Emergency Heat Light On
Short answer: The “Em Heat” or “Emergency Heat” light on your thermostat means your heat pump has been switched to backup electric resistance heating, bypassing the outdoor compressor entirely. In Florida, where heat pumps are installed in more than 70% of homes, this mode is often misunderstood. Auxiliary heat turning on automatically is normal and expected — manually switching to Emergency Heat mode is a signal your outdoor unit may be failing and you should call for service.
Emergency Heat vs. Auxiliary Heat: The Difference That Matters in Florida
Most Tampa Bay homeowners have a heat pump as their sole heating system. Unlike a gas furnace, a heat pump extracts heat from outdoor air and moves it inside — even when it is 45 degrees outside. When the outdoor temperature drops or the system needs a boost, a backup electric resistance strip heater inside the air handler kicks in. This backup heat operates in two very different ways, and confusing them is one of the most common mistakes Florida homeowners make:
Auxiliary Heat (AUX): Normal, Automatic, Expected
Auxiliary heat switches on automatically when the heat pump cannot keep up with demand on its own. This happens when outdoor temperatures drop below 35-40 degrees Fahrenheit — the “balance point” where a heat pump’s efficiency starts to fall — or when the thermostat is raised by more than 3 degrees at once, triggering a rapid warm-up cycle. The “AUX” indicator on your thermostat means the system is doing exactly what it was designed to do. No action is needed.
Emergency Heat (EM HEAT): Backup-Only Mode, Compressor Bypassed
Emergency Heat mode is a manually activated setting that completely disconnects the outdoor heat pump compressor and runs on electric resistance strips alone. According to the U.S. Department of Energy’s heat pump guidance, emergency heat uses approximately 2 to 3 times more electricity than normal heat pump operation because electric resistance heating has a Coefficient of Performance (COP) of 1.0, while a modern heat pump delivers a COP of 2.5 to 4.0 under Tampa Bay winter conditions. Running your system in Em Heat mode will roughly double or triple your electric bill compared to normal heat pump operation.
Florida’s heat pump penetration is among the highest in the nation. The U.S. Energy Information Administration’s 2020 Residential Energy Consumption Survey found that approximately 72% of Florida homes use heat pumps as their primary heating equipment — compared to just 38% nationally — because Florida’s mild winters make heat pumps far more cost-effective than gas furnaces year-round.
When the Em Heat Light Is On: Normal vs. Problem
The emergency heat indicator can appear for two completely different reasons, and your response should be different for each:
You or Someone in Your Household Switched It On Manually
Many homeowners hit the Em Heat switch accidentally or because they thought it would make the house warmer faster. It will not — electric resistance strips produce heat at the same temperature as the heat pump but far less efficiently. If the outdoor unit is running normally, switch back to standard heat pump operation immediately. Your bills will thank you.
The Outdoor Unit Has Stopped Working
Emergency heat mode is the correct temporary solution when the outdoor unit has genuinely failed: a locked compressor, a failed reversing valve (the valve that switches between heating and cooling modes), severely low refrigerant, or a damaged outdoor fan. In these cases, running Em Heat buys you time while you wait for a technician — but it is not a substitute for repair. A system running on emergency heat only in the middle of a Tampa “cold snap” (we get nights in the 40s from November through February) may struggle to maintain 68 degrees inside because electric strips are slower to respond than a functioning heat pump.
Triage Table: Emergency Heat Indicator Scenarios
| What You See on the Thermostat | What It Means | What to Check | When to Call for Service |
|---|---|---|---|
| “AUX” light on, outdoor unit running, house is warming | Normal auxiliary heat operation — heat pump is supplementing with backup strips automatically | Nothing — this is expected behavior during cold weather or after a big thermostat set-point change | Only if the AUX light stays on continuously for days even when it is above 50 degrees outside |
| “EM HEAT” light on, outdoor unit is silent and NOT running | System is in emergency heat mode — compressor bypassed, running on electric strips only | Check whether someone switched the thermostat to Em Heat manually; check if the outdoor unit is iced over or making unusual noises | Immediately if you did not switch it on yourself — the outdoor unit has likely failed or locked out |
| “EM HEAT” on, outdoor unit running, house barely warming | Thermostat may be in Em Heat but the outdoor unit is still running in cooling mode (reversing valve fault) | Feel the air coming from the supply vents — if it feels lukewarm or cool, the system is producing heat but less than expected | Yes, call immediately — a reversing valve stuck in cooling mode will fight your thermostat |
| “AUX” on continuously, outdoor unit running, but house is not reaching set temperature | Refrigerant may be low, reducing heating capacity and forcing aux strips to run constantly | Check whether the outdoor unit is icing over on the refrigerant lines; check whether the system has been serviced recently | Yes — continuous AUX on mild nights (above 45 degrees) with no temperature recovery points to a refrigerant or coil issue |
| “EM HEAT” on but house is not warming at all | Electric resistance strips have also failed, or the air handler has a separate power issue | Check the air handler breaker; listen for the air handler blower running | Immediately — no heating at all requires diagnosis |
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What Causes the Outdoor Unit to Stop Working in Florida?
