Physics of Buildings

Physics of Buildings · Heating and cooling

How does a heat pump or air conditioner move heat?

By boiling a refrigerant where it collects heat and condensing it where it gets rid of it. The compressor decides where each happens. Drag a thermometer, switch to heating, or start from a question.

A split system cut open, simulated with real properties for R-410A or R-32 (how far along each coil the refrigerant finishes boiling or condensing is drawn to a typical pattern, not measured). Solid core in a tube: liquid; dots: vapour. Blue walls: low-pressure side; vermilion: high-pressure side. Drag the red-dotted thermometer tops on either side of the wall (room air on the left, outdoor air on the right), switch the mode, or tap the reversing valve. Numbers ①–④ match the pressure–enthalpy chart below.

An air conditioner does not make cold, and a heat pump does not burn anything to make heat. Both move heat that is already there, using one trick: a liquid soaks up heat when it boils and gives it back when it condenses [1][2]. The refrigerant boils inside the coil where heat is to be collected and condenses inside the coil where heat is to be dumped. The compressor’s job is to make that possible by changing the pressure, because pressure sets the temperature at which a liquid boils. A heat pump is the same loop with a valve that swaps the two coils’ jobs [3].

Start from a question

Each question jumps to the step of the tour below that answers it. The tour runs the same simulated system one stage at a time.

Figure 1 The same simulated split system, pinned while you read. Each step highlights one part and sets the temperatures; drag a thermometer or change anything to take over, and use Return to the story to hand back.#

1. The evaporator: boiling soaks up heat

Cooling on a 95°F day with the house at 75°F. Inside the indoor coil the refrigerant is a cold mix of liquid and vapour at 118 psig. At that pressure R-410A boils at 40°F, so it boils here, 35°F colder than the room air blowing across the coil, and heat flows from the air into it [4].

While it boils, its temperature stays at 40°F: the heat it takes in goes into turning liquid into vapour, not into warming it. Only near the end of the coil, once the last liquid has boiled off, does the vapour start to warm. That extra warming is the superheat, about 10°F here. The vapour then leaves through the larger of the two copper lines, the suction line, which is why that line feels cold and sweats on a humid day.

Question 1 Prediction inside the tour.#

2. The compressor: it moves the boiling point

The compressor squeezes the vapour from 118 to 392 psig. That is the whole point of it. At 392 psig, R-410A condenses at 115°F instead of boiling at 40°F, so outdoors it is now 20°F hotter than the 95°F air and heat can flow out of it [4][5].

Squeezing a gas also heats it. The vapour leaves the compressor at about 176°F in this simulation, hotter than the temperature it will condense at; that hot gas is ② on the drawing. The electricity the compressor uses becomes heat in the refrigerant too, so the outdoor coil has to get rid of the room’s heat plus the compressor’s work.

3. The condenser: condensing gives the heat back

In the outdoor coil the hot gas first cools to 115°F, then condenses at that temperature, giving back the heat it took in when it boiled. The outdoor fan carries it away, which is why the air leaving the outdoor unit is warm when the house is being cooled.

By the end of the coil it is all liquid and has cooled a little below its condensing temperature: that is the subcooling, about 10°F here. Per pound of refrigerant, the condenser gives out 207 kJ/kg while the evaporator took in 162 kJ/kg; the difference, 45 kJ/kg, is the compressor’s work [2]. Heat out equals heat in plus work, every time.

4. The metering device: back to the low pressure

The warm liquid, about 105°F at 392 psig (③), flows down the small copper line, the liquid line, to the metering device: a fixed orifice, a thermostatic expansion valve (TXV) or an electronic valve. It is a deliberate restriction. On the far side the pressure is back to 118 psig, where the refrigerant’s boiling point is 40°F.

A 105°F liquid cannot stay liquid at a pressure where it boils at 40°F, so part of it flashes to vapour at once, and the energy for that boiling comes from the rest of the liquid, which drops to 40°F. About 28% of it leaves the valve as vapour (④). No heat is added or removed: the enthalpy is the same on both sides [2]. That cold mixture is what enters the indoor coil, and the loop closes.

