Heat Not Working? Checks to Do Before You Call

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The house is cold and the heater is not doing its job. Before you book a repair, work down this list in order. Each step takes a minute, none needs more than a flashlight, and plenty of no-heat problems turn out to be a thermostat setting, a switch or a filter. If it is still cold at the end, you can tell a technician exactly what you ruled out.

This page is the starting point of our Bay Area Winter Heating Help Center. It covers gas furnaces and heat pumps (including ductless mini-splits). If you have a wall heater or floor furnace, steps 1, 2 and 6 still apply, and the pilot section of our pilot light and ignition guide covers the rest.

The checklist: do these in order

1. Thermostat mode. It should say Heat, not Cool, Off or Fan. On a heat pump, pick Heat rather than Auto; ENERGY STAR advises avoiding Auto mode so the system does not flip between heating and cooling on mild days (ENERGY STAR). Set the fan to Auto, not On. With the fan on On, the blower runs nonstop and pushes room-temperature air between heating cycles, which feels like "cold air from the vents".
2. Setpoint and schedule. Raise the setpoint 3 to 5 degrees above the room temperature and wait five minutes. Smart thermostats often hide the real cause here: an away mode, a vacation hold, an eco setting or a schedule that someone edited in the app. If the screen is blank or dim, replace the batteries (many models have them even when wired). Our thermostat troubleshooting guide goes deeper.
3. Breaker and furnace switch. At the panel, look for a breaker labeled furnace, FAU or heat pump. A tripped breaker sits in the middle; push it fully off, then on. At the furnace (hall closet, garage or attic), find the switch that looks like a light switch on or near the unit and make sure it is on; people turn it off by mistake all the time. Heat pumps usually have a second breaker for the outdoor unit and a disconnect box on the wall beside it. If a breaker trips again, leave it off and call.
4. Furnace door. The blower door has a push-button safety switch behind it. If the panel is loose or slightly crooked, the furnace will not run at all. Seat it firmly, the way it came off. Do not open the upper burner compartment cover.
5. Read the status light before you touch anything else. Most modern furnaces have an LED on the control board, often visible through a small window in the blower door. Count the blinks and match them to the code chart printed on the inside of the door. Write the code down or take a photo before removing the door, because cutting power can clear it. A steady glow usually means normal; a code pointing to "pressure switch", "flame sense" or "limit" tells a technician a lot.
6. Filter. Slide it out and look. If you cannot see light through it, replace it. A choked filter makes the furnace overheat and shut the burners off on its limit switch, which shows up as short bursts of heat and then cool air. ENERGY STAR says to check the filter every month in heavy-use months and change it at least every 3 months (ENERGY STAR). After wildfire smoke or a remodel, it can clog much faster.
7. Gas supply (gas furnaces). Find the gas valve on the pipe entering the furnace. The handle should point along the pipe (on). Then try another gas appliance: does a stove burner light? If nothing gas in the house works, the problem is the supply, not the furnace. Call PG&E, and do not try to turn a shut-off meter or a tripped earthquake valve back on yourself unless you know exactly how.
8. Water and the float switch (high-efficiency furnaces). If your furnace has white PVC vent pipes, it is a condensing furnace that makes water and drains it through a hose, trap and sometimes a small pump. A clogged drain can trip a float switch and stop the furnace cold. Look for water in the pan or around the base and for a pump reservoir that is full. Mop up, and call; the Carrier owner's manual says the condensate system should be cleaned annually by a qualified service agency (Carrier manual). Also check that the two PVC pipes outside are not blocked by leaves, a bird nest or a stored item.
9. Registers and returns. Open the supply registers in the rooms you care about and make sure furniture, rugs or storage boxes are not covering the large return grille.
10. Give it one clean restart. Turn the furnace switch off for 30 seconds, back on, and call for heat. Listen: blower first or inducer fan hum, a click, then a glow or spark and burners. Try this once. If it goes through the motions and stops, note at which point it quit.

