Defining the 'Em Heat' Switch on Your Thermostat
There is a specific kind of dread that sets in on a 15-degree morning when you realize your home feels more like a refrigerator than a sanctuary. I’ve been there—staring at the thermostat while my family shivers, wondering if that mysterious "Em Heat" switch is the magic "turbo" button I need to finally get some warmth moving. The temptation to flip it is overwhelming, but as someone who has seen that single click turn a standard $150 utility bill into a $400 nightmare overnight, I can tell you that "Emergency Heat" is rarely the hero homeowners think it is.
In my years of troubleshooting HVAC systems and consulting with homeowners, the most common winter mistake I see isn't a lack of maintenance—it's a misunderstanding of how heat pumps actually function in the cold. We’re going to dive into exactly what "Em Heat" is on your thermostat, why your system has it, and the specific, high-stakes scenarios where you should actually use it. If you’ve been wondering "should I use heat or em heat" during the next cold snap, this guide will save you from the most expensive mistake you can make this winter.
That small switch labeled "Em Ht" on your thermostat looks harmless — but flipping it triggers one of the most energy-intensive modes your HVAC system can run.
So what is 'Em Heat' on a thermostat, exactly? Emergency Heat is a manual override that completely shuts down your outdoor heat pump compressor and forces your system to run entirely on electric resistance heating strips instead. According to HVAC.com, manually activating this setting disables the outdoor unit and shifts 100% of the heating load to those backup strips — the same technology found in a basic space heater, just built into your ductwork.
Under normal winter operation, your heat pump does the heavy lifting, pulling warmth from outdoor air even in cold temperatures. The electric resistance strips exist as a backup, not a replacement. Emergency Heat removes the heat pump from the equation entirely, which is why it carries a red indicator light on common thermostats — including popular Honeywell models — as a visual warning that something unusual is happening.
This setting exists for one specific scenario: your outdoor unit is physically damaged or frozen solid and cannot operate safely. It is genuinely a last resort, not a standard cold-weather mode. Understanding why it exists — and why the line between "Em Heat" and normal supplemental heating matters so much — is exactly where things get interesting.
Emergency Heat vs. Auxiliary Heat: The Critical Difference
Understanding what emergency heat is — and what it isn't — is the single most important thing a homeowner can do before winter arrives.
Auxiliary heat and emergency heat are not the same thing, and confusing the two is where most heating bills go wrong.
Auxiliary (Aux) heat is your system working with you automatically. When outdoor temperatures drop to a point where the heat pump alone can't maintain your target temperature — typically below 35–40°F — your thermostat triggers the backup electric heat strips to supplement. You don't flip a switch. The system decides. Once conditions improve, the heat pump resumes its primary role and the strips shut off. It's additive, temporary, and controlled.
Emergency heat, by contrast, is a manual override that completely bypasses the outdoor heat pump. The moment you flip that switch, your system stops using one of the most efficient heating mechanisms available and relies entirely on electric resistance strips — the HVAC equivalent of running a giant space heater. As David Watkins of Watkins Heating & Cooling notes via Homes & Gardens, the emergency heat function should only be used "when there is a problem with the outdoor heat pump… such as if the heat pump is not defrosting properly."
The panic scenario is familiar: temperatures plummet overnight, the house feels cold, and that "Em Ht" switch looks like a fast fix. In practice, what it actually triggers is a substitution — not a boost — that strips away your system's efficiency entirely. Aux heat adds capacity alongside the pump. Em heat replaces it. That distinction has a direct and measurable impact on your utility bill, which is exactly what we'll look at next.
The Financial Cost of Flipping the Switch
Flipping that "Em Heat" switch isn't just an operational choice — it's a decision that can quietly add hundreds of dollars to your utility bill within days.
To understand why, it helps to know what is 'Auxiliary Heat' and how it differs mechanically. A standard heat pump moves existing warmth from outdoor air into your home, which is an energy-efficient process. Electric resistance strips, by contrast, generate heat entirely from electrical current — the same principle as a toaster, just at a much larger scale. A typical 10kW strip set draws enormous wattage with no efficiency multiplier working in its favor.
The numbers make the case plainly:
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Emergency heat typically costs between $1.05 and $2.25 per hour to operate, which is 2 to 5 times more expensive than a standard heat pump, according to The Furnace Outlet.
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Running emergency heat for just 12 hours a day over one week can increase energy costs by approximately $163.80 compared to standard heat pump operation, per Charlotte Comfort Systems.
That $163.80 figure assumes moderate usage. In a colder region where the system runs longer cycles, the real-world impact could climb higher. The core reason behind this math is simple: heat pumps operate at efficiency ratings of 200–400%, meaning they deliver more heat energy than the electricity they consume. Resistance strips operate at exactly 100% — every watt in equals one watt of heat out, nothing more. That gap is where your money disappears.
Understanding the cost is only half the picture, though. Knowing when that cost is actually justified — versus when it's pure waste — is what separates a smart homeowner from an expensive one.
When Should You Actually Use Emergency Heat?
Emergency heat has exactly one legitimate use case: your heat pump's outdoor unit has physically failed or become dangerously inaccessible.
Rule of Thumb: If your heat pump is running and your home is reaching the set temperature — even slowly — leave "Em Heat" off. The switch exists for hardware emergencies, not cold weather comfort.
Understanding what is 'Em Heat' clarifies why the threshold for using it is so high. The cost difference alone makes the case. A heat pump operating at 30°F costs roughly $32.76 per week to run, but switching to emergency heat during a 10°F cold snap can spike that figure to $196.56, according to Charlotte Comfort Systems. That's not a marginal difference — it's a sixfold jump.
