clogged filter to overheating, because it cuts the return air the furnace needs.
Does closing vents in unused rooms cause problems?
It can. Carrier lists closing too many supply registers among the causes of a tripped limit switch.
How long does a furnace limit switch last?
Carrier says a quality limit switch is designed to last the life of the furnace, typically 15 to 20 years, though chronic overheating can wear it out sooner.
Is a tripping limit switch dangerous?
The trip itself is the furnace protecting itself. Ignoring the cause is the risk, since Carrier warns that repeated overheating can crack the heat exchanger.
Why does my furnace blower keep running?
A limit switch stuck in the closed position can keep the blower running, according to Carrier. A fan setting of On at the thermostat does the same thing, so check that first.
Van Bremen's Heat & Air Fixes Furnaces That Overheat
Van Bremen's Heat & Air repairs residential and commercial furnaces from its office at 214 Stoystown Rd in Somerset, serving Somerset, Fayette, Cambria and Westmoreland Counties and Morgantown, WV. A technician can test the limit circuit, check the blower and filter, and measure airflow so the fix lasts. See the heating services page and maintenance services page for more.
Visit Van Bremen's Heat & Air or call (814) 483-1735 today. Contact us for more information.
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