I get a lot of no-heat calls on the first cold night of the year, and a good number of them turn out to be something simple: a thermostat setting, a tripped breaker, a filter so clogged the system shuts itself off. Before you pay anyone to come out, here are the same checks I'd walk you through on the phone.
These steps work for the systems we see most around Raleigh, Durham and the rest of the Triangle: gas furnaces with a separate AC, heat pumps with an air handler in the attic or crawlspace, and all-electric furnaces. Go in order. If one of them fixes it, great. If you get through all ten and still have no heat, that's when you call a pro, and you'll be able to tell them exactly what you already checked.
Safety first
- Smell gas (rotten eggs)? Don't touch switches or the thermostat. Get everyone out, then call your gas company or 911 from outside.
- Carbon monoxide alarm going off? Get everyone outside into fresh air and call 911. Don't go back in to check the furnace.
- Turn the power off at the furnace or air-handler switch before you open any panel.
- Never bypass a safety switch (door switch, float switch, pressure switch, limit switch). They shut the system off for a reason.
What to check
Check the thermostat mode and setting
Make sure it says Heat (not Cool, Off or Auto with a cooling setpoint) and that the temperature is set a few degrees above what the room is now. If it's on a schedule, it may be in an "away" or overnight setback, so try a temporary hold.
If the system kicks on within a few minutes, you're done.
Replace the thermostat batteries
A lot of thermostats run on AA or AAA batteries even when they're wired to the system. A blank screen, a dim screen or a low-battery icon means new batteries first. Smart thermostats that lose power can also lose Wi-Fi and settings, so give it a minute to reconnect.
Check the breakers
Look for a tripped breaker labeled furnace, air handler, heat pump, AHU or HVAC. A heat pump usually has two: one for the outdoor unit and one for the indoor air handler (which also runs the backup heat strips). Flip a tripped breaker all the way off, then back on, once.
If it trips again right away, leave it off and call. That's an electrical problem, not something to keep resetting.
Find the furnace or air-handler switch
Near the indoor unit there's usually a switch that looks like a regular light switch. In attics and crawlspaces it's often on the side of the unit or on a nearby rafter. People flip it off by accident all the time, thinking it's a light. Make sure it's on.
Check the air filter
A filter packed with dust chokes the airflow. A furnace will overheat and shut itself off on its high-limit switch, and a heat pump will run but barely warm the house. Pull the filter out and hold it up to a light. If you can't see light through it, replace it. Most homes here need a new 1-inch filter every 1 to 3 months, more with pets.
Make sure the blower door is on tight
Furnaces and air handlers have a safety switch behind the lower front panel. If that panel isn't seated all the way (maybe after a filter change), the switch stays open and nothing will run. Push the panel firmly into place until it clicks or sits flush.
Check the drain pan and float switch
High-efficiency gas furnaces and attic air handlers make water, and they drain it through a pipe. If the drain clogs, a float switch in the pan or on the drain line cuts the power to protect your ceiling. Look for standing water in the pan under the unit or a small switch on the drain pipe.
If the pan is full, the drain is clogged. You can shop-vac the drain line from the outside end, but if water is overflowing or the ceiling is stained, call us.
Look at the gas supply (gas furnaces)
On the gas pipe going into the furnace there's a shutoff valve. The handle should run parallel with the pipe (that's open). Also check that other gas appliances like the water heater or stove are working, which tells you gas is reaching the house. Don't try to relight anything if you smell gas.
Read the error light
Most furnaces and many air handlers have a small blinking light on the control board, visible through a little window in the lower panel. Count the blinks (for example, 3 short flashes, pause, repeat) and write it down or take a video. There's usually a chart on the inside of the door that tells you what it means. See how to read your furnace or heat pump error codes.
Don't turn the power off and on before you read it. On many systems that clears the code.
Heat pumps: check the outdoor unit
If you have a heat pump, look at the outdoor unit. A light coat of frost in cold, damp weather is normal, and the unit will defrost itself every so often (it might steam and go quiet for a few minutes). A solid block of ice, a fan that isn't spinning, or a unit that's silent while the thermostat is calling for heat is not normal. See heat pump iced up.
If the thermostat shows Emergency Heat or Aux, read what emergency heat means before you leave it on.
How to keep this from happening again
Most no-heat calls I see in the fall come from three things that build up over the summer: a filter nobody changed, a drain line that slowly clogged with algae while the AC was running, and a flame sensor or igniter that's been getting weaker for a while. None of those show up until the first cold night, which is exactly when everyone else is calling too.
A fall tune-up catches all three. On a heating tune-up I check the filter and airflow, clean or test the flame sensor and igniter, flush the drain, test the safety switches, and on gas furnaces check combustion and the vent. On heat pumps I check the defrost controls, the refrigerant charge and the backup heat strips. Frost Fire members get up to 15% off repairs, and you get on the schedule before the rush.
Two habits help too: put a reminder on your phone to change the filter every one to three months, and keep the area around the furnace or air handler clear so you (or a tech) can get to the switch, the filter and the drain in a hurry.
When to call a pro
- A breaker trips again right after you reset it.
- You smell gas, burning or hot plastic, or see scorch marks.
- The furnace tries to start (you hear the inducer fan) but shuts down, or the error light keeps flashing.
- Water is overflowing the drain pan or coming through a ceiling.
- The heat pump outdoor unit is a solid block of ice or isn't running at all.
- You've done all ten checks and still have no heat.
Still not working?
Frost Fire Heating & Cooling: $59 diagnostic (waived with repair), same-day service, no weekend surcharge, 4.8-star rated. Licensed & Insured. Serving Raleigh, Durham, the Triangle, Clayton and surrounding areas.
Common questions
Why is my furnace running but blowing cold air?
On a gas furnace, the burners may not be lighting (ignition, flame sensor, gas supply or a pressure switch problem) while the blower keeps running. On a heat pump, the air is naturally cooler than a gas furnace, and it can blow cool for a few minutes during a defrost cycle. If it never warms up, have it checked.
How long should I wait after changing the thermostat before calling?
Give it about 5 to 10 minutes. Many systems have a short delay before they start, and gas furnaces run the inducer fan and igniter before the burners light and the blower comes on.
Is it safe to reset my furnace?
Turning the switch off and back on once is fine. If it locks out again, stop. Repeatedly resetting a furnace that keeps shutting down can hide a safety problem.
How much is a service call?
Frost Fire's diagnostic is $59, and it's waived if you go ahead with the repair. We charge the same on weekends, with no Sunday surcharge.
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