How Does a Heat Pump Work in Winter, and When Should I Turn It On?

If you have a heat pump, you’ve probably noticed it doesn’t behave quite like a furnace. It runs differently; it sounds different in winter than in summer, and at some point someone may have told you something about not running it below a certain temperature, or not touching it if ice forms on the outdoor unit. Some of that advice is solid. Some of it is myth. And a lot of Georgia homeowners who’ve had heat pumps for years still aren’t entirely sure what their system’s actually doing when winter arrives.

It’s not generating heat. It’s moving it.

The name is a little confusing because in winter, a heat pump isn’t generating heat the way a furnace does. It’s moving it. Specifically, it’s extracting heat energy from the outdoor air and transferring it inside your home.

That feels counterintuitive. How is there heat in outdoor air when it’s 35 degrees outside? The answer is that there’s always some heat energy in air until you reach absolute zero, which is around negative 460 degrees Fahrenheit. At 35 degrees, there’s significantly less heat energy than at 75 degrees, but there’s still enough for the refrigerant in your heat pump to absorb, compress, and release inside your home.

The process is essentially the reverse of what happens in summer. In cooling mode, your heat pump absorbs heat from inside your home and releases it outdoors. In heating mode, it absorbs heat from outdoor air and releases it inside. Same equipment, same refrigerant, same coils, just running in reverse. A component called the reversing valve is what switches the system between the two modes.

When does a heat pump struggle?

Heat pumps are highly efficient at extracting heat from outdoor air when temperatures are moderate, which describes most of Georgia’s winter. But efficiency drops as temperatures fall, and below a certain threshold, usually somewhere between 30 and 40 degrees depending on the system, a heat pump has to work significantly harder to keep up.

Most heat pump systems installed in Georgia are paired with auxiliary electric heat strips, sometimes called emergency heat or backup heat, that supplement the heat pump when outdoor temperatures drop to this range. The system manages this automatically. When it detects the heat pump alone can’t keep up, the auxiliary strips activate. You’ll often notice your energy bill ticking up during cold stretches because electric resistance heat strips are less efficient than the heat pump itself.

That’s normal and expected. It’s not a sign that something’s wrong. What would be a sign of a problem is if your home isn’t reaching the set temperature during mild winter days when the heat pump should be handling the load easily on its own.

When should you switch from cooling to heating?

There’s no rule that says you have to switch on a specific date, and in Georgia that wouldn’t make sense anyway since October can give you 80-degree days and 45-degree nights in the same week. Switch when overnight lows are consistently dropping into the 50s and you’re waking up to a house that feels cool.

With a standard heat pump thermostat, switching modes is usually just changing the setting from Cool to Heat and adjusting your target temperature. The system handles the rest. There’s no harm in switching back and forth during the shoulder season as the weather moves around.

One thing worth knowing: when a heat pump switches to heating mode for the first time after a long summer, you may notice a faint burning smell for the first few minutes. That’s typically dust that’s settled on the auxiliary heat strips burning off and is generally harmless. If the smell persists beyond the first couple of cycles, or smells like something other than dust, it’s worth having someone look at it.

What’s the defrost cycle, and why is there ice on my outdoor unit?

This is one of the most common sources of confusion for heat pump owners in Georgia. During heating mode, the outdoor unit is absorbing heat from cold air, and the coil surface can drop below freezing. When moisture in the outdoor air contacts that cold coil, it can freeze. This is normal.

What’s also normal is that your heat pump has a defrost cycle designed to periodically melt that ice. During a defrost cycle, the system temporarily reverses to cooling mode to send warm refrigerant through the outdoor coil. You may see steam rising from the outdoor unit, and you may feel cool air briefly blowing from your vents. The system’s doing exactly what it’s supposed to do.

What’s not normal is ice that covers the entire outdoor unit for several hours and doesn’t clear on its own. That can indicate a problem with the defrost controls, a refrigerant issue, or restricted airflow around the unit. If your outdoor unit’s heavily iced over and not clearing, it’s time to call someone.

Why fall is the right time to have your heat pump serviced

Heat pumps are more complex than furnaces because they handle both heating and cooling, which means there are more components that can develop issues over time. A refrigerant charge that’s slightly low won’t show up dramatically in cooling mode but can significantly affect heating capacity in winter when the system’s already working harder.

The reversing valve is another one to watch. It can fail or stick, and this tends to surface in the transition season when it’s actually being asked to change positions. A stuck reversing valve typically means the system can only operate in one mode. Discovering that in October is a much better situation than discovering it in January.

Blount’s fall preventative maintenance visit covers refrigerant levels, electrical connections, defrost controls, auxiliary heat function, and overall system condition before the heating season begins. For Monroe homeowners and anyone else in Walton County with a heat pump, getting this done in September or October means you know exactly what you’re working with before you need the heat.

Contact Blount Heating and Cooling to schedule your fall heat pump service or to learn more about the Preventative Maintenance Agreement.