If you own a heat pump, you might have noticed that sometimes your system gets louder than usual, blows cooler air for a few minutes, and the outdoor unit seems to be steaming. This is often your heat pump system entering defrost mode – a normal and essential self-care cycle that helps maintain efficient operation during cold, damp weather, such as the mild winters common in the San Francisco Bay Area.

A heat pump works by absorbing heat from the outdoor air and transferring it to the air inside your home. Unlike traditional systems that create heat, heat pumps move existing heat from one place to another, extracting heat from the cool outdoors and delivering it indoors. The process relies on components like the expansion valve, which regulates refrigerant flow and temperature, and the indoor unit, which distributes hot air or cool air throughout your home while also circulating and filtering the air inside to support indoor air quality.

Air source heat pumps are a popular and efficient type of heat pump system, especially for residential heating and cooling. They operate most effectively in moderate climates, but in colder regions, supplemental systems may be needed to provide heating during extreme cold. The heat pump’s efficiency depends on the temperature difference between the heat source (outdoor air) and the heat sink (air inside your home); a smaller temperature difference leads to higher efficiency and lower energy consumption.

During operation, heat exchange occurs between the indoor unit and the air inside your home, with the refrigerant experiencing a temperature rise during compression, enabling the transfer of heat. Heat pumps can use various heat sources, such as geothermal or air, and some advanced systems can even utilize waste heat. In addition to providing heating, many heat pumps can also heat water, supplying hot water for showers or radiant floor heating, making them a versatile solution for residential heating needs.

The air temperature produced by heat pumps can feel different from the average body temperature (98.6°F), which may affect how warm or cool the air feels to occupants. Maintaining good indoor air quality is important, and the indoor unit plays a key role in filtering and circulating air inside the home.

Heat pumps help reduce waste heat and emissions in residential heating, supporting energy efficiency and environmental goals. Their ability to extract heat from the cool outdoors and provide both heating and cooling makes them a smart choice for many homeowners.

In this article, Galaxy Heating & Air Conditioning explains what defrost mode is, why it happens, how long it lasts, and when it’s time to call a professional.

Introduction to Heat Pumps

Heat pumps are a modern, energy efficient solution for both heating and cooling homes and businesses. Unlike traditional heating systems that generate heat by burning fuel, heat pumps work by transferring heat from one place to another. This innovative approach allows heat pumps to deliver significant cost savings and reduce energy consumption, making them an increasingly popular choice for property owners seeking efficient heating and cooling.

At the heart of a heat pump’s operation is the ability to transfer heat, even when outdoor temperatures drop. In heating mode, air source heat pumps absorb heat from the outdoor air and move it indoors, providing warm air to keep your space comfortable. When summer arrives, the process reverses: the heat pump transfers heat from the indoor air to the outside, acting much like an air conditioner to provide cool air. This dual capability means heat pumps offer year-round comfort, efficiently handling both heating and cooling needs.

There are several types of heat pumps available. The most common are air source heat pumps, which extract heat from the outside air, even in extremely cold temperatures. Ground source heat pumps, sometimes called geothermal systems, transfer heat to and from the ground, taking advantage of the earth’s stable temperatures for even greater energy efficiency. Both types are designed to minimize energy costs and environmental impact, making them attractive options for those looking to upgrade their HVAC system.

One of the standout benefits of heat pumps is their impressive energy efficiency. Because they move heat rather than generate it, heat pumps can deliver more thermal energy than the electricity they consume. This translates to lower energy bills and a smaller carbon footprint. In colder climates, modern heat pumps are engineered to absorb heat from outdoor air even when temperatures drop below freezing, ensuring reliable and efficient heating performance.

However, as heat pumps work to absorb heat from the outdoor air, especially in cold weather, frost or accumulated ice can sometimes form on the outdoor coil. To maintain efficient heating and protect the system, the heat pump temporarily switches into defrost mode – a normal part of heat pump operation. During this cycle, the system may blow cold air indoors for a short time, but this is simply part of the process that keeps your heat pump running smoothly.

Whether you’re considering an upgrade to a more energy efficient system or want to better understand your current heat pump, knowing how these systems transfer heat and maintain comfort in all seasons can help you make informed decisions. With their ability to provide both heating and cooling, reduce energy costs, and operate efficiently even in colder climates, heat pumps are a smart investment for Bay Area homeowners and businesses alike.

Why Heat Pumps Frost Up in the First Place

Heat pumps operate by transferring heat between the indoor and outdoor air, using a heat transfer process that relies on a liquid refrigerant cycling through the system. In heating mode, the outdoor unit (also called the outdoor coil or condenser) acts like an evaporator, absorbing heat from the cold outdoor air. Even when temperatures drop, the heat pump extracts thermal energy from the air outside to provide efficient heating indoors.