Heat pump or mini-split: extra checks

• Is the outdoor unit running? With the thermostat calling for heat, the fan outside should spin and the compressor should hum. If it is silent, check its breaker and disconnect box.
• Steam and a pause are normal. In cold, damp weather the outdoor coil frosts over and the system defrosts itself. Bryant's heat pump manual says you may see steam or fog rising from the unit and hear a "whoosh" at the start and end of defrost (Bryant owner's manual). Indoor airflow may drop or pause for a few minutes.
• Ice that does not clear is not normal. Trane notes that ice that is not going away, or is growing, points to low refrigerant, faulty defrost controls or a faulty sensor (Trane). Do not chip at it; turn the system off and call.
• Emergency heat. If your heat pump has electric backup strips, the thermostat's Emergency or EM Heat setting can keep the house warm while you wait for service. It works, but it costs much more to run, so treat it as a bridge.
• Mini-splits: check the remote is in Heat (the sun symbol), its batteries, the indoor filters behind the front panel, and that the louvers are not closed.

More detail is in our guide to heat pump problems in a Bay Area winter.

What not to do

• Do not tape down the door switch, jumper a limit switch or bypass any safety device. They exist to stop overheating, backdrafting and gas buildup.
• Do not reset the furnace over and over. After a few failed ignition tries, most furnaces lock out for a while by design. Repeated resets can hide a flame or venting problem that needs a technician.
• Do not remove the burner compartment cover, pull wires or try to clean the flame sensor while the furnace has power and gas.
• Do not heat the house with a gas oven, a grill, a camp stove or a generator. The CDC's advice is plain: "Don't heat home with gas oven" (CDC).
• Do not keep a heat pump running with a solid block of ice on it.

When a cold house becomes an emergency

How cold "cold" gets depends on where you live. San Jose's January nights average 41.8°F, but Livermore's average 37.8°F, with about 6 nights at or below freezing in a typical January (NOAA 1991-2020 climate normals). Inland homes cool off faster overnight. For a broader middle-of-the-night plan, read what to do when your HVAC breaks at 2 AM, or see how our 24/7 emergency HVAC repair works.

Staying warm safely while you wait

• Pick one room. Gather people and pets in one smaller room, close its door, and close curtains at dusk.
• Layer up. Hats, socks and a sleeping bag or extra blankets on the beds do more than most people expect.
• Space heaters, done right: keep anything that can burn at least 3 feet away (USFA), plug it straight into a wall outlet, never an extension cord or power strip (CPSC), and never leave it running unattended or while you sleep (CPSC). Read our space heater safety guide before you plug one in.
• Test the CO alarm. If the heater has been acting strangely, make sure the carbon monoxide alarm near the bedrooms works. The CDC warns that people who are asleep can die from CO before they have symptoms (CDC).
• Protect pipes in inland valleys. On a freezing night, open cabinet doors under sinks on outside walls.

Narrowing it down

If the blower runs but the air is cold, our post on why a furnace blows cold air walks through airflow, limit switch and ignition causes. If nothing happens at all when the thermostat calls, why your furnace won't turn on covers power, controls and ignition in more depth. If it lights, runs a few minutes and quits, that is short cycling, a separate pattern with its own causes.

What a technician does next

When you call, tell us the system type, any blink code, and where the startup stops. On site, a technician confirms the call for heat reaches the furnace, then watches a full ignition sequence (inducer, pressure switch, igniter, gas valve, flame proving, blower) and tests each stage with a meter, so the failed part is found rather than guessed. We also check venting before handing the furnace back. Every repair carries our 1-year parts and labor warranty.

Still no heat? Book a heating repair

Frequently asked questions

Written and reviewed by Jim McKinny, owner of Cool Aid Air Conditioning & Refrigeration — California C-20 and C-38 licensed, EPA Universal certified, second generation of a family business serving the Bay Area since 1966. Every job we complete is backed by our 1-year parts & labor warranty.