What is AUX heat and why is it so expensive?

AUX heat (auxiliary heat) is electric resistance heating that kicks in when your heat pump can't meet demand. While heat pumps are highly efficient (300-600% efficiency), resistance heating is only 100% efficient - basically turning electricity directly into heat via coils. This makes AUX heat 3-5x more expensive per BTU than the heat pump itself.

Why do Nest and Ecobee thermostats cause high electric bills?

Many aftermarket smart thermostats come with factory defaults set to engage AUX heat at temperatures as high as 30-40°F. Since heat pumps can efficiently operate down to 15-20°F (with modern models going even lower), these aggressive AUX thresholds mean you're burning expensive electric resistance heat when your heat pump could handle the job.

How can I tell if AUX heat is running in my home?

Look for these signs: 1) Unusually high electric bills in winter (especially $200-500 higher than previous years), 2) Outdoor unit not running when it should be cold but not extremely cold (like 35-45°F), 3) Electric strips glowing red/orange through basement vents, 4) Checking your thermostat's energy history to see AUX runtime, 5) Following manufacturer instructions to view live system status (often in settings menu).

Can I save money by adjusting AUX lockout temperatures myself?

Absolutely. Reducing your AUX lockout from the default 30-40°F down to 25-30°F can save $150-300/month during winter. Just make sure your heat pump can actually handle your home's heating load at those temperatures - if it's undersized or you've added major loads (like a hot tub), consult an HVAC professional before reducing AUX engagement further.

Should I choose Nest or Ecobee for better AUX control?

Ecobee generally provides more granular control over AUX heat with customizable lockout temperatures and advanced staging options. Nest's 'Heat Pump Balance' feature is automatic but less configurable. If you want to tweak settings yourself, Ecobee wins. If you prefer letting algorithms decide, Nest's learning capabilities might suit you better.

Why is my Nest thermostat using AUX heat so much?

Factory defaults trigger AUX heat far too aggressively, often kicking in at 30-40°F even though heat pumps can efficiently heat down to 15-20°F. You need to manually lower the AUX lockout temperature in the thermostat settings.

How much more does AUX heat cost?

AUX (resistance) heat costs roughly 3-5x more per BTU than heat pump operation because it runs at 100% efficiency (COP 1.0) versus the heat pump's 250-400% efficiency.

What is the ideal AUX lockout temperature?

For northern climates, set it to 25-30°F. For moderate climates, 20-25°F. Most modern heat pumps handle 15-20°F efficiently. Verify with your specific model specs.

Should I disable AUX heat entirely?

No. AUX heat is essential when temperatures drop below your heat pump's minimum operating range, typically 15-20°F. Running the heat pump below its rating can damage the compressor.

HVAC · Updated May 2026 · 12 min read · EcoHome Intelligence

Why Your Nest or Ecobee Is Secretly Costing You $800 a Month (AUX Heat Fix)

Smart thermostats promise to slash heating bills - but many homeowners see exactly the opposite. Instead of saving $50-100/month, they're burning an extra $200-500/month on electric heat strips. This hidden spike comes from AUX heat running too aggressively. Here's how to diagnose and fix this expensive mistake in just 10 minutes.

If you are not certain which mode your system is in, start with emergency heat vs auxiliary heat — they are different settings and only one of them is chosen by you.

In This Guide

The $500/Month Hidden Problem
Why AUX Heat Costs 5x More Per BTU
How to Tell If This Is Happening in Your Home
How to Fix: 4 Steps to Save $200-500/Month
Best Thermostats for Heat Pump Homes
Frequently Asked Questions

The $500/Month Hidden Problem

For the general case across any thermostat, not just Nest: auxiliary heat that will not turn off.

You bought a Nest Learning Thermostat or Ecobee Elite because your contractor said it would slash your electric bill by 15-20%. That was months ago. Now it's January and your utility statement just showed a $687 charge for heating - $300 more than your oil furnace ever cost in peak winter.

Before blaming rate increases, double-check your electric heater box. If copper strips are glowing orange through the ventilation holes, you're paying 300-500% more per BTU than you should be.

This massive billing spike happens because modern smart thermostats are wrongly programmed by default to kick in AUX heat when temperatures drop below 30-40°F - even though heat pumps can efficiently heat homes well below freezing (to around 15-20°F).

Our HVAC Efficiency Checklist walks you through verifying heat pump operation, checking AUX engagement, and validating system staging - step by step with pictures.

Download the Complete Checklist

Why AUX Heat Costs 5x More Per BTU

In a properly functioning heat pump system, the refrigerant cycle extracts free ambient warmth from outside air and concentrates it indoors - achieving 300% efficiency (COP 3.0) at moderate temperatures. At 47°F outdoor temperature, some modern units hit 400-600% efficiency (COP 4.0-6.0).

But when AUX heat kicks in, resistive elements take over. Electricity flows through metal strips (or ceramic elements) which simply warm up like an expensive toaster. This is electrical resistance heating - literally electrons bumping against atoms in a wire until they generate heat. It's the same principle as baseboard heaters, toasters, and clothes dryers. By definition, this process is 100% efficient (COP 1.0) because all input energy gets converted to heat - with nothing recovered.

The math is brutal:

Heat pump at 35°F: ~250% efficiency = $0.048/kWh effective cost
AUX heat: 100% efficiency = $0.12/kWh actual cost
Total savings from heat pump instead of resistance: 60%

A furnace running natural gas averages $0.80/therm or about $0.008/kWh effective input cost. Heat pumps beat gas handily in moderate climates - but when they give up and let resistance strips take over, efficiency drops below what a gas furnace would achieve.