However, when the outdoor air is cool and humid, moisture can condense on the outdoor coils and freeze, creating a layer of frost or ice. This frost buildup is normal to some extent, but if it becomes too thick, it can block airflow and insulate the coils, reducing the heat pump system’s heating performance and energy efficiency. To prevent this, heat pumps are designed to periodically enter defrost mode to melt away the frost and keep the system running smoothly.

What “Defrost Mode” Actually Does

Most modern heat pumps use a method called reverse-cycle defrost to clear frost from the outdoor unit. During this process, the system temporarily reverses the refrigerant flow using a reversing valve, switching from heating mode to cooling mode. This reversal causes the outdoor coil to warm up, melting the accumulated ice.

Here’s what happens during defrost mode:

1. The heat pump temporarily reverses operation, causing the outdoor coil to heat up instead of absorbing heat.
2. The outdoor fan stops running to help the coil warm faster by reducing airflow over it.
3. As the ice melts, you may see steam rising from the outdoor unit—this is just water vapor, not smoke.
4. Inside your home, the thermostat may display “Aux Heat” or “Defrost.” This is because many systems activate auxiliary heat (such as electric heat strips or a dual-fuel furnace) to maintain comfortable indoor temperatures while the heat pump’s heating function is temporarily paused.

Some newer heat pumps also feature demand defrost, which uses sensors to detect actual frost accumulation and only initiates defrost cycles when necessary, saving energy compared to older, timed defrost controls.

When the Defrost Cycle Starts, and How Long It Lasts

The timing and frequency of defrost cycles depend on several factors, including the specific model, outdoor temperature, and humidity levels:

• Triggers for defrost mode include the outdoor coil temperature sensor detecting frost, time elapsed since the last defrost cycle (for older systems), and ambient temperature combined with humidity.
• Defrost cycles typically last between 5 and 15 minutes (the typical cycle for most systems), but in cases of heavy frost, they can extend up to 15 minutes.
• On chilly, damp days with temperatures in the mid-30s to mid-40s °F, defrost cycles may occur every 30 to 90 minutes.
• In drier or colder conditions, defrost cycles are less frequent.

If you notice defrost cycles lasting longer than about 20 minutes or occurring every few minutes, this could indicate a problem requiring professional attention.

What’s Normal vs. What Isn’t

Understanding the difference between normal and abnormal defrost behavior can help you avoid unnecessary service calls while spotting real issues early.

Normal signs of defrost mode include:

• The outdoor fan stops during defrost to allow the coil to warm.
• A distinct whoosh sound as the reversing valve shifts refrigerant flow.
• Steam (water vapor) rising off the outdoor unit for a few minutes as ice melts.
• A short burst of cooler supply air indoors; this is often masked by auxiliary heat.
• A water puddle under or around the unit after defrosting, which is melted frost draining away.

Signs that are not normal and warrant calling a professional:

• Thick ice completely encasing the outdoor unit or solid ice buildup on the top cap.
• The outdoor fan never restarts after a defrost cycle finishes.
• Defrost cycles that take more than 20 minutes repeatedly or occur every 10–15 minutes.
• Unusual noises such as grinding, screeching, or loud bangs.
• Extended periods of no heat indoors or the thermostat stuck on auxiliary or emergency heat for hours.
• Error codes displayed on the thermostat or outdoor control board.
• Frequent circuit breaker trips or visible electrical arcing or sparking.

Quick Homeowner Checklist (60-Second Sanity Check)

If you suspect an issue with your heat pump’s defrost cycle, here’s a quick way to assess the situation:

1. Look: Light frost on outdoor coils is normal; thick glazed ice is not.
2. Listen: A brief whoosh and fan pause is normal. Harsh scraping or continuous noise is not.
3. Watch: Steam for a few minutes is expected. Smoke or no fan restart is concerning.
4. Feel: Slightly cooler air indoors during defrost is normal; no heat for 20+ minutes is not.
5. Time it: Defrost cycles longer than 15–20 minutes or very frequent cycles require professional evaluation.

Comfort and Energy Efficiency Tips During Defrost Season

To keep your heat pump operating efficiently and your home comfortable during defrost season, consider these tips:

• Avoid large jumps in thermostat settings. Raising the thermostat by just 1–2° at a time prevents excessive use of auxiliary heat, which can increase energy costs.
• Maintain at least 18–24 inches of clearance around the outdoor unit. Trim back plants, remove leaves, and keep pets away to ensure proper airflow.
• If your outdoor unit sits directly on soil, ask about installing a pad or stand to allow meltwater to drain and reduce refreezing under the unit.
• Change air filters every 1–3 months. Restricted airflow caused by dirty filters can increase frost buildup and reduce indoor comfort.
• Gently rinse the outdoor coil with a garden hose during mild weather to keep it clean. Avoid using power washers, which can damage the coils.
• Keep your system set to “Heat” mode rather than “Emergency Heat,” unless instructed otherwise by a technician.

