What Is EM Heat On Thermostat? Your Complete Guide To Emergency Heat Settings
Unlock the Mystery of That "EM Heat" Button on Your Thermostat
Have you ever glanced at your thermostat and wondered, “What is EM heat on thermostat, and should I be using it?” That little “EM” or “EMER” setting can be a source of confusion—and potentially a source of a shockingly high energy bill if used incorrectly. You’re not alone. Many homeowners see this option but are unsure of its purpose, how it works, or the critical rule: it is not for daily use.
This comprehensive guide will demystify emergency heat. We’ll break down exactly what it is, how it differs from your regular heating mode, when it’s safe to flip the switch, and why using it as a convenience feature can drain your wallet. By the end, you’ll be a confident thermostat operator, ready to keep your home comfortable without unnecessary costs.
What Exactly is Emergency Heat (EM Heat)?
The Definition: A Backup, Not a Primary Source
Emergency heat—commonly shown as EM or EMH on thermostats—bypasses the heat pump entirely and relies on a secondary heat source. This is the core function. Your standard heating system, especially in milder climates, is likely a heat pump. A heat pump is incredibly efficient because it doesn’t generate heat; instead, it moves heat from the outside air (even when it’s cold) into your home. Think of it like an air conditioner running in reverse.
Emergency heat, or “EM heat,” stands for a strictly backup heating source that is intended only for situations where the primary heat pump system cannot function. It is your system’s Plan B, designed for true emergencies.
The "EM" Label: What It Means on Your Display
If your thermostat displays EM heat or EMER heat, it signals that your system’s backup heating mode is active. This is a clear visual indicator. You might also see an “aux” or “auxiliary heat” light. While “auxiliary heat” and “emergency heat” are sometimes used interchangeably, there’s a subtle technical difference we’ll clarify later. For the homeowner, seeing either light means your system is relying on its less efficient, more expensive backup heater.
How Does Emergency Heat Work? The Bypass Mechanism
The Heat Pump vs. The Backup: A Tale of Two Systems
To understand EM heat, you must first understand your primary system. When your thermostat realizes the room is getting chilly, it sends a signal to your furnace or heat pump to turn on. In a standard forced-air heating setup with a heat pump, the system then attempts to generate heat by extracting warmth from the outdoor air and transferring it indoors. This process is highly efficient until the outside temperature drops too low.
Unlike regular heat mode, emergency heat does not attempt to pull warmth from the outdoor air. It completely bypasses the heat pump. Instead, it flips a switch (often inside the air handler or on the outdoor unit) that tells the system to ignore the pump and activate the secondary heat source directly.
What is This "Secondary Heat Source"?
Emergency heat typically relies on electric resistance coils or a secondary heat. The most common backup sources are:
- Electric Resistance Heat (Heat Strips): These are coils of wire, similar to what’s in a toaster or hair dryer, that glow red-hot when electricity passes through them. They are simple, reliable, and installed inside your air handler. This is usually electric resistance heat and is the most common backup.
- A Gas Furnace: In some hybrid systems, particularly in colder climates, the backup is a full-sized, high-efficiency gas furnace. When EM heat is activated, the heat pump shuts off, and the gas burner fires up to produce heat.
- Oil or Hot Water (Hydronic) Backup: Less common in modern residential setups but possible, especially with older systems or specific geothermal integrations. The EM heat source can be electric, oil, gas, or hot water.
The em heat source... is an important tool to have at your disposal in frigid environments. Without it, your heat pump would struggle mightily or fail entirely during a deep freeze, leaving you in the cold.
When Should You Use Emergency Heat? The Golden Rule
This Mode Exists for Two Main Reasons.
First, as a safety net when your primary heat pump fails due to a mechanical issue—a faulty compressor, a refrigerant leak, or a frozen outdoor unit. Second, as a performance booster during extreme cold when the heat pump’s efficiency drops to near zero.
Emergency heat is exactly what it sounds like: A temporary backup setting, not a daily solution.It’s meant to keep your home warm when your heat pump isn’t working properly or outdoor temperatures drop too low for efficient operation.
