what does aux heat mean on thermostat
"Aux heat" on your thermostat stands for auxiliary heat, a backup system that kicks in when your primary heat pump can't keep up with the cold. It's a common sight during chilly snaps, like the ones we've seen in early 2026 across many U.S. regions, where heat pumps struggle below freezing.
Core Meaning
Aux heat activates automatically in heat pump systems as a secondary source—usually electric strips or gas—to boost warmth when outdoor temps drop too low (often around 30-35°F). Your thermostat signals this when the heat pump alone can't hit your set temperature, preventing overwork on the main unit. Think of it as your system's "sidekick," stepping in like a reliable friend during a winter storm.
"Aux heat on a thermostat activates when heat is unable to effectively move external air to the heat pump. This typically occurs when exterior temperatures hover around three to eight degrees (Fahrenheit) above freezing."
When It Turns On
Several triggers prompt aux heat based on real-world HVAC insights:
- Extreme cold : Heat pumps lose efficiency below freezing; aux provides the extra punch.
- Defrost mode : Pumps enter defrost cycles in icy conditions, temporarily relying on aux.
- Big temp gaps : Indoor air lags 3+°F behind your setpoint, signaling backup need.
- High demand : Rapid recovery after setbacks, like opening doors in winter.
No fixed temp threshold exists—it's smart, demand-driven. Recent forum chatter on Reddit (e.g., Sensi thermostat users) notes it popping up unexpectedly in milder 2025-2026 winters, often due to settings or insulation tweaks.
Aux vs. Emergency Heat
Feature| Aux Heat| Emergency Heat
---|---|---
Purpose| Automatic backup for efficiency 3| Manual override, bypasses
heat pump entirely 7
Cost| Higher than pump but optimized| Very expensive, all-electric 10
Use Case| Daily cold snaps| Pump failure only 5
Indicator| Flashes briefly on screen 7| Steady "EM Heat" mode 3
Pro tip : Avoid forcing emergency heat unless your pump's broken—it's like calling a tow truck for a flat tire instead of changing it yourself.
Why It Matters (Cost & Efficiency)
Aux heat guzzles energy (2-3x more than the pump), spiking bills—users report 20-50% jumps in January cold waves. It's not a fault ; it's designed-in smarts. But constant use? Check for:
- Poor insulation or dirty filters.
- Low refrigerant.
- Thermostat miswiring (common in smart models like Nest/Honeywell).
Trending tip : With 2026's erratic weather (per recent HVAC forums), pros recommend lockout settings on newer thermostats to limit aux below 25°F, saving cash.
Quick Fixes & Prevention
- Inspect filters : Clogged ones force aux overuse—swap monthly.
- Adjust settings : Raise temp 1-2°F or use "hold" wisely.
- Add insulation : Seal drafts for less demand.
- Service call : If stuck on, techs check staging (e.g., balance point).
- Smart tweaks : Apps like Sensi let you monitor runtime—users share hacks online.
Real story : One homeowner in a 2025 Reddit thread fought endless aux on a new heat pump—turned out to be a 1° calibration error, fixed in minutes.
TL;DR Bottom Line
Aux heat means backup power for your heat pump during tough cold—normal, but watch for overuse to cut costs. It's efficient short-term but pricey long- haul; tweak habits or call a pro if persistent. Information gathered from public forums or data available on the internet and portrayed here.