Special Bay Area Note

The Bay Area’s unique microclimates with cool, damp mornings and mild afternoons are ideal conditions for light frost on heat pump coils. It’s common to see a defrost cycle mid-morning on foggy days. Modern air source heat pumps with demand defrost manage this efficiently, while older timed systems may defrost more often than necessary.

When to Call Galaxy Heating & Air Conditioning

If you notice any of the abnormal symptoms described above, or if:

• Ice does not clear after a defrost cycle.
• Defrost cycles are very frequent and indoor heating suffers.
• Your winter energy bills spike unexpectedly while comfort declines.
• You find yourself relying heavily on emergency heat to stay warm.
• You suspect a faulty reversing valve, which can prevent your heat pump from switching properly between heating and cooling modes.

Contact Galaxy Heating & Air Conditioning. Our experienced technicians can diagnose issues with sensors, defrost control logic, the reversing valve (including troubleshooting a faulty reversing valve), refrigerant charge, outdoor fan motors, and airflow. We also assess duct static pressure and filter sizing, since poor indoor airflow can worsen outdoor icing.

Serving Contra Costa, Alameda, and the San Francisco Bay Area, Galaxy Heating & Air Conditioning offers licensed workmanship, transparent pricing, and reliable service. Call us at 📞 (925) 578-3293 to schedule service or get a free estimate.

FAQ

Is steam coming off my unit dangerous?

No. The steam you see is simply water vapor created as the warm outdoor coil melts frost. It is completely normal and safe.

My outdoor fan stopped: did it fail?

During defrost mode, the outdoor fan intentionally stops to allow the outdoor coil to warm more quickly. It should restart once defrost is complete.

Why is my indoor air cooler during defrost?

Since the heat pump temporarily reverses to warm the outdoor coil, it stops heating the indoor air. Many systems activate auxiliary heat to compensate, but without it, you may feel cooler air for a few minutes.

Why is my heat pump blowing cold air when it should be heating?

If your heat pump is blowing cold air instead of warm air, it could be due to incorrect thermostat settings, the system being in defrost mode, low refrigerant, or a faulty component like the reversing valve. Check your thermostat and fan settings, and if the issue continues, contact a professional for diagnosis.

Why does my heat pump sometimes blow hot air, and how do fan settings affect this?

A heat pump blows hot air when it is heating properly, but if the fan is set to ‘on’ instead of ‘auto,’ it may circulate air even when the system isn’t actively heating, causing it to feel like it’s blowing cold air. Setting the fan to ‘auto’ ensures the system only blows hot air when heating is on.

How often should defrost happen?

Defrost cycles can vary from rarely to every 30–90 minutes on damp, chilly days. Very frequent or prolonged cycles usually indicate a problem.

Can I disable defrost?

No. Defrost mode is essential to maintain system efficiency and prevent damage from frost buildup.

If you’re ever unsure whether your heat pump’s defrost behavior is normal, snap a quick photo or video and contact Galaxy Heating & Air Conditioning. We’re here to help you understand your system and keep your home comfortable all year round.

Galaxy Heating & Air Conditioning Craftsmanship, reliability, and care season after season. 📞 (925) 578-3293If you own a heat pump, you might have noticed that sometimes your system gets louder than usual, blows cooler air for a few minutes, and the outdoor unit seems to be steaming. This is often your heat pump system entering defrost mode – a normal and essential self-care cycle that helps maintain efficient operation during cold, damp weather, such as the mild winters common in the San Francisco Bay Area.

A heat pump works by absorbing heat from the outdoor air and transferring it to the air inside your home. Unlike traditional systems that create heat, heat pumps move existing heat from one place to another, extracting heat from the cool outdoors and delivering it indoors. The process relies on components like the expansion valve, which regulates refrigerant flow and temperature, and the indoor unit, which distributes hot air or cool air throughout your home while also circulating and filtering the air inside to support indoor air quality.

Air source heat pumps are a popular and efficient type of heat pump system, especially for residential heating and cooling. They operate most effectively in moderate climates, but in colder regions, supplemental systems may be needed to provide heating during extreme cold. The heat pump’s efficiency depends on the temperature difference between the heat source (outdoor air) and the heat sink (air inside your home); a smaller temperature difference leads to higher efficiency and lower energy consumption.

During operation, heat exchange occurs between the indoor unit and the air inside your home, with the refrigerant experiencing a temperature rise during compression, enabling the transfer of heat. Heat pumps can use various heat sources, such as geothermal or air, and some advanced systems can even utilize waste heat. In addition to providing heating, many heat pumps can also heat water, supplying hot water for showers or radiant floor heating, making them a versatile solution for residential heating needs.

