Heat Pump Running Constantly in Cold Weather? In North Alabama, That Is Usually Correct

If your heat pump has been running without stopping since the temperature dropped, the short answer is that in North Alabama it is probably doing exactly what it is supposed to do. A heat pump that runs for long stretches on a cold night is not a heat pump that is broken. It is a heat pump that is working.

That is not a comforting generality. There is a real test behind it, and it comes down to one number and one question. The number is the outdoor temperature. The question is whether the house is holding the temperature you set while the system runs.

Here is the problem with most of what you will find on this subject: nearly all of it is written for cold climates, and it tells you that constant running becomes normal "below 30 degrees." That advice is useless here. In Cullman, 30°F is not the exception. It is the average January night. Measured against the cold-climate rule, a Cullman heat pump should almost never run constantly, and yet yours is running constantly at 38°F, which leaves you with no answer at all. So let us give you the one that actually fits the weather outside your window.

The Short Answer

• Long run times in cold weather are normal. Heat pumps are designed to run long and steady, not in short blasts like a furnace.
• In a Cullman winter, overnight is the expected time for it to run nonstop. Our January nights sit right on top of the temperature where that behavior begins.
• The thing to watch is not the clock. It is the thermostat. If the house is holding the temperature you set, the length of the cycle does not matter.
• It is a problem when it runs constantly and loses ground, or when it runs constantly in mild weather, or when auxiliary heat is on while it is 50°F outside.

Why a Heat Pump Runs Long and a Furnace Runs Short

A gas furnace makes heat. It burns fuel, produces a large amount of very hot air quickly, satisfies the thermostat, and shuts off. You get short, strong bursts with quiet gaps between them. Most people grew up with that rhythm and learned to read it as "working."

A heat pump does not make heat. It moves it. It pulls warmth out of the outdoor air and carries it inside, which is why it is so much cheaper to run than electric strip heat. Moving heat is a gentler process than burning fuel, so the air coming out of your vents is warmer than the room but nowhere near furnace hot. To deliver the same amount of heat, the system has to run longer.

That is the design, not a compromise. Long, steady run times are how a heat pump keeps a house at an even temperature without the swings a furnace produces. A heat pump that short cycles, firing up and shutting off every few minutes in cold weather, is the one we worry about.

The Number That Decides It: Your Balance Point

Every heat pump has a temperature at which the heat it can pull from outside exactly equals the heat your house is losing. That temperature is called the balance point.

• Above the balance point, the heat pump produces more heat than the house needs. It satisfies the thermostat and shuts off. You see normal cycling.
• At the balance point, production and demand match. Run times stretch out. The system may not shut off at all.
• Below the balance point, the heat pump alone cannot keep up, so the backup heat strips step in to cover the gap. The heat pump keeps running the whole time, because its heat is still the cheap heat.

For most houses around Cullman, the balance point lands somewhere between 30°F and 40°F, depending on insulation, windows, ductwork and how the system was sized. Your installer can set it, and a thermostat can be configured around it, but the physics underneath is the same in every house.

If you want the full explanation of what the backup strips are, when they should come on and what they cost per hour at Cullman Electric rates, we wrote that up separately in auxiliary heat versus emergency heat.

Why Cullman Winters Sit Right on Top of the Balance Point

This is the part the national articles cannot tell you, and it is the whole answer.

Here are the 1991 to 2020 climate normals for Cullman:

Those figures come from the NOAA climate normals for Cullman.

Now put them next to a balance point of 30°F to 40°F and look at what happens over a single winter day here.

Overnight, the temperature falls into the low 30s, which is at or just under the balance point for a typical Cullman house. Your heat pump runs continuously, and on the coldest nights the strips help. That is correct operation.

By afternoon, the temperature climbs to around 50°F, comfortably above the balance point. The same heat pump that ran all night now cycles on and off normally.

That daily swing is the tell, and it is the single most useful thing you can watch. A healthy heat pump in Cullman runs nonstop at 3 a.m. in January and cycles normally at 3 p.m. the same day. Nothing is wrong with the one in the morning, and nothing has been repaired by the afternoon. The weather crossed the balance point, twice, the way it does most days from December through February.

A homeowner in Minnesota reading the "below 30 degrees" rule gets a sensible answer, because their winter lives well below that line and continuous running is simply the season. A homeowner in Cullman gets nonsense, because our winter spends every single day crossing back and forth across that line.

How Long Should It Actually Run?

Stop counting minutes and run this instead. It takes about two minutes.

1. Look at the outdoor temperature. Phone weather is close enough.

2. Look at what the thermostat reads versus what it is set to.

That is the test. Here is how to read it:

• It is in the 30s and the house is holding its setting. Working correctly. Long or continuous running is expected at this temperature. Leave it alone.
• It is in the 30s and the house is one or two degrees under the setting. Usually still fine, especially overnight or after you raised the thermostat. A heat pump catches up slowly by design. Watch it rather than reacting.
• It is in the 30s and the house is drifting steadily further from the setting. Worth a call. The system is running flat out and still losing.
• It is 50°F or warmer and the system still will not shut off. This is the clearest warning sign on the list. At that temperature a healthy heat pump should be cycling comfortably, so something is keeping it from doing its job.
• Auxiliary heat shows on the thermostat in mild weather. The strips are covering for the heat pump, and they are expensive. Have it looked at.

