The calls I dread most are the ones I get in January during a cold snap, when someone has been without heat for a day and a half and every HVAC company in Cowlitz County has a three-day backlog. Often, the problem was there in October. The system was giving signals for weeks that something was wrong, and nobody caught it.
This post is about those signals. What to look for in the fall before your heat pump is working hard every night. Some of what you see or hear will be completely normal. Some of it will be worth calling about. And some of it means call today, not next week.
Start Here: The Defrost Cycle Is Not a Problem
Before anything else, I want to address the most common reason homeowners think their heat pump is broken when it is actually working correctly.
In heating mode, when outdoor temperatures are cold and humid, your heat pump will periodically go into a defrost cycle. During this cycle, the system temporarily reverses itself to melt ice that has built up on the outdoor coil. Here is what that looks like from the homeowner’s side:
- The outdoor unit may produce visible steam or vapor as ice melts
- You may hear a whooshing sound as the refrigerant reverses direction
- The air from your supply vents will feel cooler than normal for several minutes
- If you have auxiliary or backup heat, it should kick in to keep your indoor temperature stable
A defrost cycle typically lasts 5 to 15 minutes. It happens roughly every 30 to 90 minutes during sustained heating operation, though the system only triggers it when sensors detect ice, not on a fixed schedule. After it completes, the system returns to normal heating mode.
If what you are observing matches this description, your heat pump is working. The cool air is intentional. The steam is normal. The whooshing is normal. A lot of service calls in December turn out to be homeowners who saw steam rising from their outdoor unit and thought something was on fire.
Now, here is what is not normal.
Ice That Won’t Go Away
Some frost on the outdoor coil in cold, wet weather is expected and is handled by the defrost cycle. What is not normal is ice that builds up and does not clear, or that builds up again immediately after clearing.
If your outdoor unit is substantially encased in ice for more than an hour or two, or if you can see that ice is building up faster than the defrost cycle can handle it, something is wrong. The three most common causes:
Low refrigerant. When refrigerant charge is low, the system cannot generate enough heat during the defrost cycle to melt the ice it has accumulated. The outdoor coil surface gets colder than it should, ice builds faster, and the defrost cycle cannot keep up. If you let this go, the coil can freeze solid and the system will shut down or run in emergency mode.
Defrost sensor or control board failure. The system decides when to defrost based on sensor readings. If a sensor is reading incorrectly, the system may not trigger defrost at the right time, or may not complete it properly. This is a parts and labor repair, usually in the range of a few hundred dollars for the part, plus labor.
Dirty outdoor coil. A coil coated in dirt, cottonwood seed, or debris transfers heat less efficiently. This forces the outdoor coil to run colder, which increases ice formation. Sometimes cleaning the coil resolves persistent ice problems. This is one where a fall tune-up helps: a technician will clean the outdoor coil as part of a standard maintenance visit.
If you see ice that is not clearing, do not try to chip it off. You can damage the coil fins, which are fragile aluminum. Gentle water from a garden hose is okay if the system is off and you are trying to help it along, but persistent ice that keeps returning needs a technician to find the cause.
Signs of a Refrigerant Leak
Refrigerant leaks are worth calling about promptly, especially before winter. A heat pump with low refrigerant will struggle to heat your home, will run constantly trying to compensate, and will have the ice problem I described above.
Signs a homeowner can observe:
- Frost or ice on the refrigerant lines. The two copper lines that run from your outdoor unit into the house should not have ice on them. If you see frost or white coating on those lines, that is a sign the refrigerant charge is low.
- Hissing or bubbling sounds from the outdoor unit. Refrigerant escaping through a leak can make a hissing sound. Bubbling or gurgling is less common but also possible. These are not sounds your system should make.
- System runs constantly but cannot reach setpoint. If your heat pump is running all the time and your house is still not reaching 68 degrees on a 35-degree day, something is limiting its output. Low refrigerant is one of the most common causes.
- Steadily declining heating performance. If the system used to keep up easily and now seems to struggle on the same type of day, gradual refrigerant loss is one possibility.
Refrigerant does not deplete normally during operation. If the charge is low, there is a leak somewhere. Finding and repairing the leak is part of the fix; just recharging without addressing the source is a temporary solution that will need to be repeated.
What Different Sounds Mean
Sound is one of the most useful diagnostic tools homeowners have access to. Here is a guide to what common heat pump sounds actually indicate:
| Sound | What it suggests | How urgent |
|---|---|---|
| Grinding | Motor bearing failure; metal-on-metal contact | Stop the system, call immediately |
| Squealing or screeching | Failing motor bearing or compressor pressure issue | Call within 24 hours |
| Banging or clanking | Loose component inside the unit, possibly a fan blade | Call within 24 hours |
| Chattering or buzzing | Contactor struggling to make contact | Call within 24 hours |
| Hissing | Refrigerant leak or pressure relief | Call within 24-48 hours |
| Clicking every few seconds | Capacitor failing; system trying to start but can’t | Call within 24 hours |
| Loud humming without startup | Capacitor or contactor failure | Call within 24 hours |
| Whooshing | Normal; refrigerant reversing during defrost cycle | No action needed |
Grinding is the sound I take most seriously. If you hear metal grinding from your outdoor unit, turn the system off and call. Running a system with a failing bearing damages other components fast, and what might be a motor replacement becomes a much larger repair.
