The defrost cycle is a normal and essential function of air-source heat pumps. Understanding it helps you distinguish normal operation from potential problems.
When a heat pump extracts heat from cold outdoor air, moisture in the air freezes on the outdoor coil surface. Frost buildup reduces airflow and heat transfer, forcing the system to work harder. The defrost cycle temporarily reverses the refrigerant flow to send hot gas through the outdoor coil, melting accumulated frost. This is more common in Metro Vancouver's humid winter climate where temperatures hover around freezing.
The defrost cycle typically lasts 5 to 15 minutes and occurs every 30 to 90 minutes in frost-prone conditions. During defrost, you may notice steam rising from the outdoor unit as frost melts. The indoor system may switch to auxiliary or backup heat during this period, and you might hear a slight whooshing sound as the system reverses. A temporary drop in indoor temperature is normal. After defrost completes, the system resumes normal heating.
Excessive frosting between cycles, defrost cycles running too frequently or too long, or ice remaining after a complete cycle indicates a problem. Possible causes include low refrigerant charge, a faulty defrost control board, a defective reversing valve, or outdoor coil airflow obstruction. If your heat pump runs defrost cycles more than every 30 minutes or produces a solid sheet of ice on the coil, call a technician for diagnosis.
When the outdoor coil temperature drops below freezing, moisture in the air condenses and freezes on the coil as frost. Frost restricts airflow and insulates the coil, reducing heat transfer efficiency. The defrost cycle removes this frost to restore performance.
The heat pump briefly reverses to cooling mode: hot refrigerant flows to the outdoor coil, melting frost. The outdoor fan stops to retain heat. Indoor backup heat (electric strips or gas furnace) activates to prevent cold air delivery. A typical defrost cycle lasts 2-10 minutes.
Demand-defrost systems monitor coil temperature and runtime, initiating defrost only when needed. Timer-based systems defrost at fixed intervals regardless of frost buildup. Demand-defrost is more efficient, reducing unnecessary cycles. Most modern heat pumps use demand-defrost logic.
During normal operation, visible steam rising from the outdoor unit on cold, humid days is normal — it's water vapor from the defrost cycle, not smoke. Excessive or continuous defrost cycles indicate a problem: stuck reversing valve, faulty defrost sensor, or low refrigerant charge.
In Metro Vancouver's cool, damp climate, expect 1-4 defrost cycles per day during heating season. More frequent defrost in freezing rain or high humidity is normal. Excessive defrost (every 10-20 minutes) indicates a problem — likely a faulty defrost sensor, low refrigerant, or stuck reversing valve.
Heat pumps temporarily stop heating during defrost. Without backup heat, indoor temperatures would drop. Electric heat strips activate automatically to maintain comfort. Properly sized backup heat ensures you don't notice the defrost cycle. Inadequate backup heat causes noticeable cold drafts.
A stuck defrost relay keeps the unit in defrost mode, causing continuous heating of the outdoor coil and cooling indoors. A failed defrost sensor prevents defrost initiation, allowing frost to accumulate until the coil is completely blocked. Both require professional diagnosis and repair.
Frost on the coil during defrost is normal and melts in 2-10 minutes. Ice buildup that persists after defrost or extends beyond the coil indicates a problem: blocked drainage, low refrigerant, or failing defrost controls. Ice on the outdoor unit cabinet (not just the coil) is abnormal.
In Vancouver's climate, 1-4 defrost cycles per day is normal. More frequent cycles during high humidity, freezing rain, or fog are expected. Excessive defrost (every 10-20 minutes) indicates a malfunction. If you're unsure, note the interval between defrost cycles and consult a technician.
Yes. Steam or vapor rising from the outdoor unit during cold weather is water vapor from the defrost cycle. It's completely normal and indicates the defrost system is working correctly. This is a common concern from new heat pump owners but is not smoke or a malfunction.
During defrost, the system briefly switches to cooling mode while backup electric heat activates. The air may feel cooler briefly as backup heat ramps up. This is normal. If you consistently feel cold air for more than a few minutes, your backup heat system may not be functioning.
No — defrost is a necessary function to maintain efficiency and prevent damage. However, demand-defrost systems minimize unnecessary cycles. Keeping the area around the outdoor unit clear of snow and debris ensures proper drainage of melted frost and reduces refreeze issues.
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