The air temperature produced by heat pumps can feel different from the average body temperature (98.6°F), which may affect how warm or cool the air feels to occupants. Maintaining good indoor air quality is important, and the indoor unit plays a key role in filtering and circulating air inside the home.

Heat pumps help reduce waste heat and emissions in residential heating, supporting energy efficiency and environmental goals. Their ability to extract heat from the cool outdoors and provide both heating and cooling makes them a smart choice for many homeowners.

In this article, Galaxy Heating & Air Conditioning explains what defrost mode is, why it happens, how long it lasts, and when it’s time to call a professional.

Introduction to Heat Pumps

Heat pumps are a modern, energy efficient solution for both heating and cooling homes and businesses. Unlike traditional heating systems that generate heat by burning fuel, heat pumps work by transferring heat from one place to another. This innovative approach allows heat pumps to deliver significant cost savings and reduce energy consumption, making them an increasingly popular choice for property owners seeking efficient heating and cooling.

At the heart of a heat pump’s operation is the ability to transfer heat, even when outdoor temperatures drop. In heating mode, air source heat pumps absorb heat from the outdoor air and move it indoors, providing warm air to keep your space comfortable. When summer arrives, the process reverses: the heat pump transfers heat from the indoor air to the outside, acting much like an air conditioner to provide cool air. This dual capability means heat pumps offer year-round comfort, efficiently handling both heating and cooling needs.

There are several types of heat pumps available. The most common are air source heat pumps, which extract heat from the outside air, even in extremely cold temperatures. Ground source heat pumps, sometimes called geothermal systems, transfer heat to and from the ground, taking advantage of the earth’s stable temperatures for even greater energy efficiency. Both types are designed to minimize energy costs and environmental impact, making them attractive options for those looking to upgrade their HVAC system.

One of the standout benefits of heat pumps is their impressive energy efficiency. Because they move heat rather than generate it, heat pumps can deliver more thermal energy than the electricity they consume. This translates to lower energy bills and a smaller carbon footprint. In colder climates, modern heat pumps are engineered to absorb heat from outdoor air even when temperatures drop below freezing, ensuring reliable and efficient heating performance.

However, as heat pumps work to absorb heat from the outdoor air, especially in cold weather, frost or accumulated ice can sometimes form on the outdoor coil. To maintain efficient heating and protect the system, the heat pump temporarily switches into defrost mode – a normal part of heat pump operation. During this cycle, the system may blow cold air indoors for a short time, but this is simply part of the process that keeps your heat pump running smoothly.

Whether you’re considering an upgrade to a more energy efficient system or want to better understand your current heat pump, knowing how these systems transfer heat and maintain comfort in all seasons can help you make informed decisions. With their ability to provide both heating and cooling, reduce energy costs, and operate efficiently even in colder climates, heat pumps are a smart investment for Bay Area homeowners and businesses alike.

Why Heat Pumps Frost Up in the First Place

Heat pumps operate by transferring heat between the indoor and outdoor air, using a heat transfer process that relies on a liquid refrigerant cycling through the system. In heating mode, the outdoor unit (also called the outdoor coil or condenser) acts like an evaporator, absorbing heat from the cold outdoor air. Even when temperatures drop, the heat pump extracts thermal energy from the air outside to provide efficient heating indoors.

However, when the outdoor air is cool and humid, moisture can condense on the outdoor coils and freeze, creating a layer of frost or ice. This frost buildup is normal to some extent, but if it becomes too thick, it can block airflow and insulate the coils, reducing the heat pump system’s heating performance and energy efficiency. To prevent this, heat pumps are designed to periodically enter defrost mode to melt away the frost and keep the system running smoothly.

What “Defrost Mode” Actually Does

Most modern heat pumps use a method called reverse-cycle defrost to clear frost from the outdoor unit. During this process, the system temporarily reverses the refrigerant flow using a reversing valve, switching from heating mode to cooling mode. This reversal causes the outdoor coil to warm up, melting the accumulated ice.

Here’s what happens during defrost mode:

1. The heat pump temporarily reverses operation, causing the outdoor coil to heat up instead of absorbing heat.
2. The outdoor fan stops running to help the coil warm faster by reducing airflow over it.
3. As the ice melts, you may see steam rising from the outdoor unit—this is just water vapor, not smoke.
4. Inside your home, the thermostat may display “Aux Heat” or “Defrost.” This is because many systems activate auxiliary heat (such as electric heat strips or a dual-fuel furnace) to maintain comfortable indoor temperatures while the heat pump’s heating function is temporarily paused.

Some newer heat pumps also feature demand defrost, which uses sensors to detect actual frost accumulation and only initiates defrost cycles when necessary, saving energy compared to older, timed defrost controls.

