January 8, 2026

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: 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. 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