Key Takeaways
- Heat pumps lose heat output in cold weather because of restricted heat exchange, icing, refrigerant issues or airflow blockages. Inspect outdoor coils, refrigerant lines and vents prior to blaming system failure.
- Defrost cycles are normal, but too much or too long defrosting cuts down on available heat. Check the outdoor unit for frost and ensure defrost controls function properly.
- Weak or cold air from vents indicates low refrigerant and blocked airflow. Keep an eye out for ice on lines, extended run times, and change or clean filters and remove blockages immediately.
- Check thermostat settings, batteries, and calibration. Try a programmable or smart thermostat to minimize misreadings and unnecessary cycling.
Heat pump not heating in cold weather is a frequent problem as temperatures fall out of its efficient operating window. Reasons include low refrigerant, frozen coils, defective defrost controls or a system too small for your climate.
Symptoms are weak airflow, long run times, or warm air that cools fast. Troubleshooting steps vary from inspecting filters and airflow to calling a certified technician for diagnostics and refrigerant or control repairs.
Why It Fails
Heat pumps transfer heat from the outdoor air into a building. Cold weather has less heat to pull, so output falls. Efficiency typically drops off steeply under around 35°F (2°C) and units have a much harder time once temperatures near or dip lower than 0°F (-18°C).
Older systems approaching the standard 12-year life expectancy are often depleted in cold snaps, and leaky homes occasionally bleed as much as 40% of heating energy, which compounds the issue.
1. Defrost Mode
Defrost mode reverses the refrigeration cycle to melt ice on the outdoor coils. This is normal, but excessive or prolonged defrost cycles reduce heat provided indoors as the system heats the outdoor coil rather than the home.
Check for heavy frost or a rim of ice on the outdoor unit, which is visible during cold, wet weather and indicates too much cycling. Make sure defrost controls and sensors work.
Failed sensors or incorrect defrost timing leave the heat pump in defrost too often, lowering indoor comfort and increasing runtime.
2. Low Refrigerant
Reduced refrigerant decreases heat transfer and evaporator output. Indicators include extended heating cycles, vented cold air, and ice on refrigerant lines or the outdoor coil.
A slow leak allows pressure to drop while working the compressor, putting it at risk of expensive failure. Leak detection, such as gauges and leak detectors, is important.
Do not add refrigerant without repairing the leak. If basic checks do not resolve the performance, arrange for a trained technician for repair, leak repair, evacuation, and proper recharge.
3. Blocked Airflow
Flow restriction damages capacity and can lead to icing. Verify filters and vents, and clean coils from dust, pet hair, or even snow.
Change clogged filters every 90 days, as a rule. Dirty filters not only make the fan work harder, they actually decrease the amount of heat moved indoors.
Outside, remove snow and debris and maintain a minimum of 60 cm (two feet) of clearance so air can circulate. Within, your heat pump goes to work longer.
Keep supply and return vents open to ensure air flow is balanced throughout your home.
4. Thermostat Issues
Thermostat errors cause bad cycle control. Check it’s on heat and a constant desired temperature. Defective sensors or inadequate calibration generate incorrect readings and short cycling.
Try not to make manual setpoint adjustments and turnoffs. Starting and stopping the system causes it to work harder and reduces efficiency.
Think about investing in a programmable or smart thermostat for reliable regulation and possible cost efficiency.
5. Outdoor Unit
Snow, ice and surrounding obstructions restrict outdoor heat transfer. Give them a de-gunking and check the fan and coils for damage or odd noise indicating mechanical issues.
Maintain an open space and listen and feel for any vibrating or humming that lets you know a motor or fan blade is failing.
In extreme cold, certain air-source heat pumps flat out can’t keep up. You’ll need some supplemental heat or a cold-climate model.
Your First Steps
Start with a nice, methodical to-do list to run through prior to phoning a tech. This quick triage helps divide simple user-mendable problems from malfunctions requiring professional repair. The checklist below spans power, controls, airflow, and obvious physical issues. Follow it to save time and junk repair bills.
Check Thermostat
Make sure the thermostat mode is set to ‘heat’ or ‘auto’ and that the desired temperature is above room temperature. See if any error codes or warning icons are being displayed and note them. Replace batteries or reset if the unit is unresponsive or errors out. Fifty percent of faults are due to low battery or corrupted settings.
Verify preset schedules to make sure the system isn’t controlled by a surprise crash time. Test response by turning the setpoint up several degrees and listen for the system to start. Observe if the indoor fan starts and if the outdoor unit powers on. Note any disconnect between thermostat commands and system behavior. That aids in determining if the problem is control related or with the heat pump.
