Heat Pump Not Working in Cold Weather: Troubleshooting, Fixes, and Efficiency Tips

When a heat pump struggles on a freezing day, it can be frustrating and confusing. This guide explains how heat pumps behave in low temperatures, what “normal” looks like, and how to troubleshoot safely. It also covers common cold-weather faults, when to call a pro, costs, and upgrades that keep homes warm in winter.

Why Heat Pumps Seem To Struggle In The Cold

Heat pumps move heat using refrigeration, not fuel combustion. As outdoor temperatures drop, there is less available heat, so capacity falls and the unit runs longer. Long run times in cold weather are normal, especially near a home’s “balance point.”

The balance point is the temperature at which a heat pump’s output equals the home’s heat loss. Below that point, systems typically rely on auxiliary heat, like electric strips or a furnace in dual-fuel setups, to maintain comfort.

Modern cold-climate heat pumps can provide meaningful heat at single-digit temperatures. Many inverter-driven models maintain 70–100% of rated capacity at 5°F and operate down to -5°F or -15°F. Older single-stage systems often lose capacity faster below 25–30°F.

Efficiency, expressed as COP (coefficient of performance), also falls with temperature. A heat pump might reach a COP of 3 or more at 47°F, but around 1.5–2.0 at 17°F. This is still more efficient than electric resistance heat, which has a COP of about 1.

Expect normal defrost cycles in freezing, humid conditions. The outdoor coil will frost over and the unit will reverse briefly to melt ice, sometimes emitting steam. Short, periodic defrosts are expected in winter.

Quick Symptoms And What They Usually Mean

• Runs Constantly, House Barely Warm: Low outdoor temperature near/below balance point, dirty filter, outdoor coil frost, undersized system, or poor home insulation.
• Blowing Cool Or Lukewarm Air: In defrost mode, thermostat set too low for aux heat, low refrigerant, or reversing valve issue.
• Outdoor Unit Iced Over: Defrost fault, sensor problems, bad fan motor, low refrigerant, or blocked airflow; thick ice sheets are not normal.
• Aux Heat Not Coming On: Thermostat programming, wiring, tripped breaker, blown fuse, or failed heat strips/relays.
• Outdoor Fan Not Spinning: Normal during defrost, failed fan motor/capacitor, or iced blades; shut power before inspection.
• High Bills Suddenly: Aux heat running constantly, clogged filter, severe icing, failing components, or very low outdoor temps.
• System Short-Cycles Or Shuts Off: Safety lockout, thermostat issues, low voltage, or iced coil causing high pressure.

Step-By-Step Troubleshooting You Can Safely Do

Confirm Thermostat Modes And Setpoints

Ensure the thermostat is in Heat mode, not Cool or Off. If the heat pump cannot maintain setpoint, set the temperature 2–3°F higher to encourage auxiliary heat. For dual-fuel setups, confirm switchover settings are reasonable based on local climate.

In many thermostats, O/B settings control the reversing valve. If recently replaced, verify the O/B polarity matches your equipment. Incorrect settings can make the system cool instead of heat. Consult the thermostat’s manual for the correct configuration.

Check Airflow: Filters, Vents, And Returns

Dirty filters starve the system of air, reduce heating capacity, and can trigger defrost more often. Replace or clean filters every 1–3 months in winter. Confirm all supply registers are open and return grilles are unobstructed by furniture or rugs.

Restricted ductwork or closed dampers can cause low airflow. If some rooms are cold, check for blocked vents or broken dampers. In multi-story homes, balance registers seasonally to improve distribution during winter.

Inspect The Outdoor Unit For Snow, Ice, And Debris

Clear snow from around the outdoor unit, including the back and sides. Maintain at least 18–24 inches of clearance for airflow. Remove leaves or debris from the coil surface using gentle water flow if weather allows.

If the unit is encased in thick ice, turn power off at the disconnect before addressing it. Use lukewarm water to melt ice. Do not use sharp objects or de-icers. If icing returns quickly, schedule service—there may be a defrost or refrigerant issue.

Recognize Defrost Mode Behavior

In defrost, the system may stop the outdoor fan, reverse to cooling briefly, and produce steam. You might hear a whoosh as the reversing valve shifts. Short defrosts every 30–90 minutes in freezing conditions are normal.