Blowing Cold Air in Heating Mode
This is one of the most common sources of confusion I encounter. The question is always the same: “My heat pump is set to heat but it’s blowing cold air. Is something wrong?”
The answer depends on timing.
If the cool air lasts 5 to 15 minutes and then warm air returns: This is a defrost cycle. The system reversed itself temporarily to melt the outdoor coil and will resume heating shortly. This is normal.
If the system is blowing cool air and has been for more than 15 to 20 minutes, or if it blows cool air every single time you turn on heating: Something is wrong. The most likely cause is a stuck reversing valve.
The reversing valve is the component that switches the heat pump between heating and cooling modes. If it gets stuck, the system may run in cooling mode when you have set it to heat. The result is a heat pump that actively cools your house while your thermostat is set to 70. You may also hear hissing, gurgling, or clicking from the outdoor unit as the valve struggles to shift.
A stuck reversing valve is a repair that requires a technician. The valve itself is not particularly expensive, but replacement involves recovering the refrigerant, replacing the valve, and recharging the system, so it is a meaningful labor job.
If you are not sure whether what you are experiencing is normal defrost or a valve problem, the simplest test is to wait it out. If warm air returns within 15 minutes, you are fine. If you are still getting cold air after 20 minutes with no sign of changing, call.
A Simple Performance Check You Can Do
If you have a non-contact infrared thermometer (they cost about $20 to $30 at any hardware store), you can do a basic heat pump performance check yourself.
Turn the system on in heat mode and let it run for at least 15 minutes without cycling or going into defrost. Make sure emergency or auxiliary heat is off. Then:
- Find a supply vent (where warm air comes out) and measure the air temperature
- Find a return vent (where air goes back into the system) and measure the air temperature
- Subtract the return temperature from the supply temperature
In heat mode, your supply air should be in the range of 90 to 110 degrees Fahrenheit. The difference between supply and return, called the temperature differential or delta T, should be somewhere in the range of 16 to 22 degrees.
If your supply air is consistently below 85 degrees or your delta T is below 16 degrees, the system is not producing the heat it should. This can indicate low refrigerant, a dirty coil, airflow restriction from a clogged filter, or other issues worth investigating.
This is not a diagnostic tool that replaces a technician, and results vary with airflow and home conditions. But it gives you a real data point to bring to a service call, or to judge whether the system is clearly underperforming.
Two Components That Fail in Fall: Capacitors and Contactors
These are the two most common individual component failures I see, and fall is when they show up.
Capacitors store electrical energy and provide the starting surge for motors. Every heat pump has at least one, and they wear out over time. Summer cooling puts heavy demands on them. They often fail in late summer or fall, right when you are switching to heating.
Signs a capacitor is failing:
- The outdoor unit hums or buzzes but the fan or compressor does not start
- You hear clicking every few seconds as the system tries and fails to start
- The fan spins slowly or not at all when the unit is calling for operation
- The system starts, runs for a moment, then shuts off
Capacitors are relatively inexpensive parts ($15 to $100 for the part itself), but they store enough electricity to be dangerous to someone who does not know how to handle them safely. This is not a DIY repair.
Contactors are electrical switches that connect power to the compressor and outdoor fan motor. They take a mechanical beating over years of opening and closing, and the contacts can burn or pit.
Signs a contactor is failing:
- The outdoor unit is completely unresponsive despite power at the breaker
- The unit operates intermittently, working sometimes and not others
- You hear a loud buzzing or chattering at startup, then nothing
- The system runs but the compressor and fan are not both operating at the same time
A bad contactor is usually a straightforward repair, in the same general cost range as a capacitor replacement. The reason to take it seriously before winter is that a failing contactor can fail completely at any time, and a complete failure means no heat.
When to Call Now vs. When to Schedule
Some of this can wait for a regular appointment. Some of it cannot.
Call the same day:
- Grinding sounds from the outdoor unit
- No heat at all and outdoor temperatures are dropping
- Circuit breaker trips and then trips again after you reset it
- Burning smell from the outdoor unit or air handler
- Outdoor unit completely unresponsive
Schedule within the next few days:
- Ice that is not clearing from the outdoor coil
- Hissing or bubbling sounds
- System running constantly without reaching setpoint
- Squealing, clicking, or chattering sounds
- System blowing cool air in heat mode for more than 15 minutes
Monitor, but can wait for a routine appointment:
- Slightly longer run times than last year (worth mentioning at your next service call)
- Minor performance differences that are not getting worse
- Frost that forms and clears on schedule
The Timing Argument for Fall Service
If you have been meaning to schedule a tune-up and have been putting it off, September and October are better than December and January. The main practical reason is availability: HVAC technicians are not nearly as backed up in the fall as they are in the middle of a cold spell when half the county is calling at once.
The other reason is that a fall tune-up catches exactly the kinds of problems this post describes. A technician will check refrigerant charge, inspect the coil, test the capacitors and contactor, check the defrost controls, and measure system performance. If something is borderline, you find out now rather than at 9 pm on a Tuesday in January when it’s 28 degrees and you have no heat.
If you want to schedule a fall checkup or you have noticed any of the signs above, call me and we will get it sorted out before the cold weather arrives.