Check Filters
Find the air filter in the indoor air handler or return grille and see if it looks dirty and clogged. A clogged, dusty filter decreases airflow, decreases heating output, and can cause the system to bog down on cold days. Swap out throwaway filters or clean washable filters every 1 to 3 months throughout the heating season.
Make a replacement schedule on a calendar. If you notice decreased airflow, excessive indoor dust, or hot and cold spots in your home, consider filter maintenance as a prime culprit. Write down the filter size and type so replacements are accurate, as an ill-fitting or wrong MERV rating can restrict flow.
Check Vents
- Living room ceiling vent
- Master bedroom floor vent
- Hallway return grille near stairs
- Kitchen supply vent
- Basement or lower-level registers
Check each vent above for warm air when the system operates in heat mode. Push furniture, curtains, or rugs that may be obstructing registers. Take vent covers off and clean visible dust. Check ducts where accessible for loose connections or heavy buildup.
If certain rooms are cold and others warm, write down which registers blow little or no air. That aids in figuring out whether the issue is distribution or the heat pump unit.
Check Unit
Check indoor and outdoor units for obvious damage, leaks, or ice on coils. Make sure all unit power switches and circuit breakers are on. A tripped breaker is a common first fix. Listen for rattling, buzzing, or grinding sounds and note where they are coming from.
If you come across ice, refrigerant hissing, or breakers tripping constantly, document this for the technician and don’t run the system any more.
The Cold Truth
While heat pumps work great in many climates, their heating ability decreases with outside temperatures. It describes the physics limits, the technological advances that shrink the gap, and how design and installation decisions affect actual winter performance.
Performance Limits
Typical air-source heat pumps utilize outdoor air as their heat source and become less efficient below 0°C. Refrigerant pressure and compressor work increase, so the unit shifts less heat per kilowatt of input. Most systems have electric resistance or gas backup that kicks on automatically when the heat pump can’t keep up with the thermostat setpoint.
Keep an eye on the heating capacity ratio, which is output heat over rated heat at standard conditions, to check how your unit fares on frigid days. A ratio of 50–70% at −10°C is typical for legacy units. Anticipate increased energy bills and sporadic cold spots as one outdoor unit works hard to supply warm air evenly amongst several rooms.
Long cold snaps reveal weaknesses: duct losses, undersized units, and poor insulation magnify comfort problems.
Modern Technology
Variable-speed compressors and inverter drives allow compressors to run at partial speed and maintain output during conditions. These systems minimize short cycling, increase part-load efficiency, and maintain a more consistent indoor temperature.
Cold-climate heat pumps are designed with bigger heat exchangers, refrigerant charges, and controls optimized for low-temperature operation, and manufacturers may publish performance curves down to −25°C. Optimized defrost algorithms minimize how often and how long defrost cycles run, so indoor comfort doesn’t drop when the outdoor coil frosts.
New refrigerants and two-stage compressors drive the effective operating range down. Replacing old equipment with something modern can reduce electric backup run time and decrease seasonal energy consumption. Real savings vary with climate, electricity costs, and installation quality.
Design Differences
- Air-source single-speed has a lower cost and poorer low-temperature performance.
- Air-source variable-speed provides smoother control and better part-load efficiency.
- Cold-climate air-source has a higher upfront cost and is reliable at lower temperatures.
- Ground-source (geothermal): high efficiency across seasons, costly install.
- Ductless mini-split offers zoned control, reduces duct loss, and provides variable outdoor performance.
Proper sizing matters: oversizing short cycles and undersizing causes constant struggle. Enhance insulation and airtightness. Efficient heat pump performance travels farther in tightly sealed homes.
Consider site factors: prevailing wind, elevation and shading affect outdoor coil frost and capacity. Match system style to home design. Big open plans lend themselves to central systems, while retrofit rooms can favor multi-head mini-splits.
Balance advantages and disadvantages with your wallet and your long-term energy objectives.
The Backup Plan
A backup plan gives you an alternative heating source when a heat pump can’t keep up on a bitterly cold day. It safeguards your loss of comfort, minimizes the risk of frozen pipes, and provides homeowners the peace of mind that they’ll have heat if their primary system is hard-pressed.
Here’s the lowdown on what backup options are, how they work, and what to look for when backup heat is called into action.