Tampa Bay winters are mild — our average January low is 53 degrees Fahrenheit and genuine hard freezes (below 28 degrees for 4+ hours) happen only once every several years. This means Florida heat pump failures in winter are almost never caused by cold weather itself. The most common culprits are:
- Reversing valve failure: The reversing valve is the component that switches your heat pump between cooling (summer) and heating (winter) mode. In Tampa Bay, heat pumps spend most of their lives in cooling mode — the reversing valve may go years without switching, and when it finally needs to switch for heating season, a stuck or leaking valve fails. The symptom is a system that blows cool air in heating mode or one that loses heating efficiency suddenly.
- Low refrigerant: Refrigerant does not get “used up,” but coil leaks can develop over time, especially in homes near the coast where salt air accelerates corrosion on outdoor coil fins. Low refrigerant reduces heating capacity significantly and causes the compressor to work harder and run longer, eventually tripping the system into lockout.
- Capacitor failure: The start and run capacitors that help the compressor and outdoor fan motor start are wear items. Florida’s heat — outdoor units in the Tampa area can see 95-degree ambient air for six months of the year — shortens capacitor lifespan. A failed start capacitor means the compressor tries to start, cannot, and locks out. The system switches to emergency heat automatically on some thermostat models.
- Outdoor unit coil iced over: If the outdoor coil is covered in ice that is not clearing through normal 15-90 second defrost cycles, the heat pump cannot extract heat from the air. See our related problem guide on heat pumps stuck in defrost mode for the full explanation of when ice is normal vs. when it signals a problem.
Is It Safe to Run Your Heat Pump on Emergency Heat?
Yes, it is safe — emergency heat mode is a designed-in backup for exactly these situations. The only downsides are efficiency (your electric bill goes up significantly) and the fact that you are not solving the underlying problem. For Florida winters where nighttime lows rarely stay below 45 degrees for more than a night or two, emergency heat can keep the house comfortable while you wait for a scheduled service call. If temperatures are forecast to drop into the 30s overnight, prioritize getting the heat pump repaired sooner rather than later, since electric strips alone may not keep pace with heat loss in a larger home on a genuinely cold Florida night.
Need your heat pump diagnosed or repaired? See our heating repair service page for the full scope of what we service, or our heat pump installation guide if the system is aging and repairs are adding up.
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Frequently Asked Questions
Is it OK to run my heat pump on emergency heat?
It is safe to run on emergency heat temporarily, but it is expensive and is not a long-term solution. Emergency heat uses electric resistance strips with a Coefficient of Performance of 1.0, meaning you get 1 unit of heat for every unit of electricity consumed. A functioning heat pump delivers 2.5 to 4.0 units of heat per unit of electricity — so emergency heat costs 2.5 to 4 times more per hour of operation. Use it to stay warm while you schedule a repair, but do not let the underlying compressor issue go unaddressed.
Why is my emergency heat not keeping the house warm in Florida?
Electric resistance strips are slower to respond and deliver less total heating capacity than a functioning heat pump, especially in larger Tampa Bay homes (2,000 sq ft and above). If your home is not reaching the set temperature on emergency heat alone during a cold Florida night (40-50 degree lows), it may mean the electric strip capacity is undersized for the home, a breaker for the strip heater has tripped, or the strips themselves have a failed element. A technician can check both the strip capacity and the condition of the heating elements.
What is the difference between the AUX and EM HEAT indicators on my thermostat?
AUX (auxiliary heat) comes on automatically when the heat pump needs supplemental heating assistance — this is normal and expected. The thermostat controls it based on outdoor temperature and the gap between current and set temperature. EM HEAT (emergency heat) is a manual mode you switch on yourself, or that some thermostats activate when they detect the compressor is not responding. When EM HEAT is on, the outdoor unit is intentionally bypassed and the system runs on backup strips only. If you see EM HEAT and did not turn it on yourself, your outdoor unit has likely failed or locked out.
Can a dirty air filter cause the emergency heat light to come on?
Indirectly, yes. A severely clogged air filter restricts airflow across the indoor coil, which can cause the system to overheat and trip a high-limit safety switch. On some systems, this lockout registers as a fault that causes the thermostat to fall back to emergency heat mode. More commonly, a dirty filter reduces heating efficiency enough that the aux heat runs far more than normal. Replacing the filter (every 1-3 months in Tampa Bay, where systems run year-round) is always the first step before calling for service.
How long does a heat pump repair take, and what does it typically cost in Tampa Bay?
Most heat pump repairs are diagnosed and completed in a single visit. Capacitor replacement (a very common cause of outdoor unit lockout) typically runs $150-250 installed. Reversing valve replacement is a more involved repair ranging from $400-800 depending on the system. Low refrigerant correction requires finding and repairing the leak first, then recharging — costs vary from $300-700 depending on leak location and refrigerant type. Home Therapist provides a FREE diagnosis so you know the exact scope and cost before authorizing any work. The $279 minimum labor figure applies to approved repair work only — the diagnosis itself is always free.
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