Prolonged defrosts, frequent defrosts with little ice, or no defrost when iced suggest a sensor or control board problem. Excessive wind can also trigger more defrost; wind baffles sometimes help.

Verify Breakers, Disconnects, And Power

Check the indoor air handler and outdoor unit breakers. Heat strips often have separate breakers or fuses. Restore any tripped breaker once; repeated trips indicate a fault and require an HVAC technician.

Ensure the outdoor service disconnect is fully seated. Low voltage circuits can drop in extreme cold or during storms; if the thermostat goes blank, check the furnace/air handler door switch and low-voltage fuse.

Look For Condensate And Drain Issues

Some air handlers shut down if the condensate drain pan float switch trips. In winter, blocked drains or frozen condensate lines can cause intermittent operation. Verify the drain is clear and sloped properly.

Mini-Split Specific Checks

Confirm each indoor head is set to Heat, not Auto, if you need consistent heating. Ensure louvers direct air downward in heating mode. Outdoor icing on mini-splits should clear during defrost; persistent ice buildup requires service.

Auxiliary And Emergency Heat Basics

Auxiliary heat engages automatically when the heat pump cannot meet demand. Emergency heat bypasses the heat pump entirely and uses backup heat only. Use Emergency Heat for short-term failures, not as a season-long strategy due to higher cost.

If the air feels cold and the thermostat indicates “Aux Heat,” but the home still cools off, verify the heat strips have power and appropriate breakers are on. Faulty relays, contactors, or burned elements require a technician.

Quick DIY/Pro Guide

Frost, Ice, And Defrost: What’s Normal Vs Not

Light frost or a thin glaze on the outdoor coil in humid, freezing weather is expected. The unit should enter defrost and clear frost within a few minutes. Steam or a whooshing sound is common. Brief steam does not mean a fire.

Not normal: a solid ice block covering the coil, fan, or base pan; ice that returns quickly; or repeated defrosts without clearing the coil. These point to a defrost sensor issue, a faulty control board, low refrigerant, or a fan problem.

Some units include a base-pan heater to keep meltwater from refreezing. If the base pan is a solid ice sheet, the pan heater may be failed or missing. In windy areas, baffles or repositioning can reduce wind chill on the coil and cut defrost frequency.

If defrost never engages yet frost keeps growing, the outdoor sensor or its wiring may be damaged. Do not bypass sensors. A technician can test sensor resistance, the reversing valve coil, and defrost control logic.

Performance Expectations By Temperature

Even a working heat pump feels different from a furnace in winter. Supply air may be 85–100°F rather than 120–140°F, so air can feel “cool” to the skin yet still heat the home. Long, steady runs are efficient and reduce drafts.

Numbers vary by model and installation. Consult manufacturer capacity tables for your unit. Many modern systems publish 5°F capacity ratings and minimum operating temperatures.

Common Cold-Weather Faults And How They’re Fixed

• Dirty Outdoor Coil: Reduces heat absorption, increases defrost frequency. Remedy: Gentle cleaning with water; avoid pressure washers. Pro coil clean during maintenance.
• Failed Outdoor Fan Motor Or Capacitor: Causes rapid icing. Remedy: Professional diagnosis and replacement; verify fan blade integrity.
• Defrost Sensor Or Control Board Failure: Prevents proper defrost timing. Remedy: Replace faulty sensor/board; check wiring harness and connectors.
• Low Refrigerant Charge: Can cause frosting and poor heating. Remedy: Leak find-and-fix, weigh-in charge to specifications. Charging requires EPA-certified technician.
• Stuck Or Leaking Reversing Valve: System may cool when calling for heat or underperform. Remedy: Professional replacement; confirm coil voltage and control logic.
• Base-Pan Ice And Drainage Issues: Freezing meltwater blocks airflow. Remedy: Verify pan heater operation; clear drain exits; adjust unit level or install risers.
• Crankcase Heater Not Working: Cold start issues, compressor noise, trip on overload. Remedy: Verify heater power; replace if failed; use delay start in severe cold.
• Heat Strips/Relays Failed: No auxiliary heat, long run times. Remedy: Replace elements, sequencers, or contactors; confirm circuit sizing and breakers.
• Thermostat Misconfiguration: O/B reversing setting wrong or lockouts too aggressive. Remedy: Reconfigure to manufacturer specs; update firmware on smart stats.
• Wind Exposure: Increases icing and heat loss. Remedy: Add wind baffles or relocate unit; maintain clear airflow.
• Poor Duct Insulation Or Leaks: Heat lost in attics or crawlspaces. Remedy: Seal and insulate ducts; target R-8 in unconditioned spaces.