How to Tell If This Is Happening in Your Home

These 5 red flags indicate AUX heat is running when it shouldn't be:

1. Bill Analysis

Pull 3 months of statements. Look for:

Electric usage jumping >300% above summer baseline
Natural gas dropping simultaneously (if you dual-fuel)
Heating kWh increasing 3-5x vs previous years

2. Visual Verification

In the basement or outside condenser unit:

Check for orange/red glow in electric strip heater boxes
Listen for loud mechanical humming (compressors run quieter)
Feel exterior unit - if it's warm despite frigid air, compressor likely running correctly

3. Thermostat Inspection

In Nest/Ecobee apps:

Nest: Settings Equipment View Live Status (shows which stage is active)
Ecobee: Menu Settings About Scroll to see live system status
Look for "Auxiliary", "Emergency", or second-stage activation

How to Fix: 4 Steps to Save $200-500/Month

Step 1: Confirm Wiring Type

Many "smart thermostat" problems begin with installation confusion. Before adjusting settings, confirm you have:

Heat Pump System: One reversing valve, variable-speed refrigerant loop, no visible ducted gas burner or boiler tank
Fossil Fuel Backup: Separate gas/oil furnace or boiler wired as discrete backup source
Electric Backup: Resistance strips integrated into heat pump system (common in mobile/manufactured homes)

If you're unsure, check your unit model numbers - heat pumps end in "-H" suffixes while AC-only units end in "-C" or numerical designations.

Step 2: Adjust AUX Lockout Temperatures

Factory defaults trigger AUX heat far too liberally. Optimal ranges depend on climate:

Northern Climates: Lower to 25-30°F
Moderate Climates: Lower to 20-25°F
Heat Pump Manufacturers Say: Most systems should handle 15-20°F efficiently (verify with your model specs)

Adjustment Process:

Nest: Settings Equipment (scroll to bottom) Auxiliary Heat Lockout Temperature Set to lowest available (usually 25°F)
Ecobee: Menu Settings Installation Scroll to "Auxiliary Heat" Change "Auxiliary Lockout Temperature" (25°F recommended)

Step 3: Enable Heat Pump Optimization Features

Both platforms offer algorithmic assistance for minimizing AUX waste:

Nest Heat Pump Balance: Uses machine learning to detect when home is warming due to heat pump operation vs AUX strips. Prevents unnecessary boost engagement. Settings Comfort Heat Pump Balance Always On
Ecobee Follow Me: Integrates occupancy sensors to reduce load on rooms where people aren't present. Helps heat pump maintain setpoint without boosting to resistance heat. Menu Settings Home Follow Me = Enabled
Cold Climate Mode: Increases minimum compressor runtimes on most systems, preventing short cycling that triggers AUX heat

Step 4: Add Proper Temperature Sensing

Without accurate outdoor readings, thermostats can't properly judge when AUX is actually needed. Solutions:

Ecobee Remote Sensors: Wirelessly report room temperatures. Allows thermostat to "follow" occupied areas rather than averaging zones where nobody sits
Nest Temperature Sensor: Works similarly but less flexible zoning options versus Ecobee's multiple wireless sensors
Outdoor Temperature Monitoring: Many newer models integrate this automatically. Older units may require add-on sensors or better thermostat placement away from heat sources
Verify Placement: Move thermostat away from drafts, direct sunlight, or heat-producing appliances. Even 6 inches can impact performance

If your setup problem starts before AUX heat settings — for example a new Amazon thermostat is stuck in setup or delayed start — use the Amazon Smart Thermostat C-wire checklist before changing heat pump lockout settings.

Best Thermostats for Heat Pump Homes

These are selected on published specifications and documented heat pump behaviour — principally whether aux heat lockout is user-configurable, and how the unit handles staging on a variable-speed system. We do not test these in-house; where a manufacturer claim is load-bearing we say so and link the source.

Budget: Nest Learning Thermostat (4th Gen)

Price: $169.99 | Rating: 4.4/5 stars | Payback Time: 3-12 months savings from AUX reduction
Learning algorithms excel at detecting human schedule patterns. Heat Pump Balance works reliably in real-world conditions. Setup wizard helps prevent common configuration mistakes. Lacks advanced zoning but includes single remote sensor. Best bang-for-buck among premium smart thermostats.

View on Amazon

Performance: Ecobee Smart Thermostat Premium

Price: $229.99 | Rating: 4.6/5 stars | Payback Time: 2-6 months from AUX savings
Superior AUX control with customizable lockout temp, occupancy-aware Follow Me technology, and support for six additional wireless sensors. Built-in Alexa speaker allows voice adjustment without smartphone. Complex interface intimidates novice users but delivers granular optimization possibilities via hidden menus. Best for heat pump homeowners who want configurability.

View on Amazon

Eco-Premium: Mysa Smart Thermostat for Electric Baseboard / Heat Pumps

Price: $159.99-$179.99 (multi-unit packs) | Rating: 4.2/5 stars | Payback Time: 6+ months savings
Designed specifically for electric baseboard/wall heaters or heat pump auxiliary systems. Integrates occupancy and daylight awareness directly in the wall unit. No cloud dependency or phone required for basic operation. Bluetooth mesh networking means no WiFi dead zones. Perfect for secondary heating zones (basement, bonus rooms) where central system struggles. Excellent pairing companion to primary Nest/Ecobee smart controllers.

View on Amazon

Related Guides

Affiliate Disclosure: EcoHome Intelligence participates in the Amazon Associates Program. Where our savings figures come from — estimates on this page are built on published ENERGY STAR, DOE and EIA figures, and your result will vary. Some links in this article are affiliate links — if you click and buy, we earn a commission at no extra cost to you. We only recommend products we have researched against published specifications, manufacturer documentation and owner reports. How we choose products.