✅ Appropriate Times to Switch to EM Heat:
- Your Heat Pump Has Failed: You hear loud noises, see ice buildup that doesn’t thaw, or your home isn’t warming despite the thermostat calling for heat. Emergency heat must be manually activated and is used if the heat pump itself is malfunctioning.
- Severe Weather Alert: A polar vortex is forecast, and temperatures are predicted to plummet well below your heat pump’s “balance point” (often around 25-35°F). Proactively switching can prevent the pump from working overtime and freezing up.
- During a Power Outage (with a Generator): If you have a whole-home generator, you might switch to EM heat if your heat pump’s outdoor unit is damaged by the outage’s surge.
❌ When You Should NEVER Use EM Heat:
- As Your Primary Heat Source: This is the cardinal sin. Running your system on EM heat for regular, daily operation is wildly inefficient.
- When Your Heat Pump Is Working Fine: If your home is maintaining temperature and the outdoor unit is running normally, keep it on “Heat” or “Auto.”
- For Mildly Cold Weather: Modern cold-climate heat pumps are effective down to very low temperatures. Let them do their job.
However, EM heat (which stands for emergency heat) should only be used in emergencies because of the added strain on both your unit and your bank account.
The High Cost of Convenience: Why Emergency Heat Drains Your Wallet
The Efficiency Chasm: 300% vs. 100%
A modern heat pump can have a Coefficient of Performance (COP) of 3.0 or higher in mild weather. This means for every 1 unit of electricity it consumes, it moves 3 units of heat into your home—a 300% efficiency.
Emergency heat relies entirely on the backup heating source and should only be used when necessary, as it is typically less energy efficient. Electric resistance heat (the most common backup) has a COP of 1.0. For every 1 unit of electricity, you get exactly 1 unit of heat. It’s 100% efficient at converting electricity to heat, but it’s 300% less efficient than a well-performing heat pump at moving heat.
Why does auxiliary heat increase energy bills? The math is brutal. Even if you didn't touch your thermostat, your heat pump likely switched into emergency heat (those expensive heat strips) just to keep up on a very cold night. That usage is what you’re paying for right now. Your electric meter spins faster because it’s powering massive heating elements directly.
💡 What can you do about it? First, understand your system. Check your thermostat settings. If the “EM” or “Aux” light is on constantly during cold weather, your heat pump may be undersized, malfunctioning, or your thermostat’s balance point setting (the outdoor temperature at which it switches to backup) may be set too high.
If your auxiliary heat or EM heat light is staying on, your bill will stay high. This is a red flag that needs investigation.
How to Tell If Your System Is Stuck in Emergency Heat
Diagnosing the Problem
Sometimes, a system can get “stuck” in EM heat due to a faulty sensor, a bad outdoor unit, or a misconfigured thermostat. Here’s how to check:
- Observe the Outdoor Unit: When your thermostat calls for heat on a cold day, does the outdoor unit’s fan turn on? Does it sound like it’s running? If it’s silent and covered in ice, your heat pump is likely offline, and the system has defaulted to EM heat.
- Check Your Thermostat Display: Is the “EM,” “Aux,” or “Emergency Heat” indicator lit solidly? If it’s flashing, it might be normal staging. A solid light usually means it’s active.
- Feel the Air: Air from your vents during EM heat (with electric strips) is often hotter and drier than air from the heat pump, which is typically a warmer, more humid breeze.
- Review Your Energy Bill: A sudden, significant spike in your electric or gas bill during winter is a classic symptom of prolonged EM heat use.
If you suspect your system is stuck in emergency heat, try these steps:
- Power Cycle: Turn the thermostat off for 60 seconds, then back on. Set to “Heat” and lower the temperature setting below your room’s current temp. Wait 5-10 minutes to see if the outdoor unit kicks on and the EM light goes off.
