If your heat pump is blowing cool air on a cold night, it’s probably not broken. Heat pumps lose heating capacity as outdoor temperatures drop, and below a certain point they run the backup heat or just can’t keep up. That said, some cold-weather calls do turn out to be real failures: low refrigerant, a stuck reversing valve, or a defrost board that won’t cycle. Here’s how to tell the difference.
How a Heat Pump Actually Works in the Cold
A heat pump moves heat from outside air into your home. Even at 35°F there’s usable heat in the air, but extracting it takes more work than at 55°F. Every unit has a “balance point,” typically somewhere between 30°F and 45°F depending on the system’s size and efficiency rating, where the outdoor heat available roughly equals what your home needs. Below that point, the system can’t carry the full load on its own and either the electric resistance strips or a gas backup kicks in.
So if it’s 38°F outside and your supply air feels only warm (not hot), that may be exactly what the system is designed to do. This is not a failure.
Defrost Mode Looks Wrong But Usually Isn’t
When the outdoor coil gets cold enough, frost builds up on it. The unit has to periodically reverse itself and push hot refrigerant through the outdoor coil to melt that frost. During defrost, the outdoor fan shuts off, steam sometimes rises from the unit, and the air coming out of your vents feels cool or even cold for a few minutes. The backup heat is supposed to come on during this cycle to cover the gap.
If your heat pump seems to “stop heating” for 5 to 15 minutes at a time and then recovers, you’re probably just watching defrost cycles. Completely normal in cold weather.
What’s NOT normal: defrost that never ends, the unit cycling in and out of defrost every few minutes, or the outdoor unit iced over solid (coil buried, not just light frost on the fins).
When There’s a Real Problem
Low refrigerant. If the refrigerant charge is low, the outdoor coil gets colder than it should and ices over faster. You’ll see thick ice buildup that doesn’t go away, and the system won’t heat well at any outdoor temperature, not just cold ones. A refrigerant shortage doesn’t fix itself. It means there’s a leak somewhere that a tech needs to find and repair before recharging the system.
Reversing valve failure. The reversing valve is what lets a heat pump switch between heating and cooling modes. When it gets stuck, the system can keep running but in the wrong mode, or it can lose efficiency dramatically. Symptoms vary: sometimes it blows cold air in heat mode, sometimes it heats weakly, occasionally it makes a hissing or gurgling sound near the valve itself. This part requires a refrigeration tech to diagnose and replace.
Defrost control board issues. If the board that initiates and ends defrost cycles isn’t working right, the unit can get stuck in defrost, or never defrost at all and ice over completely. A tech can test the board and the sensors it relies on (typically a thermistor mounted on the outdoor coil) to figure out which component has failed.
Oversized or undersized system. This one isn’t a breakdown but it matters. A heat pump that was sized wrong for the house may work fine at mild temperatures and fall apart when it actually gets cold. If you’ve had complaints about heating performance since the system was installed, that’s worth bringing up.
What You Can Check Yourself
Check your air filter. A clogged filter restricts airflow across the indoor coil, which tanks system efficiency and can cause the unit to perform poorly. If you haven’t changed it in a few months, do that first before calling anyone.
Look at the outdoor unit. A light coating of frost on the coil fins is normal in cold, humid weather. Ice that covers the entire coil and doesn’t go away after the system has been running is not normal. If you see that, turn the unit off and let it thaw before running it again, then call a tech.
Check your thermostat. Make sure it’s set to HEAT, not COOL or FAN ONLY. Sounds obvious but it happens. Also confirm the temperature setting is actually above the current room temperature.
Make sure the outdoor unit isn’t buried in leaves, blocked by something you set against it, or sitting in standing water that froze. Clearance around the unit matters.
What Requires a Tech
Don’t try to add refrigerant yourself. The equipment requires EPA Section 608 certification, and putting refrigerant into a system with an active leak just means it leaks out again. A proper repair involves finding the leak, fixing it, pulling a vacuum on the system, and then recharging to the correct spec.
Reversing valve replacement is a refrigerant-side repair. The system has to be recovered, the valve brazed in or out, and the system recharged. Not a DIY job.
Defrost board diagnosis requires the right meter, the unit’s wiring diagram, and knowing what’s normal for that specific board. Leave it to a tech.
When to Call a Pro
If the outdoor unit is iced over solid and won’t clear after thawing, that’s a tech call. Same if the system runs constantly but the house won’t get warm on a mild day, or you’re hearing a loud hiss or gurgle near the outdoor unit. Blowing cold air in heat mode while the unit sounds like it’s running normally is another one that needs a diagnosis, not a reset.
Systems 10 to 12 years or older that start struggling in cold weather are worth a full inspection before next winter. Low refrigerant, a failing defrost board, and a worn reversing valve don’t fix themselves, and they get worse.
We cover the Tri-Valley and East Bay. If something on this list matches what you’re seeing, call us. We’ll get you on the schedule fast, often same or next day when we can, and we’ll tell you exactly what we found and what it takes to fix it. adriumservice.com.