When the Defrost Cycle Starts, and How Long It Lasts

The timing and frequency of defrost cycles depend on several factors, including the specific model, outdoor temperature, and humidity levels:

• Triggers for defrost mode include the outdoor coil temperature sensor detecting frost, time elapsed since the last defrost cycle (for older systems), and ambient temperature combined with humidity.
• Defrost cycles typically last between 5 and 15 minutes (the typical cycle for most systems), but in cases of heavy frost, they can extend up to 15 minutes.
• On chilly, damp days with temperatures in the mid-30s to mid-40s °F, defrost cycles may occur every 30 to 90 minutes.
• In drier or colder conditions, defrost cycles are less frequent.

If you notice defrost cycles lasting longer than about 20 minutes or occurring every few minutes, this could indicate a problem requiring professional attention.

What’s Normal vs. What Isn’t

Understanding the difference between normal and abnormal defrost behavior can help you avoid unnecessary service calls while spotting real issues early.

Normal signs of defrost mode include:

• The outdoor fan stops during defrost to allow the coil to warm.
• A distinct whoosh sound as the reversing valve shifts refrigerant flow.
• Steam (water vapor) rising off the outdoor unit for a few minutes as ice melts.
• A short burst of cooler supply air indoors; this is often masked by auxiliary heat.
• A water puddle under or around the unit after defrosting, which is melted frost draining away.

Signs that are not normal and warrant calling a professional:

• Thick ice completely encasing the outdoor unit or solid ice buildup on the top cap.
• The outdoor fan never restarts after a defrost cycle finishes.
• Defrost cycles that take more than 20 minutes repeatedly or occur every 10–15 minutes.
• Unusual noises such as grinding, screeching, or loud bangs.
• Extended periods of no heat indoors or the thermostat stuck on auxiliary or emergency heat for hours.
• Error codes displayed on the thermostat or outdoor control board.
• Frequent circuit breaker trips or visible electrical arcing or sparking.

Quick Homeowner Checklist (60-Second Sanity Check)

If you suspect an issue with your heat pump’s defrost cycle, here’s a quick way to assess the situation:

1. Look: Light frost on outdoor coils is normal; thick glazed ice is not.
2. Listen: A brief whoosh and fan pause is normal. Harsh scraping or continuous noise is not.
3. Watch: Steam for a few minutes is expected. Smoke or no fan restart is concerning.
4. Feel: Slightly cooler air indoors during defrost is normal; no heat for 20+ minutes is not.
5. Time it: Defrost cycles longer than 15–20 minutes or very frequent cycles require professional evaluation.

Comfort and Energy Efficiency Tips During Defrost Season

To keep your heat pump operating efficiently and your home comfortable during defrost season, consider these tips:

• Avoid large jumps in thermostat settings. Raising the thermostat by just 1–2° at a time prevents excessive use of auxiliary heat, which can increase energy costs.
• Maintain at least 18–24 inches of clearance around the outdoor unit. Trim back plants, remove leaves, and keep pets away to ensure proper airflow.
• If your outdoor unit sits directly on soil, ask about installing a pad or stand to allow meltwater to drain and reduce refreezing under the unit.
• Change air filters every 1–3 months. Restricted airflow caused by dirty filters can increase frost buildup and reduce indoor comfort.
• Gently rinse the outdoor coil with a garden hose during mild weather to keep it clean. Avoid using power washers, which can damage the coils.
• Keep your system set to “Heat” mode rather than “Emergency Heat,” unless instructed otherwise by a technician.

Special Bay Area Note

The Bay Area’s unique microclimates with cool, damp mornings and mild afternoons are ideal conditions for light frost on heat pump coils. It’s common to see a defrost cycle mid-morning on foggy days. Modern air source heat pumps with demand defrost manage this efficiently, while older timed systems may defrost more often than necessary.

When to Call Galaxy Heating & Air Conditioning

If you notice any of the abnormal symptoms described above, or if:

• Ice does not clear after a defrost cycle.
• Defrost cycles are very frequent and indoor heating suffers.
• Your winter energy bills spike unexpectedly while comfort declines.
• You find yourself relying heavily on emergency heat to stay warm.
• You suspect a faulty reversing valve, which can prevent your heat pump from switching properly between heating and cooling modes.

Contact Galaxy Heating & Air Conditioning. Our experienced technicians can diagnose issues with sensors, defrost control logic, the reversing valve (including troubleshooting a faulty reversing valve), refrigerant charge, outdoor fan motors, and airflow. We also assess duct static pressure and filter sizing, since poor indoor airflow can worsen outdoor icing.

Serving Contra Costa, Alameda, and the San Francisco Bay Area, Galaxy Heating & Air Conditioning offers licensed workmanship, transparent pricing, and reliable service. Call us at 📞 (925) 578-3293 to schedule service or get a free estimate.

FAQ

Is steam coming off my unit dangerous?

No. The steam you see is simply water vapor created as the warm outdoor coil melts frost. It is completely normal and safe.

My outdoor fan stopped: did it fail?