- Check the Outdoor Unit: Ensure it’s not iced over. Gently clear any snow or ice blocking the unit (never use sharp objects). If it’s iced, the system may have a defrost cycle—listen for a reversing valve clicking.
- Call a Professional: If the problem persists, you need a licensed HVAC technician. Expert emergency furnace repair in Davenport or your local area can diagnose if it’s a faulty thermostat, a bad reversing valve, a refrigerant issue, or a failed compressor. Fixing [a system] stopped working on [the] coldest night is their specialty.
Choosing the Right Thermostat for Your System
Not All Thermostats Are Created Equal
Your thermostat is the brain that decides when to use the heat pump and when to switch to EM heat. A basic, non-programmable thermostat might not handle this logic well. A modern smart or programmable thermostat is essential for optimal efficiency.
Click now for the full guide! (In a real blog, this would be a CTA linking to a thermostat buyer’s guide). When shopping, look for thermostats explicitly compatible with heat pumps that auxiliary/emergency heat. They will have settings for:
- Balance Point/Aux Heat Lockout: The outdoor temperature below which the thermostat will allow the auxiliary heat to turn on. Set this as low as your system allows (e.g., 15°F or lower) to force the heat pump to work harder in cold weather.
- Compressor Delay: Prevents the heat pump and auxiliary heat from running simultaneously, which wastes energy.
- Defrost Control: Manages the outdoor unit’s ice-melting cycle.
Find flair aux/emergency heat pump thermostats at Lowe's today. Shop thermostats and a variety of heating & cooling products online at lowes.com. While brand-specific, retailers like Lowe's offer filters to find thermostats compatible with your system type (heat pump with aux heat). Learn how your thermostat controls each mode and when to switch for best results by reading the manual or consulting an HVAC pro during installation.
Proactive Maintenance: Your Best Defense Against EM Heat Emergencies
Prevent the Need for Emergency Heat
The best way to avoid the high cost of EM heat is to keep your primary heat pump in top shape.
- Schedule Annual Professional Maintenance: Have an HVAC tech inspect and tune up your entire system (both indoor and outdoor units) before the heating season. They’ll check refrigerant levels, electrical connections, and the reversing valve.
- Keep Your Outdoor Unit Clear: Maintain at least 18-24 inches of clear space around your outdoor heat pump unit. Remove leaves, snow, and debris regularly to ensure proper airflow.
- Change Your Air Filter Monthly: A clogged filter restricts airflow, causing the indoor coil to freeze and the system to malfunction.
- Understand Your System’s Limits: Know your heat pump’s lowest efficient operating temperature (found in the manual). If you live in a region with frequent sub-20°F weather, a hybrid system with a gas furnace backup is far more cost-effective than electric resistance strips.
- Insulate and Seal Your Home: The less heat your home loses, the less work your heat pump—and potentially your EM heat—has to do. Check doors, windows, and ductwork for leaks.
Conclusion: EM Heat is Your Safety Net, Not Your Comfort Blanket
Emergency heat (em heat) is exactly what it sounds like: a critical, last-resort feature designed to prevent you from freezing when your primary heat pump fails or faces temperatures beyond its design capability. This mode exists for two main reasons: system failure and extreme cold survival.
Understanding the fundamental difference between your efficient heat pump and the power-hungry emergency heat backup is the key to managing comfort and cost. The em heat source can be electric, oil, gas, or hot water, but regardless of the fuel, it will always be more expensive to operate than your heat pump when the pump is functioning properly.
Remember the golden rule: EM heat is for emergencies only. Using it as a primary or even secondary setting adds strain on both your unit and your bank account. If you find your system frequently calling for auxiliary heat during cold snaps, it’s not a sign to lean on EM heat—it’s a sign to call a professional. Your heat pump may need service, your thermostat settings may need adjustment, or your system might be inadequate for your climate.
By respecting the “E” in EM, you ensure this vital backup is there when you truly need it, while keeping your regular heating operations as efficient and affordable as possible. Stay warm, stay informed, and keep that EM switch in the “off” position unless it’s an absolute emergency.
