How a Heat Pump Actually Works

A furnace makes heat. It burns natural gas and turns the chemical energy in that fuel into warm air. A heat pump does something different. It collects heat that already exists in the outdoor air and carries it indoors. That sounds impossible in January, and it is what trips people up. Heat is just molecular motion, and 30 degree air still holds plenty of it. Thirty degrees Fahrenheit sits roughly 490 degrees above the point where molecular motion stops entirely. A heat pump gathers that low grade heat, concentrates it, and releases it inside.

You already own one. Your refrigerator is a heat pump. It does not make cold, it pulls heat out of the food compartment and dumps it into your kitchen, which is why the coils behind it are warm. An air conditioner is the same machine pointed at your house. A heat pump in heating mode is that identical machine run in the other direction, pulling heat out of outdoor air and releasing it inside. Once you picture it as a pump rather than a heater, the rest of its behavior makes sense.

The working fluid is called refrigerant, a chemical chosen because it boils and condenses at convenient temperatures and pressures. Most systems installed in the last fifteen years use R-410A, and new equipment is shifting to blends such as R-454B. The chemistry differs, the trick does not. A liquid absorbs a large amount of heat when it boils into a gas, and gives that heat back when it condenses into a liquid again. The entire machine exists to make the refrigerant boil where you want heat picked up and condense where you want heat delivered.

Follow one lap in heating mode. Refrigerant reaches the outdoor coil as a cold, low pressure liquid, colder than the outdoor air. Heat flows from warmer to colder, so the outdoor air gives up heat to it and the refrigerant boils into a gas. That gas travels to the compressor, a pump that squeezes it into a much smaller volume. Squeezing a gas raises its temperature, so refrigerant that left the outdoor coil barely above freezing leaves the compressor genuinely hot. It then runs indoors to the indoor coil, where the blower pushes house air across it.

House air is cooler than that compressed gas, so heat moves out of the refrigerant and into your rooms. Losing heat makes the gas condense back into a liquid, and that phase change is where most of the energy transfer happens. The still pressurized liquid then arrives at the metering device, a deliberate restriction in the line, sometimes a fixed orifice and on better equipment an electronic expansion valve. Dropping the pressure drops the temperature sharply, and the refrigerant leaves cold enough to absorb heat from winter air all over again.

Electric resistance heat, the kind in a baseboard heater or in an air handler strip heater, converts electricity into heat at essentially 100 percent. One unit of electricity in, one unit of heat out. A heat pump is not bound by that ceiling, because it is not converting electricity into heat at all. It spends electricity running a compressor and fans, and the heat itself arrives from outdoors for free. In mild weather a modern heat pump commonly delivers two to three units of heat for every unit of electricity it draws.

That multiplier is what makes the comparison against gas work at all. Most of Montgomery and Prince George's County buys electricity from Pepco and natural gas from Washington Gas, and per unit of raw energy, gas has generally been cheaper here. A heat pump still competes because of that multiplier, and loses ground as it falls in colder weather. The honest comparison is to look at both of your own bills, since rates move and the crossover point moves with them. It is also why a heat pump quietly running on resistance heat gets expensive fast.

An air conditioner and a heat pump contain almost identical parts. The meaningful addition is a component called the reversing valve, a four port valve on the outdoor unit that swaps which coil boils refrigerant and which coil condenses it. In cooling, the indoor coil is the cold one and the outdoor coil throws heat away. Shift the valve and those roles trade. Nothing physically turns around. The refrigerant simply takes a different path through the same components, and the coil that was dumping heat outside starts collecting it instead.

This matters because of how it fails. A reversing valve that sticks, or a solenoid coil that has burned out, leaves a system heating when it should cool, cooling when it should heat, or stuck weakly between the two. The loud whoosh from the outdoor unit a few times a day in winter is that valve shifting, which is normal. It also means one machine covers the entire year, which is why heat pumps are common in the postwar ramblers and townhouses around White Oak and Beltsville.

Two things change as the outdoor temperature drops. There is less heat available in the air, so the system delivers less of it, and a heat pump that heats your house easily at 45 degrees may produce only a fraction of its rated capacity in the teens. At the same time the efficiency multiplier falls, because the compressor is working across a much wider temperature gap. Every house has a balance point, the outdoor temperature at which the heat pump can just barely keep up with the heat the house is losing, and below that something has to help.

The ice is a separate issue and it is not a malfunction. In heating mode the outdoor coil runs colder than the outdoor air, so when that air is damp, moisture condenses on the coil and freezes. Frost insulates the coil and blocks airflow, so the system periodically runs a defrost cycle: it flips the reversing valve into cooling for a few minutes, sends hot gas to the coil, shuts off the outdoor fan and melts the frost. Steam rising off the outdoor unit on a damp 35 degree morning is the system working as designed.

What is not normal is ice that never clears, a solid block encasing the coil, or a unit that defrosts every fifteen minutes. Those point to a failed defrost board, a bad coil sensor, a stuck reversing valve, a low refrigerant charge, or a unit set too low where meltwater collects. Maryland winters press on this in a specific way: our worst stretches are often wet and near freezing rather than bitterly cold and dry, and damp air near freezing is what grows frost fastest.

Below the balance point the system calls for auxiliary heat, usually electric resistance strips inside the indoor air handler, or in a dual fuel setup a gas furnace. Auxiliary heat is supplemental. It runs alongside the heat pump to cover the shortfall, and on a normal Maryland winter night it may cycle on for a stretch and then back off. That is designed behavior. What you want to avoid is a system leaning on those strips constantly at temperatures where the compressor should still be carrying most of the load.

Emergency heat is a different setting, and homeowners switch to it by mistake all the time. It shuts the compressor off entirely and heats the house on resistance strips alone. Remember the arithmetic: the heat pump gave you two or three units of heat per unit of electricity, and the strips give you one. Running on emergency heat can multiply the heating portion of a Pepco bill for as long as it is left there. It exists for one job, keeping a house warm when the outdoor unit is broken, and it should go back to normal once the repair is done.

The old reputation of heat pumps, lukewarm air and a system that gives up when it gets truly cold, came from single speed equipment with an early cutoff. Cold climate models changed that with variable speed inverter driven compressors that spin faster as the weather gets colder instead of only switching on and off. Many now hold a meaningful share of their rated capacity down to 5 degrees Fahrenheit. In a climate sitting mostly in the 20s and 30s with occasional single digit snaps, that covers nearly the whole season on the compressor alone.