When To Use Emergency Heat

Use Emergency Heat if the outdoor unit is iced over, failed, or unsafe to operate. This mode uses electric strips or a furnace only. It restores heat quickly but costs more to run.

Turn Emergency Heat off once the heat pump is repaired or outdoor icing resolves. Continuous use will raise energy bills and may stress electrical circuits. If emergency heat cannot hold setpoint, schedule service to restore full system capacity.

Costs, Savings, And Rebates

Repair costs vary by region and model. Typical ranges: diagnostic visit $100–$200; capacitor $100–$300; fan motor $300–$700; defrost board $250–$600; heat strip element $200–$500; reversing valve $800–$1,800; refrigerant leak repair $400–$1,500+.

Operating costs rise in deep cold, especially if auxiliary heat runs. Improving insulation, sealing air leaks, and maintaining filters reduce runtime. Smart thermostat staging can limit unnecessary aux heat calls.

Federal incentives can offset upgrades. Many high-efficiency heat pumps qualify for tax credits under the Inflation Reduction Act (25C) and utility rebates. Check ENERGY STAR heat pumps and local utility programs.

Cold-climate systems on the NEEP cold-climate list are designed for winter performance. They often deliver substantial heat at 5°F with inverter compressors and enhanced vapor injection.

Preventive Maintenance Checklist For Winter

• Replace Filters: Every 1–3 months, more often with pets or construction dust.
• Clear Snow And Debris: Maintain 18–24 inches around the outdoor unit; keep above drifting snow with risers.
• Inspect Coil And Base Pan: After storms, check for ice; verify defrost clears coil fully.
• Confirm Thermostat Settings: Proper mode, O/B polarity, schedules, and lockouts; update firmware.
• Seal Duct Leaks: Use mastic or UL-181 tape; insulate ducts in unconditioned spaces.
• Test Auxiliary Heat: Early in season, verify strips or furnace stage responds to calls.
• Professional Tune-Up: Annual check: refrigerant levels, electrical, sensors, defrost logic, and airflow measurement.
• Protect From Wind: Install baffles where needed; do not block coil airflow.
• Keep Drains Clear: Ensure condensate lines and traps are unobstructed and protected from freezing.

Upgrading To A Cold-Climate Heat Pump

If the heat pump cannot meet winter needs or is older and inefficient, consider upgrading. Cold-climate models maintain capacity in freezing weather using inverter-driven compressors and advanced controls.

What to look for: published capacity at 5°F, operation down to -5°F or below, variable-speed indoor blowers, and defrost strategies that minimize comfort swings. Many brands offer ducted and ductless options for different home layouts.

Efficiency metrics have recently shifted to SEER2 and HSPF2. For cold climates, HSPF2 around 8.1–9.5+ is strong, but capacity at low temperature matters more for comfort. Check manufacturer extended performance data, not just nameplate ratings.

In ducted homes, confirm ductwork is sized for variable-speed airflow. In homes without ducts, ductless mini-splits can provide zoned heating with excellent low-temperature performance and high COPs at 17°F.

Dual-fuel setups pair a heat pump with a gas furnace. The thermostat or controls switch to gas below a chosen temperature. This can optimize comfort and cost in very cold regions with low gas prices.

Home Envelope Improvements That Help In Cold Weather

Before blaming the heat pump, evaluate the building envelope. Air sealing and insulation reduce heat loss, lowering the balance point and auxiliary heat usage. Start with attic insulation, air sealing around penetrations, and weatherstripping doors.

Energy audits with blower-door testing identify leaks and duct losses. Utilities often subsidize audits and insulation upgrades. The combination of a tuned heat pump and a tighter envelope provides the most reliable cold-weather comfort.

Safety Notes And What Not To Do

• Do not chip ice with screwdrivers or shovels; you can puncture the coil.
• Turn power off before clearing ice or working near the fan blades.
• Avoid covering the outdoor unit with plastic; it traps moisture and worsens icing.
• Do not bypass defrost sensors or short controls; it risks compressor damage.
• Use only lukewarm water on coils; boiling water or salt can damage components.

