How a Heat Pump Heats a Home with Cold Outdoor Air

A heat pump uses work to move heat from a colder place to a warmer one. That is why an air conditioner is already a heat pump: it takes heat out of a room. A reversible air conditioner can send the heat the other way and warm the room. In this 2021 video, Technology Connections explains the cycle for an audience familiar with air conditioning but perhaps less familiar with using it for heating.

A freezer offers a starting point. Heat naturally flows from the warmer room into its colder interior. Insulation slows that flow, but keeping the inside cold requires a refrigeration system to remove heat and reject it to the room. The warm exterior is evidence that even the cold interior supplies heat. The presenter calls temperature a concentration of heat energy as a teaching analogy; the relevant point is the direction of heat flow between bodies at different temperatures. Watch the freezer explanation.

A refrigerant is a working fluid whose boiling temperature changes with pressure. The presenter gives R-134a as an example: it boils around −15°F (−26°C) at ordinary pressure, yet can remain liquid in a pressurized can. Lower pressure lowers its boiling point; higher pressure raises it. The system uses that relationship to make the same fluid evaporate in one place and condense in another.

A phase change also transfers latent heat. Water at its boiling point can keep absorbing energy while turning into vapor without getting hotter. As an illustration, the presenter says vaporizing a quantity of water takes roughly seven times the energy needed to warm it from room temperature to boiling: with constant heating, ten minutes to reach boiling would mean about another 70 minutes to boil away. His other example is a reusable sodium acetate hand warmer: crystallization releases heat, and melting it in hot water absorbs that heat again. These are explanatory examples, not measurements of the heat pump.

The student-provided footage of a refrigeration training rig makes the circuit visible. The presenter credits a student at Front Range Community College's Center for Integrated Manufacturing, where the equipment is used for hands-on learning and troubleshooting. The rig has two heat exchangers, a sealed compressor and unusually visible refrigerant piping; ordinary systems may have sight glasses without transparent pipe throughout.

08:28View full image

The training rig's two coils can exchange roles because its cycle is reversible. The footage was supplied by a student at Front Range Community College's Center for Integrated Manufacturing.

Follow one trip around the loop. The compressor draws in low-pressure refrigerant gas and compresses it, raising its pressure and temperature. At high pressure its boiling point is higher. In the condenser, air passes over finned tubing; the hot refrigerant gives up heat and condenses to liquid, releasing latent heat. For illustration, the presenter says R-134a at the assumed high pressure would boil around 130°F. The delivered heat includes heat previously collected by the refrigerant and energy supplied through compression.

Next a metering device restricts flow and helps maintain the pressure difference. A capillary tube is one possible restriction. On the low-pressure side the refrigerant's boiling point falls below the surrounding air temperature. Some liquid initially boils using energy from the refrigerant itself, cooling what remains; then, in the evaporator, it absorbs heat from the air as it evaporates. The compressor draws the vapor back in, and the loop repeats. Actual pressures depend on the refrigerant and conditions. Watch the rig explanation.

In cooling mode, the indoor coil is the evaporator: it absorbs room heat. The outdoor coil is the condenser and rejects that heat. The presenter's central air conditioner and garage mini-split share the compressor, restriction, piping, two coils and fans; he estimates they are about 95% mechanically alike. His mini-split also has variable-speed compression and more sophisticated controls, while its wall unit has its own blower and the central system uses the furnace blower.

The demonstrated mini-split is described as 1.5 tons, 18,000 BTU, or about 5,250 watts of capacity. The presenter thinks its indoor metering device is a capillary tube but explicitly asks to be corrected. That identification is a guess. His thermal camera illustrates relative changes; its displayed temperatures may be inaccurate because different materials have different emissivities.

15:45View full image

The thermal image shows the indoor unit during the cooling sequence. Color and displayed temperatures should be read with the presenter's emissivity warning in mind.

To heat, a reversing valve and extra piping change refrigerant flow through the rest of the system; the compressor itself still pumps in one direction. In the demonstrated arrangement, the restriction is now downstream of the indoor coil, which becomes the high-pressure condenser. The outdoor coil becomes the low-pressure evaporator. In the demonstration the unit pauses for pressure to equalize before switching, then restarts with the coils' roles exchanged.

18:57View full image

The sped-up thermal sequence shows the indoor coil warming as the system changes modes; it is a visual demonstration, not a calibrated temperature measurement. Watch the reversal.

The compressor and fans consume electricity, but the system also collects environmental heat. That is why heat delivered can exceed the electrical energy consumed without creating energy from nothing. Even on a cold day, heat flows from outdoor air into the evaporator if the coil is colder still; the presenter illustrates this with outdoor air around 40°F. Compression then enables the refrigerant to release heat indoors.