What Is It
| Backup Type | Typical Use Case | Pros | Cons |
|---|---|---|---|
| Electric resistance (heat strips) | Small to medium homes, retrofit to existing heat pump | Fast response, simple control, no combustion | High operating cost, lower efficiency |
| Gas furnace (dual fuel) | Cold climates, homes with gas supply | Efficient at low temps, lower fuel cost per kWh equivalent | Requires gas line, higher install cost, maintenance |
| Hybrid / dual fuel system | Homes seeking efficiency across a wide temp range | Optimizes cost by switching between heat pump and furnace | Complex controls, higher upfront cost |
| Secondary boiler or radiant | Large homes, hydronic systems | Even heat distribution, works with existing infrastructure | High install cost, slower response |
Backup heat can be electric resistance coils, gas furnaces, or other. Emergency heat settings are common on many thermostats. They cause supplemental systems to operate when outside temperatures become frigid.
Emergency heat is typically much less energy efficient than the heat pump in its normal operation, so use should be restricted to genuine cold snaps or system breakdown.
How It Works
Thermostats detect either outdoor temperature or a decrease in system output and automatically engage backup or emergency mode. In easy systems, a changeover switch or the thermostat’s emergency heat setting switches on electric heat strips, or a dual-fuel controller activates a gas furnace.
Heat strips and furnaces add sensible heat directly to the air stream and heat indoor air rapidly when the heat pump’s capacity drops. Backup systems bridge the gap when the heat pump cannot keep up, maintaining room setpoint and safeguarding pipes.
Keep an eye on energy consumption during backup runs. Electric resistance can skyrocket energy costs to two or three times your usual bills, and even a gas backup increases consumption and cost.
When It Fails
No heat on a frigid day, constantly tripping breakers, or a skyrocketing electric bill indicate backup failure. For electric backups, examine breakers, fuses, and burned heating elements. These are common faults and often easy to repair.
For furnace backups, check gas supply, pilot or ignition systems, and condensate or venting concerns. Gas supply interruptions and ignition failure are common causes.
If backup doesn’t engage, arrange for professional service immediately. A well-serviced heat pump and routine backup component checks minimize the risk of simultaneous failure and limit emergency heat usage.
Winter Preparation
Winter months mean higher heating loads and more stress on a heat pump. Getting the system and the home ready minimizes the risk the pump will fall behind in colder weather and limits expensive emergency repairs. The steps below address sizing, placement and service, as well as air-sealing and easy user actions that maintain a heat pump’s efficiency.
Proper Sizing
| Insulation Level | Recommended Heat Pump Output (W per m²) | Example: 100 m² Home (kW) |
|---|---|---|
| High (well insulated) | 60–80 W/m² | 6.0–8.0 kW |
| Medium (average) | 90 to 120 W/m² | 9.0 to 12.0 kW |
| Low (poor insulation) | 130 to 160 W/m² | 13.0 to 16.0 kW |
Undersized systems can’t heat up or maintain comfortable temperatures. If your heat pump operates all the time and can’t bring the inside temperature up, then it’s the wrong size or it’s degraded.
Reference manufacturer capacity tables and a qualified contractor to align output to square footage and the home’s insulation grade. Provide ceiling height, window area, and local design temperature for accurate sizing. Replace with a larger or more efficient model if calculations indicate current capacity is insufficient for winter loads.
Unit Placement
Locate the outdoor unit to avoid exposure to wind, snow, and falling ice as much as possible. Find it by a wall or windbreak that breaks the wind but does not stop it.
Raise the unit on a concrete pad or bracket so the cabinet is above normal snow depth, exhaust blockage, and ice formation around coils. Space at least 0.6 to 1.0 meters clearance on all sides and more above the unit for heat exchange and service access.
Don’t locate the unit under roof runoff or where drifting snow collects. It can make a big difference in performance to move the unit just a few meters. Remove leaves, sticks, and snow frequently so defrost cycles function as designed.
Regular Service
Plan for annual pro maintenance before heating season. A technician ought to check refrigerant charge, test electrical connections, inspect defrost controls and confirm backup heat operation where fitted.
Clean coils, fans and swap out or clean air filters as dirty filters block airflow and reduce efficiency. Test integrated controls that shift to backup heat, so it operates seamlessly even in extreme cold.
Document each service visit, parts replaced, and performance observations for warranty and troubleshooting. Basic homeowner behaviors, like keeping filters clean, setting a steady thermostat and avoiding frequent large setpoint changes, help the system run more efficiently and extend component life.
