The first cold evening of the year, you switch the thermostat to Heat and nothing happens. No blower, no hum from outside, no warm air. Before you assume the worst, know that a good share of the no-heat calls we run on that first night turn out to be something the homeowner could have found with a flashlight.
This guide is for nothing happening at all. If air is blowing but it is cold, that is a different problem, covered in heat pump blowing cold air in heat mode.
1. The thermostat mode and setpoint
It sounds obvious. It is also the most common fix. Check that the mode is Heat and not Cool, Off or Auto, and that the setpoint is at least three degrees above the room temperature. Check for a schedule or a "Hold" that is overriding what you just set. If the thermostat was replaced recently, check that Heat appears as an option at all. A thermostat configured as cool-only will not offer it.
2. A blank thermostat screen
A blank screen means no power. Replace the batteries if it takes them. A smart thermostat installed without a common (C) wire can run its internal battery flat and go dark, typically after months of seeming fine. A blank screen can also mean a blown fuse on the air handler's control board, which is number six.
3. Give it five minutes
Heat pumps have a built-in delay of three to five minutes after a mode change or a power blip, to protect the compressor. Many thermostats show "Wait" or a flashing icon during it. Switch to Heat, then go and make a coffee.
4. Every breaker, and there are more than you think
A typical Florida system has three or four places it can lose power:
- One breaker for the air handler blower and controls
- Often a second breaker, 30–60 amps, just for the heat strips. It is frequently unlabelled.
- A breaker for the outdoor unit
- A pull-out disconnect or switch on the wall next to the outdoor unit
Reset a tripped breaker once. If it trips again, leave it off and call. A breaker that trips twice is telling you something is drawing too much current, and resetting it again is how wires overheat.
5. The float switch
Most air handlers have a safety switch that cuts power when the condensate drain backs up. It sits in the drain line or in a pan under the unit. A clog left over from summer can hold that switch open, and the system will not run in any mode until the water is gone. If you see standing water in the pan or around the unit, a wet-vac on the outdoor end of the drain line often clears it. Then give the switch a few minutes to reset.
6. The air handler panel and the low-voltage fuse
If you changed a filter at the air handler, make sure the access panel is seated properly. Some units have a safety switch behind it and will not run with it loose.
Inside, the control board has a small 3- or 5-amp fuse that blows when a thermostat wire shorts. This happens after a thermostat swap, or when something has chewed a wire. Replacing the fuse without finding the short just blows the new one, so this one is usually worth a service call.
7. A filter so clogged the heat shuts itself off
If the heat runs for a minute or two, then quits, then tries again, look at the filter. A filter packed with a summer's worth of dust starves the strips of airflow, and their high-limit switch shuts them off to stop them overheating. Change it and give the system half an hour. Some strip kits also have a one-time fusible link. Once that blows, the strip stays dead until a technician replaces it.
8. A thermostat replaced since last winter
A new smart thermostat that cooled perfectly all summer has never been asked to heat. Common setup misses are: the heat wire (W or Aux) left unconnected, the system type set to conventional instead of heat pump, or the wrong O/B reversing-valve setting. The thermostat's installer settings menu shows all three. If you are not sure what you are looking at, take a photo of the wiring and the settings screen. We can usually tell you over the phone.
When to stop and call
- A breaker trips a second time
- You smell something electrical, like hot plastic or scorched wiring, rather than dust
- Water in the pan keeps coming back
- You have been through all eight and there is still nothing
Phone triage is free. The heating diagnostic is $89 and credited to the repair, and there is no after-hours surcharge for the night it happens to be cold. If you would rather not find out on the coldest night, our pre-winter checklist covers all of this on a mild afternoon.
Written by
The Eco Air Orlando team — licensed Florida HVAC contractors, #CAC1814992. Everything here comes from work we actually do in Central Florida homes.