A furnace or heat pump doesn’t quit working without a reason. In the Upstate it tends to quit on the coldest night of a mild winter, when the system is asked to do something it has not done since February. The fastest way to a warm house is to start from what you notice rather than from a list of everything that could be wrong, so this post is sorted that way. Find the symptom that matches your house and work through the checks. If the house is still cold afterward, you will know what the technician is likely to find.
Before anything else, three checks
- Start at the thermostat, and confirm it is set to heat rather than cool or auto, the set point is above the room temperature, and the batteries are fresh if the display looks dim. A thermostat with dead batteries or a wiring problem stops a working furnace from ever getting the signal to run.
- Look at the breaker panel, since heating systems draw significant power, and a breaker can trip from a surge, a short, or an overload. Reset a tripped breaker labeled furnace, air handler, or heat pump once, and if it trips again, leave it off, since it is protecting a fault, and our electricians can find it.
- Pull the filter, because many furnaces have a safety switch that shuts the unit down when airflow drops too low, which protects the equipment and leaves the house cold. If the filter is visibly dirty, replace it and give the system twenty minutes.
If you smell gas at any point, leave the house and call the gas utility from outside immediately.
Nothing runs at all
When the thermostat calls for heat and nothing happens, no fan and no sound, the cause is usually power or control, which is why the thermostat and the breaker come first above. Beyond them, a dead system points to a failed transformer, a control board that took a surge, or a float switch that shut the system down because the condensate drain backed up. Each is a routine repair that a technician confirms with a meter in a few minutes.
It runs, but the air is cool
Most Upstate homes heat with a heat pump, and one that runs while blowing cool air is the most common no-heat call we take. On a cold night, a heat pump leans on its electric auxiliary heat strips to make up for the heat it cannot pull from 25-degree air, and a strip that failed last winter sits unnoticed until the first night it is needed. A reversing valve stuck in cooling, a defrost control that let the outdoor coil ice over, and a low refrigerant charge produce the same symptom. A look at the outdoor unit helps here, since frost on the coil is normal and clears in a defrost cycle, while a solid block of ice is not. Each is heat pump repair work, and each is caught in a fall tune-up that tests the strips under load.
It starts, runs a minute, and stops
A gas furnace that lights, runs briefly, and shuts down is locking out on a safety control, which is the system doing its job. The usual cause is a dirty flame sensor, the small rod that confirms the burners lit and cuts the gas when it cannot detect a flame. A failed igniter, a pilot that will not stay lit, a pressure switch held open by a blocked vent, and a limit switch tripping on restricted airflow produce the same pattern. A cracked heat exchanger is the one to take seriously, because it can let carbon monoxide into the house, and when a technician finds one, the furnace stays off until it is addressed. Our furnace technicians see each of these every winter.
There is heat, but not enough
A house that warms slowly, or never reaches the set point on a cold night, has a system working at less than its capacity, and the filter is the first suspect. After that, a duct joint that has separated in the attic or crawl space is pouring heated air into a 40-degree space, a blower motor has slowed, or the heat strips have partly failed, and rooms that heat unevenly point the same way. A smart thermostat that reports runtime often shows this trend building for weeks before the cold night makes it obvious.
Why it happens on the coldest day
Parts such as igniters, flame sensors, capacitors, and blower motors wear gradually, and a component near the end of its life fails during heavy use, which is why breakdowns cluster on the coldest days. A fall tune-up tests the strips, the igniter, the flame sensor, the safety switches, and the connections under load, records the readings on a system report card, and catches the part that is drifting before the night it is needed.
Keeping it from happening again
- Replace the filter every one to three months through the heating season, since restricted airflow trips a furnace’s safety switch and pushes a heat pump onto its strips.
- Have the system tuned up each fall and keep the report cards, since manufacturers ask for proof of maintenance before honoring a parts warranty.
- Treat short cycling, new sounds, and rooms that no longer heat evenly as early warnings rather than quirks.
- If the system is past fifteen years and needs a repair costing a third of a replacement, put the money toward the replacement, which carries a 10-year parts and labor warranty.
Carolina Heating Service has served Greenville and the Upstate since 1981. If the checks above did not restore the heat, same-day repair service is available by phone, and members of the Carolina Comfort Club receive priority service and reduced rates on the repair. You will receive a photo of your technician and an on-the-way message before arrival, the price is presented before any work begins, and the visit is recorded on video.