The coefficient of performance (COP) is the ratio of useful heat delivered to energy consumed. A heating COP of 1 is comparable to electric resistance heat; COP 4 means four units of heat for one unit of input under those conditions. The presenter says 4 is attainable in decent conditions a few degrees above freezing. That is not a guarantee of performance in every condition: air-source performance falls as outdoor conditions become more demanding.

The cold outdoor evaporator collects moisture. Condensation releases latent heat, but near or below freezing frost can build on the fins. Dry air can slow the buildup; humid air can accelerate it. Ice insulates the coil and can block airflow, reducing heat collection.

22:12View full image

The close-up comes from a sped-up sequence of frost forming on the outdoor coil.

This particular unit monitors indoor and outdoor temperatures and refrigerant pressure to judge when to defrost. The presenter observes compressor speed rising as frost accumulates. It then stops the compressor and both fans, restarts in cooling mode to send heat from indoors to the outdoor coil, and melts the ice. Afterward it reverses again, restarts the outdoor fan and compressor, and waits for the indoor coil to warm before restarting the indoor blower. To an occupant it may feel like a pause in heat; frequent defrost on cold, humid days reduces output and efficiency.

24:11View full image

The outdoor coil is shown during the defrost demonstration, which returns heat outdoors to clear the ice. Watch the defrost sequence.

The presenter's unit has a stated SEER 19 for cooling, which he equates to COP 5.5. That is a cooling seasonal measure; it is not the unit's heating-season COP. Its HSPF 10 is the heating seasonal measure, which he converts to an average heating COP of about 2.9 under the test method. On that basis he estimates roughly one third the electricity of resistance heat for the same seasonal heat, while noting that individual days will be better or worse. These are the unit's reported ratings and the presenter's interpretation, not a promise for another installation.

The garage unit was rated down to 5°F. At −10°F it still ran and produced some heat, but the presenter describes it as tepid and says it was working hard. He guesses its COP might have been around 1 or slightly higher; he did not measure it. He installed the unit primarily to remove summer moisture after stored items developed mold. Heating makes the garage useful on milder days, and a 45°F freeze-protection setting helps melt snow off his car.

He says this unit alone could not reliably heat his space all winter. Auxiliary or emergency heat can be electric resistance, natural gas or another fuel, and a thermostat may coordinate both systems. He also notes that many local winter days are above the unit's limit and cites other heat pumps rated to −20°F. Different refrigerants, designs or dedicated electric defrost heaters are possible cold-weather approaches; the video does not test those alternatives.

The presenter was not offered a reversible option when his central air conditioner was installed. He argues that its 2.5-ton capacity might be useful on mild days, although he says it is roughly half the furnace's output. He recalls a −10°F day when his 70,000 BTU furnace ran about ten hours and speculates that 30,000 BTU continuously would cover that energy. He immediately acknowledges that the heat pump could not deliver that output at that outdoor temperature and calls it a rough thought. The source does not state time units for those BTU figures; this is not equipment-sizing advice.

A channel patron farther north shows a comparable reversible outdoor unit that defrosts periodically. The presenter says such systems were already common in the southern US, where heating demand is generally milder. He also warns that simultaneous demand for electric resistance backup during extreme cold can strain a grid. In discussing the Texas event immediately before the video's 2021 publication, he explicitly says heat pumps did not cause the failures, points to generation-side problems, and qualifies his observation about gas backup as based on what he had read.

The presenter gives a historical calculation, not a current assessment of every grid. He cites a US Energy Information Administration figure of 44% average efficiency for US natural-gas power plants in 2019, then assumes about 40% of the gas energy reaches the user after transmission losses. He compares that with an onsite gas furnace exceeding 90% efficiency. At a heat-pump COP of 2.5, 40% multiplied by 2.5 gives heat delivered equivalent to 100% of the original gas energy.

That result includes heat collected from the environment; it does not mean the power plant converts gas to electricity at 100% efficiency. The conclusion changes with power generation, losses, actual COP and the comparison system. In his 2021 assessment the presenter argues efficient heat pumps can reduce resource use and emissions even on a fossil-fired grid, and can make wind or solar heating and storage more feasible. He suggests very efficient units might reduce electricity demand to around a quarter or less of resistance heating in his scenario; this is a projection, not a measured universal saving.

This video focuses on air-source systems. The presenter mentions ground-source heat pumps as a cold-climate alternative and reserves their detailed operation, other applications, refrigerants, and financial and climate comparisons for a second video. Watch the historical calculation.