Helpful Resources

For fundamentals and buying guidance, see the U.S. Department of Energy’s overview of heat pump systems. For product criteria and rebates, check ENERGY STAR air-source heat pumps. For cold-climate listings, visit the NEEP CC-ASHP database.

FAQs: Short Answers

Why Is My Heat Pump Not Working In Cold Weather? It may be at or below its balance point, in defrost mode, or restricted by ice or dirty filters. Thermostat settings and auxiliary heat wiring also commonly cause issues.

How Cold Is Too Cold For A Heat Pump? Many cold-climate systems operate to -5°F or -15°F. Older models lose significant capacity below 25–30°F. Check your model’s low-temperature rating and 5°F capacity data.

Is It Normal For The Outdoor Unit To Steam? Yes. During defrost, steam is normal. Brief whooshing and temporary cool air inside are expected. Continuous steam or ice that never clears is not normal.

Should I Switch To Emergency Heat? Use it if the outdoor unit is iced over or failed. Turn it off once the heat pump is restored to avoid high energy costs.

Why Is My Thermostat Showing “Aux Heat”? The thermostat is calling for backup heat because the heat pump alone cannot meet demand. This is normal during very cold snaps or rapid temperature changes.

Can Low Refrigerant Cause Icing? Yes. Low charge reduces coil temperature and can cause frost or ice buildup. Only a licensed HVAC technician should check and correct refrigerant levels.

How Often Should My Heat Pump Defrost? In humid freezing weather, every 30–90 minutes is typical, lasting a few minutes. Frequency depends on humidity, temperature, and model control strategy.

What Maintenance Helps Most In Winter? Clean filters, clear snow and debris from the outdoor unit, verify proper thermostat mode, and schedule annual professional service to check defrost and airflow.

Why Is My Heat Pump Blowing Cold Air? It may be in defrost, the reversing valve may be misconfigured, or auxiliary heat is disabled. Verify thermostat O/B setting and heat strip breakers.

Will A Bigger Heat Pump Fix Cold-Weather Problems? Not always. Improper sizing can cause humidity and cycling issues. Improving insulation and selecting a cold-climate, inverter-driven unit is often better than oversizing.

What To Expect From A Professional Service Visit

A qualified technician will measure static pressure and airflow, check filters and coils, verify thermostat configuration, and review defrost operation. They will also inspect electrical components, sensors, relays, and contactors.

For suspected refrigerant issues, they will perform leak checks, recover and weigh charge, and recharge per specifications. For defrost faults, they will test the ambient and coil sensors and the control board, and verify reversing valve operation and voltage.

Expect clear documentation of findings, part recommendations, and cost estimates. Ask for model-specific low-temperature performance data and guidance on lockout or balance point settings for auxiliary heat.

Smart Controls And Staging Tips

Smart thermostats can optimize comfort and cost in cold weather. Use heat pump balance or staging features to limit unnecessary auxiliary heat, especially during small setpoint changes. Avoid large setbacks on the coldest days.

Outdoor temperature sensors and utility time-of-use schedules can trigger smart staging. In dual-fuel systems, select an economic switchover setpoint based on gas and electricity prices and manufacturer guidance.

Regional Considerations In The U.S.

In the Southeast and Mid-Atlantic, heat pumps typically handle most winter days without backup. In the Midwest and Northeast, cold-climate models or dual-fuel strategies provide reliable heat during extended cold snaps.

In high-altitude or windy plains areas, consider wind baffles, elevated mounting, and careful sizing. Local code and utility incentives may influence equipment choice and savings.

Key Takeaways For Fast Fixes

• Check thermostat mode, setpoint, and O/B setting if the air feels cool.
• Replace filters and clear snow to restore airflow and reduce defrosting.
• Recognize defrost as normal; steaming is expected for a few minutes.
• Use emergency heat briefly if the outdoor unit is iced over or failed.
• Call a pro for persistent icing, refrigerant issues, electrical faults, or control failures.

With the right maintenance, settings, and, when needed, a cold-climate upgrade, a heat pump can deliver consistent, efficient heat even when temperatures plunge.