During defrost mode, the outdoor fan intentionally stops to allow the outdoor coil to warm more quickly. It should restart once defrost is complete.

Why is my indoor air cooler during defrost?

Since the heat pump temporarily reverses to warm the outdoor coil, it stops heating the indoor air. Many systems activate auxiliary heat to compensate, but without it, you may feel cooler air for a few minutes.

Why is my heat pump blowing cold air when it should be heating?

If your heat pump is blowing cold air instead of warm air, it could be due to incorrect thermostat settings, the system being in defrost mode, low refrigerant, or a faulty component like the reversing valve. Check your thermostat and fan settings, and if the issue continues, contact a professional for diagnosis.

Why does my heat pump sometimes blow hot air, and how do fan settings affect this?

A heat pump blows hot air when it is heating properly, but if the fan is set to ‘on’ instead of ‘auto,’ it may circulate air even when the system isn’t actively heating, causing it to feel like it’s blowing cold air. Setting the fan to ‘auto’ ensures the system only blows hot air when heating is on.

How often should defrost happen?

Defrost cycles can vary from rarely to every 30–90 minutes on damp, chilly days. Very frequent or prolonged cycles usually indicate a problem.

Can I disable defrost?

No. Defrost mode is essential to maintain system efficiency and prevent damage from frost buildup.

If you’re ever unsure whether your heat pump’s defrost behavior is normal, snap a quick photo or video and contact Galaxy Heating & Air Conditioning. We’re here to help you understand your system and keep your home comfortable all year round.

Galaxy Heating & Air Conditioning Craftsmanship, reliability, and care season after season. 📞 (925) 578-3293If you own a heat pump, you might have noticed that sometimes your system gets louder than usual, blows cooler air for a few minutes, and the outdoor unit seems to be steaming. This is often your heat pump system entering defrost mode – a normal and essential self-care cycle that helps maintain efficient operation during cold, damp weather, such as the mild winters common in the San Francisco Bay Area.

A heat pump works by absorbing heat from the outdoor air and transferring it to the air inside your home. Unlike traditional systems that create heat, heat pumps move existing heat from one place to another, extracting heat from the cool outdoors and delivering it indoors. The process relies on components like the expansion valve, which regulates refrigerant flow and temperature, and the indoor unit, which distributes hot air or cool air throughout your home while also circulating and filtering the air inside to support indoor air quality.

Air source heat pumps are a popular and efficient type of heat pump system, especially for residential heating and cooling. They operate most effectively in moderate climates, but in colder regions, supplemental systems may be needed to provide heating during extreme cold. The heat pump’s efficiency depends on the temperature difference between the heat source (outdoor air) and the heat sink (air inside your home); a smaller temperature difference leads to higher efficiency and lower energy consumption.

During operation, heat exchange occurs between the indoor unit and the air inside your home, with the refrigerant experiencing a temperature rise during compression, enabling the transfer of heat. Heat pumps can use various heat sources, such as geothermal or air, and some advanced systems can even utilize waste heat. In addition to providing heating, many heat pumps can also heat water, supplying hot water for showers or radiant floor heating, making them a versatile solution for residential heating needs.

The air temperature produced by heat pumps can feel different from the average body temperature (98.6°F), which may affect how warm or cool the air feels to occupants. Maintaining good indoor air quality is important, and the indoor unit plays a key role in filtering and circulating air inside the home.

Heat pumps help reduce waste heat and emissions in residential heating, supporting energy efficiency and environmental goals. Their ability to extract heat from the cool outdoors and provide both heating and cooling makes them a smart choice for many homeowners.

In this article, Galaxy Heating & Air Conditioning explains what defrost mode is, why it happens, how long it lasts, and when it’s time to call a professional.

Introduction to Heat Pumps

Heat pumps are a modern, energy efficient solution for both heating and cooling homes and businesses. Unlike traditional heating systems that generate heat by burning fuel, heat pumps work by transferring heat from one place to another. This innovative approach allows heat pumps to deliver significant cost savings and reduce energy consumption, making them an increasingly popular choice for property owners seeking efficient heating and cooling.

At the heart of a heat pump’s operation is the ability to transfer heat, even when outdoor temperatures drop. In heating mode, air source heat pumps absorb heat from the outdoor air and move it indoors, providing warm air to keep your space comfortable. When summer arrives, the process reverses: the heat pump transfers heat from the indoor air to the outside, acting much like an air conditioner to provide cool air. This dual capability means heat pumps offer year-round comfort, efficiently handling both heating and cooling needs.

There are several types of heat pumps available. The most common are air source heat pumps, which extract heat from the outside air, even in extremely cold temperatures. Ground source heat pumps, sometimes called geothermal systems, transfer heat to and from the ground, taking advantage of the earth’s stable temperatures for even greater energy efficiency. Both types are designed to minimize energy costs and environmental impact, making them attractive options for those looking to upgrade their HVAC system.

