Is My Furnace Overheating?

A furnace that starts up, runs for a few minutes and quits before the house is warm may be overheating. Every modern furnace has a high-limit switch that watches the temperature inside the heat exchanger area. If the temperature climbs too high, the switch cuts off the burners, so what looks like a breakdown is often a protective shutdown.

Understanding why the limit switch trips can save you from a cold house and from repeated wear on the equipment.

What is going on inside

A furnace burns fuel to heat a metal heat exchanger, and the blower pushes household air across it and out through the ducts. That moving air carries the heat away. If less air flows than the furnace expects, the heat exchanger gets hotter than it should. The limit switch senses this and shuts off the burners. The blower usually keeps running for a while to cool things down, and then the cycle starts over.

That pattern of firing, shutting off and restarting is called short cycling, and it is hard on igniters, controls and the heat exchanger itself.

The common reasons air gets restricted

The most frequent cause is a dirty filter. Next come closed or blocked vents, a blocked return, ducts that are collapsed or leaking, a blower motor that is running slowly, and a dirty blower wheel. Sometimes the furnace is oversized for the ducts, so it pushes out more heat than the ductwork can move.

Less often, a failing limit switch trips too early, or a cracked heat exchanger changes how the air flows through the cabinet. A cracked heat exchanger is a serious safety concern because it can allow combustion gases into the airstream. Working carbon monoxide detectors on every level of the home are a good idea regardless, and you should never ignore an alarm.

Age plays a part too. As a furnace gets older, the blower motor slows, capacitors weaken and dust builds up in places you cannot see. A unit that used to run without a fuss may begin tripping the limit switch, and that is worth mentioning to a technician even if the trouble comes and goes, since intermittent faults tend to become constant ones.

Warning signs to watch for

• The furnace cycles on and off rapidly without warming the home.
• A burning or scorched smell that lasts beyond the first run of the season.
• A humming, rattling or buzzing from the cabinet.
• Vents that blow only lukewarm air, or rooms that heat unevenly.
• A cabinet that feels much hotter than usual.

Safe steps before you call

Turn the thermostat down or off so the furnace stops firing. Check the filter and replace it if it is dirty. Walk through the house and open any closed vents, and clear anything sitting in front of a return grille. Then, if the furnace is behaving normally, keep an eye on it over the next few cycles.

Do not open the burner compartment, bypass the limit switch or reset the furnace over and over. Those safety controls exist for a reason, and gas and electrical components are for a licensed technician only. If you smell gas, get everyone out of the house and call your gas utility or emergency services from a safe distance.

Why the fix is usually straightforward

When a technician arrives, they will check airflow, filter, blower, ductwork and the limit switch, then confirm the furnace is running within its safe range. Many overheating problems come down to airflow and are resolved quickly, and a yearly tune-up makes them much less likely to recur.

Our crew works on all brands, and our phones are answered around the clock for emergencies. If your furnace keeps shutting itself down, call Air-Tech on (509) 392-2021 and let us take a look.