When to Call
A quick check can nab easy errors. Recognize when you need an expert. If simple checks — thermostat settings, circuit breakers, filter condition, outdoor unit clear of debris — don’t restore the heat, call a licensed HVAC pro.
We can have technicians run system diagnostics, measure refrigerant pressures, test electrical components and verify that the problem is repairable or if efficiency loss indicates a near-term replacement.
Contact a professional HVAC contractor if basic troubleshooting does not resolve heating issues.
Call when the unit still doesn’t work after you change the filter, reset the thermostat and clear the outdoor unit. Anticipate the technician to inspect airflow, blower operation, refrigerant charge and control boards.
For instance, intermittent no heat with the fan on usually indicates a cracked heat exchanger. If your system is older than 10 to 15 years, mention the age in the call. The older the unit, the more likely it has component fatigue that makes repair financially imprudent.
Seek expert heat pump repair for persistent problems like low refrigerant, defrost failures, or backup system malfunctions.
Low refrigerant shows as reduced output and can cause long run times and higher bills. Defrost circuit failures make the outdoor coil ice up and lead to short cycling or cold air delivery.
A pro will test sensors, timers, and heaters. Backup electric strip heaters or gas furnaces that don’t kick in require electrical and control checks. Note that efficiency drops notably below 35°F (about 1.7°C) and can fall sharply near or below 0°F (−17.8°C), so technicians will evaluate whether the heat pump and backup system match local climate demands.
Arrange for routine maintenance services to prevent unexpected failures during winter.
Book annual tune-ups before the cold weather hits. A service visit includes cleaning coils, checking refrigerant, tightening electrical connections, lubricating motors and calibrating controls.
Routine care keeps efficiency high and minimizes surprises like unexpected breakdowns or spiking energy consumption. If you’re already experiencing uneven heating, cold spots, weird sounds or escalating energy bills, schedule a service soon instead of waiting for a disastrous breakdown.
Trust experienced technicians for complex HVAC repairs and advanced heating solutions tailored to your home.
Seasoned techs know when to call in reinforcements, recommending upgrades like higher-efficiency heat pumps, properly sized units, or improved ductwork and zoning to combat chronic shortfall or frequent repairs.
If your unit requires repair constantly, think replacement. Constant repairs are a sure sign of a fading lifespan and diminished value. Call in a professional and be sure to call a qualified contractor who can evaluate performance, offer costed options, and safely make repairs.
Conclusion
A heat pump can stop giving heat in cold weather for a few clear reasons: low refrigerant, frozen coils, weak defrost cycles, or a worn compressor. You can inspect the outdoor unit, clear snow and ice, switch to emergency heat if necessary, and reset breakers. Keep filters clean, get a pro tune-up every year, and consider a cold-climate model or a little bit of electric backup for very low temps. If you hear strange noises, smell burning, or see unmelted frost, call a professional technician. Respond quickly to obvious symptoms to prevent more significant damage. For hands-on help or a reliable local tech, reach out to a licensed HVAC pro today.
Frequently Asked Questions
Why is my heat pump not heating when it’s very cold outside?
Heat pumps become inefficient below their intended operating range. Ice buildup, low refrigerant, or a failing defrost cycle can halt heating. First, check error codes and frost on the outdoor unit.
Can I keep using my heat pump in temperatures below freezing?
Yes, it becomes less efficient. Anticipate greater electrical consumption and sluggish warm-up. Go for a backup heat source for very cold times to keep folks comfortable and to keep the system safe.
What quick checks should I do before calling a technician?
Check filters, outdoor unit airflow, and thermostat setting to heat and correct mode. Reset power at breaker if safe. Observe for any error codes or strange noises.
Does refrigerant level affect cold-weather performance?
Yes. Low refrigerant diminishes heat transfer and results in heat pumps not heating well in cold weather. A professional technician needs to inspect and fix leaks and recharge the system.
How often should I schedule professional maintenance for winter?
Have a tech check your heat pump once a year before winter. They will test refrigerant, defrost cycle, electrical components, and airflow to avoid cold-weather breakdowns.
What is the defrost cycle and why does it matter?
The defrost cycle melts ice on your outdoor coil. If it breaks down, ice restricts air flow and heating ceases. A technician can troubleshoot sensors, valves, or control problems.
When should I replace my heat pump instead of repairing it?
Replace if your unit is over 12 to 15 years old, requires frequent repairs, or still struggles in cold weather even after repairs. Newer models are more efficient and better at low temperatures.