One of the standout benefits of heat pumps is their impressive energy efficiency. Because they move heat rather than generate it, heat pumps can deliver more thermal energy than the electricity they consume. This translates to lower energy bills and a smaller carbon footprint. In colder climates, modern heat pumps are engineered to absorb heat from outdoor air even when temperatures drop below freezing, ensuring reliable and efficient heating performance.

However, as heat pumps work to absorb heat from the outdoor air, especially in cold weather, frost or accumulated ice can sometimes form on the outdoor coil. To maintain efficient heating and protect the system, the heat pump temporarily switches into defrost mode – a normal part of heat pump operation. During this cycle, the system may blow cold air indoors for a short time, but this is simply part of the process that keeps your heat pump running smoothly.

Whether you’re considering an upgrade to a more energy efficient system or want to better understand your current heat pump, knowing how these systems transfer heat and maintain comfort in all seasons can help you make informed decisions. With their ability to provide both heating and cooling, reduce energy costs, and operate efficiently even in colder climates, heat pumps are a smart investment for Bay Area homeowners and businesses alike.

Why Heat Pumps Frost Up in the First Place

Heat pumps operate by transferring heat between the indoor and outdoor air, using a heat transfer process that relies on a liquid refrigerant cycling through the system. In heating mode, the outdoor unit (also called the outdoor coil or condenser) acts like an evaporator, absorbing heat from the cold outdoor air. Even when temperatures drop, the heat pump extracts thermal energy from the air outside to provide efficient heating indoors.

However, when the outdoor air is cool and humid, moisture can condense on the outdoor coils and freeze, creating a layer of frost or ice. This frost buildup is normal to some extent, but if it becomes too thick, it can block airflow and insulate the coils, reducing the heat pump system’s heating performance and energy efficiency. To prevent this, heat pumps are designed to periodically enter defrost mode to melt away the frost and keep the system running smoothly.

What “Defrost Mode” Actually Does

Most modern heat pumps use a method called reverse-cycle defrost to clear frost from the outdoor unit. During this process, the system temporarily reverses the refrigerant flow using a reversing valve, switching from heating mode to cooling mode. This reversal causes the outdoor coil to warm up, melting the accumulated ice.

Here’s what happens during defrost mode:

1. The heat pump temporarily reverses operation, causing the outdoor coil to heat up instead of absorbing heat.
2. The outdoor fan stops running to help the coil warm faster by reducing airflow over it.
3. As the ice melts, you may see steam rising from the outdoor unit—this is just water vapor, not smoke.
4. Inside your home, the thermostat may display “Aux Heat” or “Defrost.” This is because many systems activate auxiliary heat (such as electric heat strips or a dual-fuel furnace) to maintain comfortable indoor temperatures while the heat pump’s heating function is temporarily paused.

Some newer heat pumps also feature demand defrost, which uses sensors to detect actual frost accumulation and only initiates defrost cycles when necessary, saving energy compared to older, timed defrost controls.

When the Defrost Cycle Starts, and How Long It Lasts

The timing and frequency of defrost cycles depend on several factors, including the specific model, outdoor temperature, and humidity levels:

• Triggers for defrost mode include the outdoor coil temperature sensor detecting frost, time elapsed since the last defrost cycle (for older systems), and ambient temperature combined with humidity.
• Defrost cycles typically last between 5 and 15 minutes (the typical cycle for most systems), but in cases of heavy frost, they can extend up to 15 minutes.
• On chilly, damp days with temperatures in the mid-30s to mid-40s °F, defrost cycles may occur every 30 to 90 minutes.
• In drier or colder conditions, defrost cycles are less frequent.

If you notice defrost cycles lasting longer than about 20 minutes or occurring every few minutes, this could indicate a problem requiring professional attention.

What’s Normal vs. What Isn’t

Understanding the difference between normal and abnormal defrost behavior can help you avoid unnecessary service calls while spotting real issues early.

Normal signs of defrost mode include:

• The outdoor fan stops during defrost to allow the coil to warm.
• A distinct whoosh sound as the reversing valve shifts refrigerant flow.
• Steam (water vapor) rising off the outdoor unit for a few minutes as ice melts.
• A short burst of cooler supply air indoors; this is often masked by auxiliary heat.
• A water puddle under or around the unit after defrosting, which is melted frost draining away.

Signs that are not normal and warrant calling a professional:

• Thick ice completely encasing the outdoor unit or solid ice buildup on the top cap.
• The outdoor fan never restarts after a defrost cycle finishes.
• Defrost cycles that take more than 20 minutes repeatedly or occur every 10–15 minutes.
• Unusual noises such as grinding, screeching, or loud bangs.
• Extended periods of no heat indoors or the thermostat stuck on auxiliary or emergency heat for hours.
• Error codes displayed on the thermostat or outdoor control board.
• Frequent circuit breaker trips or visible electrical arcing or sparking.