Mechanical failure is the clearest valid trigger. A frozen fan blade, a broken compressor, or a refrigerant leak that prevents the outdoor unit from cycling at all — these are the scenarios where flipping the switch makes sense. In practice, you'll likely notice your home cooling down despite the thermostat demanding heat, which is a reliable signal something is mechanically wrong.
Environmental hazards can also justify it temporarily. An ice storm that buries the outdoor unit under a solid sheet of ice — not frost, which the system can defrost on its own — may warrant switching over while you safely clear the unit. However, if the heat pump is simply running its normal defrost cycle, the system is working exactly as designed. Leave it alone.
If the heat pump isn't defrosting properly, the right move is to call an HVAC technician, not reach for the emergency heat switch. Using "Em Heat" as a long-term workaround masks the underlying problem and runs up your utility bill while doing nothing to fix the actual fault. The switch is a bridge, not a solution — a detail worth keeping in mind before you encounter the myths that often send homeowners toward it unnecessarily.
Common Thermostat Myths and Misunderstandings
Misconceptions about the "Em Heat" switch are widespread — and they cost homeowners real money every winter.
If you've ever searched "what is the 'Em Ht' switch on my heat pump thermostat," you've likely landed on conflicting advice. Honeywell thermostats, along with most other major brands, label this setting slightly differently — "Em Ht," "Emergency Heat," or "E Heat" — but the function is identical across all of them. The confusion in labeling leads many homeowners to treat it as a premium comfort feature rather than a last-resort backup.
Myth 1: Accidentally turning it on will damage your system. Reality: No damage occurs, but your wallet takes the hit immediately. At $0.15/kWh, running a 10kW electric resistance strip costs roughly $1.50 per hour compared to just $0.35–$0.53 for your heat pump delivering the same warmth, according to The Furnace Outlet. Flip it back off and your system returns to normal operation.
Myth 2: Em Heat blows hotter or faster than Aux Heat. Reality: Both modes draw from the same electric resistance strips. The air temperature and delivery speed are virtually identical. Em Heat simply bypasses the outdoor unit entirely — it delivers no added comfort advantage.
Myth 3: Below-freezing temps mean you should manually switch to Em Heat. Reality: Heat pumps are engineered to extract warmth from outdoor air even well below 32°F. Your system's Aux Heat engages automatically when extra output is needed. Manual intervention isn't just unnecessary — it's expensive.
With these myths cleared up, it's worth stepping back to review everything you now know before making any thermostat decisions this season.
The Bottom Line: What You Need to Know
Understanding when to ask "should I use heat or em heat" comes down to one core principle: Em Heat is a mechanical emergency switch, not a cold-weather upgrade.
Here's what every heat pump homeowner should keep in mind:
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Em Heat is for emergencies only. If your outdoor unit is physically broken, frozen solid, or otherwise out of commission, flip the switch — then call a technician immediately.
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Aux Heat is automatic. It kicks in on its own when your heat pump needs a boost. You don't need to manage it manually; that's the entire point of a two-stage system.
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Unnecessary Em Heat use is expensive. Running it when you don't need to can add over $160 to your weekly energy bill — a cost that compounds quickly through a full winter season.
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Heat pumps are built for cold weather. Standard systems are engineered to operate efficiently in freezing temperatures without manual intervention. Trust the technology.
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A physically damaged or frozen unit is the threshold. That's the one scenario that justifies the switch.
The simplest rule: if your heat pump is running, leave Em Heat alone. The system knows what it's doing. Overriding it doesn't make your home warmer faster — it just makes your energy bill larger. If something feels off about how your system is performing, that's a signal worth paying attention to before reaching for that switch.
Protecting Your HVAC System with Hyvoxa
The "Em Heat" switch is a symptom indicator, not a heating upgrade — and treating it like one is the fastest way to a costly repair bill. Every section of this article has pointed toward the same conclusion: your heat pump is designed to handle winter efficiently on its own, and when it can't, something needs fixing.
Before you ever flip that switch, take one practical step. Walk outside and check your outdoor unit. A thick layer of ice, a pile of debris blocking airflow, or a visibly damaged coil tells you far more than any thermostat indicator light ever could. That 60-second check can help you decide whether you need to wait out a normal defrost cycle or pick up the phone and call a technician.
At Hyvoxa, the focus is on giving homeowners the clarity they need to make smart decisions about their home climate systems — from understanding what each thermostat setting actually does to knowing when a professional needs to step in. Confusing controls and vague warning signs shouldn't leave you guessing in the middle of a cold snap.
If your Aux heat is running constantly or you find yourself reaching for Em Heat, that's your system telling you something is wrong. Schedule a professional HVAC inspection before the problem compounds. Don't wait for a complete breakdown — act on the warning signs early, and your heat pump will reward you with reliable, efficient warmth all season long.
Final Thoughts: Wisdom from the Field
Over the years, I’ve responded to dozens of frantic service calls where the homeowner thought their system was failing because the "Aux" light was on, only for them to flip the "Em Heat" switch and accidentally triple their heating costs for the month. My professional advice is always the same: treat that switch like the "In Case of Emergency, Break Glass" box on a fire alarm. I’ve stood in snowy backyards inspecting units that were perfectly fine, only to find the homeowner had overridden a perfectly healthy defrost cycle out of a fear that the system wasn't keeping up.
Expertise in home climate isn't just about knowing how to fix a compressor; it's about knowing how to trust the technology already in your home. Your heat pump is a remarkably resilient piece of engineering, designed to battle through sub-freezing temperatures without you ever needing to touch a manual override. If you find yourself reaching for that switch more than once a season, don't just accept the higher bills—use it as a signal to get a professional eye on your system. At Hyvoxa, we believe the best heating system is the one you don't have to think about, and keeping your hands off the "Em Heat" switch is the first step toward that peace of mind.