The trap to avoid is the opposite reflex: switching the system to emergency heat because it seems to be struggling. That shuts the heat pump off entirely and runs the house on the most expensive heat you own, and it does not make the house any warmer. Emergency heat is for a broken heat pump, not for cold weather.

The Steam Is Not Smoke, and the Ice Is Usually Normal

Two of the most alarming things you can see on a winter morning are both part of normal operation, and both happen because of the same thing: the defrost cycle.

When a heat pump pulls heat out of cold, damp air, frost forms on the outdoor coil. Frost blocks airflow, so every heat pump periodically reverses itself for a few minutes to melt it off. During that reversal the outdoor unit gets hot, the frost turns to water and steam, and the fan stops.

So if you walk outside and find the unit wrapped in steam with the fan still, that is a defrost cycle. It is not smoke and nothing is burning. It lasts a few minutes and ends on its own. You may also hear a loud whoosh when it starts and again when it finishes, which is the reversing valve changing direction.

A light coat of frost or a thin layer of ice on the outdoor coil between defrost cycles is also normal in our damp winters.

What is not normal is a unit encased in a solid block of ice, with the coil buried and the fan blades iced in. That means the defrost cycle is not doing its job, and it will not fix itself. Shut the system off at the thermostat and call us. Running it that way can damage the compressor, and a compressor is the single most expensive part of the system.

One related note for heat pump owners: if the air coming from your vents smells musty when the system runs in the cold, that is a separate and very common problem with its own cause, and we covered it in dirty sock syndrome.

Air That Feels Cool Is Not Proof of a Problem

Heat pump air typically comes out in the 90s, which is warmer than your house but cooler than your skin. Hold your hand to a vent and it can feel like the air conditioning is on, even while the system is heating the house correctly.

Judge it by the thermostat, not by your hand. If the room temperature is holding, the system is delivering heat. The exception is air that is genuinely cold, not merely cool, which usually means a defrost cycle is running right now, a reversing valve is stuck, or the refrigerant charge is low.

When Constant Running Is Actually a Problem

Long run times plus any of the following are worth a service call rather than a wait-and-see:

• The house keeps losing ground against the thermostat setting
• It runs nonstop at 50°F or warmer outdoors
• Auxiliary or emergency heat shows on the thermostat in mild weather
• Your power bill jumped sharply compared with the same month last year
• Defrost cycles are running very frequently or lasting a long time
• The outdoor coil is solid ice, the fan will not turn, or there is grinding or screeching
• Some rooms are much colder than others

When we get called out for a heat pump that runs constantly and still cannot keep up, the causes we find most often are a slow refrigerant leak that has quietly reduced the system's capacity, restricted airflow from a clogged filter or a dirty coil, a defrost control that is misbehaving, or a thermostat with its backup heat settings configured wrong. All four are repairs, not replacements, and all four get worse and more expensive the longer they run.

What You Can Check Yourself in Ten Minutes

Before you call anyone, there are four things worth doing, and they cost nothing.

1. Pull the filter and hold it up to a light. If light does not pass through easily, replace it. A choked filter is the single most common cause of a system that runs and runs without keeping up, and it is the cheapest thing on this list by a wide margin.
2. Walk the house and check your vents and return grilles. Rugs, furniture and closed registers all restrict airflow. Closing vents in unused rooms does not save money, it raises duct pressure and makes the system work harder.
3. Look at the outdoor unit. Clear leaves, pine straw and grass clippings from around it, and give it at least two feet of room. Do not cover it or wrap it for winter. A heat pump has to breathe all season.
4. Check the thermostat for AUX or EM HEAT. Note what the outdoor temperature is when you see it. That one observation tells a technician a great deal before anyone opens a panel.

What is not a homeowner job: checking the refrigerant charge, setting the balance point or the aux lockout, testing the defrost board, and anything involving the heat strips, which run on high voltage circuits. Those need gauges, meters and a license.

The Cheapest Version of This Is the One You Handle in October

Almost every heat pump that cannot keep up in January was showing signs in the fall. A charge that drifted low over the summer, a coil that needed cleaning, a defrost board starting to fail: all of it is visible on a fall heating tune-up, weeks before the weather turns it into an emergency call and a high power bill. Our fall and spring maintenance checklist covers what you can do yourself and what a professional visit should include. And if your system is old enough that you are weighing repairs against replacement, that decision has its own four questions.

Still Not Sure Whether Yours Is Normal?

Veterans Cooling and Heating is a veteran-owned, licensed Alabama HVAC contractor (AL#24210) with 15+ years in the trade, serving Cullman, Hanceville, Good Hope, Vinemont, Holly Pond, Fairview, Baileyton, West Point, Joppa and Arab. We will tell you straight whether your heat pump is behaving the way it should for the weather, and you get a written price before any work begins.

Call (256) 915-2826 or request service online. We are open Monday through Friday, 7 AM to 6 PM, and same-day appointments are available. Right now our $89 Heating Tune-Up covers the fall checks above, and if a repair turns into a bigger conversation, we can point you at financing.