Quick Homeowner Checklist (60-Second Sanity Check)

If you suspect an issue with your heat pump’s defrost cycle, here’s a quick way to assess the situation:

1. Look: Light frost on outdoor coils is normal; thick glazed ice is not.
2. Listen: A brief whoosh and fan pause is normal. Harsh scraping or continuous noise is not.
3. Watch: Steam for a few minutes is expected. Smoke or no fan restart is concerning.
4. Feel: Slightly cooler air indoors during defrost is normal; no heat for 20+ minutes is not.
5. Time it: Defrost cycles longer than 15–20 minutes or very frequent cycles require professional evaluation.

Comfort and Energy Efficiency Tips During Defrost Season

To keep your heat pump operating efficiently and your home comfortable during defrost season, consider these tips:

• Avoid large jumps in thermostat settings. Raising the thermostat by just 1–2° at a time prevents excessive use of auxiliary heat, which can increase energy costs.
• Maintain at least 18–24 inches of clearance around the outdoor unit. Trim back plants, remove leaves, and keep pets away to ensure proper airflow.
• If your outdoor unit sits directly on soil, ask about installing a pad or stand to allow meltwater to drain and reduce refreezing under the unit.
• Change air filters every 1–3 months. Restricted airflow caused by dirty filters can increase frost buildup and reduce indoor comfort.
• Gently rinse the outdoor coil with a garden hose during mild weather to keep it clean. Avoid using power washers, which can damage the coils.
• Keep your system set to “Heat” mode rather than “Emergency Heat,” unless instructed otherwise by a technician.

Special Bay Area Note

The Bay Area’s unique microclimates with cool, damp mornings and mild afternoons are ideal conditions for light frost on heat pump coils. It’s common to see a defrost cycle mid-morning on foggy days. Modern air source heat pumps with demand defrost manage this efficiently, while older timed systems may defrost more often than necessary.

When to Call Galaxy Heating & Air Conditioning

If you notice any of the abnormal symptoms described above, or if:

• Ice does not clear after a defrost cycle.
• Defrost cycles are very frequent and indoor heating suffers.
• Your winter energy bills spike unexpectedly while comfort declines.
• You find yourself relying heavily on emergency heat to stay warm.
• You suspect a faulty reversing valve, which can prevent your heat pump from switching properly between heating and cooling modes.

Contact Galaxy Heating & Air Conditioning. Our experienced technicians can diagnose issues with sensors, defrost control logic, the reversing valve (including troubleshooting a faulty reversing valve), refrigerant charge, outdoor fan motors, and airflow. We also assess duct static pressure and filter sizing, since poor indoor airflow can worsen outdoor icing.

Serving Contra Costa, Alameda, and the San Francisco Bay Area, Galaxy Heating & Air Conditioning offers licensed workmanship, transparent pricing, and reliable service. Call us at 📞 (925) 578-3293 to schedule service or get a free estimate.

FAQ

Is steam coming off my unit dangerous?

No. The steam you see is simply water vapor created as the warm outdoor coil melts frost. It is completely normal and safe.

My outdoor fan stopped: did it fail?

During defrost mode, the outdoor fan intentionally stops to allow the outdoor coil to warm more quickly. It should restart once defrost is complete.

Why is my indoor air cooler during defrost?

Since the heat pump temporarily reverses to warm the outdoor coil, it stops heating the indoor air. Many systems activate auxiliary heat to compensate, but without it, you may feel cooler air for a few minutes.

Why is my heat pump blowing cold air when it should be heating?

If your heat pump is blowing cold air instead of warm air, it could be due to incorrect thermostat settings, the system being in defrost mode, low refrigerant, or a faulty component like the reversing valve. Check your thermostat and fan settings, and if the issue continues, contact a professional for diagnosis.

Why does my heat pump sometimes blow hot air, and how do fan settings affect this?

A heat pump blows hot air when it is heating properly, but if the fan is set to ‘on’ instead of ‘auto,’ it may circulate air even when the system isn’t actively heating, causing it to feel like it’s blowing cold air. Setting the fan to ‘auto’ ensures the system only blows hot air when heating is on.

How often should defrost happen?

Defrost cycles can vary from rarely to every 30–90 minutes on damp, chilly days. Very frequent or prolonged cycles usually indicate a problem.

Can I disable defrost?

No. Defrost mode is essential to maintain system efficiency and prevent damage from frost buildup.

If you’re ever unsure whether your heat pump’s defrost behavior is normal, snap a quick photo or video and contact Galaxy Heating & Air Conditioning. We’re here to help you understand your system and keep your home comfortable all year round.

Galaxy Heating & Air Conditioning Craftsmanship, reliability, and care season after season. 📞 (925) 578-3293