5. The reversing valve: a heat pump runs the loop backwards

Now it is 35°F outside and the system is heating. The compressor still pumps the same way. The four-way reversing valve has slid across, so the hot gas from the compressor goes to the indoor coil and the outdoor coil connects to the compressor’s suction [3]. The indoor coil is now the condenser and the outdoor coil the evaporator.

The outdoor coil has to be colder than the outdoor air to collect heat from it, so the refrigerant there boils at about 15°F, 20°F below the air [6]. Freezing air still holds plenty of heat; it just has to flow into something colder. Indoors the refrigerant condenses at about 105°F, warm enough to heat 70°F room air.

6. A cold day: less heat, more work

At 5°F outdoors the outdoor coil has to boil the refrigerant at about −15°F. At that temperature the suction pressure falls to 31 psig, and the vapour the compressor draws in is thin: in this simulation each cubic foot of it weighs less than half what it does at 47°F. A compressor moves a fixed volume per turn, so it moves less refrigerant and less heat, while the pressure it has to climb is larger, which costs work and raises its discharge temperature [7].

So a single-speed heat pump loses capacity as it gets colder outside, just when the house needs more: Oak Ridge National Laboratory gives 20 to 40% of the 47°F rating as typical at −13°F [7]. Long run times on a very cold day are normal for a heat pump; they are not by themselves a sign of a fault. The section on COP below has the numbers.

Refrigerant work is for certified technicians. In the United States, Section 608 of the Clean Air Act prohibits knowingly releasing refrigerant, and only technicians certified under it may service equipment that contains refrigerant or buy it [8]. The outdoor unit is also wired to line voltage [9]. This page explains how the system works so you can understand what a technician measures and describe a problem accurately; it is not a repair or charging procedure.

The same cycle on a pressure–enthalpy chart

Engineers and technicians draw the cycle on a pressure–enthalpy (P-h) chart: pressure up the side on a log scale, and enthalpy, the heat content of each pound or kilogram of refrigerant, along the bottom. Every point of the loop above is a point on the chart. The dome is where liquid and vapour coexist: its left edge is saturated liquid, its right edge saturated vapour, and inside it every horizontal line is a single boiling temperature.

The cycle becomes a four-sided figure. ①→② is the compressor, climbing steeply to the high pressure. ②→③ is the condenser, a horizontal line moving left as heat leaves. ③→④ is the metering device, a vertical drop at constant enthalpy. ④→① is the evaporator, a horizontal line moving right as heat comes in. The width of the bottom line is the heat collected per kilogram; the extra width of the top line is the compressor’s work.

Figure 2 The cycle of the figure at the top on a pressure–enthalpy chart, computed from the refrigerant’s equation of state. Drag the blue low-pressure line or the vermilion high-pressure line up and down (each follows an air temperature), or use the controls. The dimension bars under the chart are the heat collected (blue), the compressor work (black) and the heat given out (vermilion) per kilogram; the dotted line from ① is an ideal (isentropic) compressor, and the dot at the top of the dome is the critical point.#

Two things show up on the chart that are hard to see in the hardware. The first is why the metering device makes vapour: point ④ sits inside the dome, from under a tenth to about half of the way across depending on the conditions (28% in the default cooling case). That vapour arrives already boiled and can carry no more heat, which is one reason a cooler liquid at ③ (more subcooling) helps. The second is that the compressor’s line leans to the right, because the work done on the vapour raises its enthalpy, and a real compressor needs more work than the ideal, reversible one, so ② lands to the right of where an ideal compressor would put it [2].

COP: how much heat for each unit of electricity

The coefficient of performance (COP) is heat moved divided by the work it costs. For cooling, it is the heat collected indoors over the compressor work; for heating, it is the heat delivered indoors over the work, which is always exactly one more, because the work itself ends up indoors as heat [1][2]. In the simulation at 47°F outdoors, heating a 70°F house, the cycle delivers about 4.4 units of heat per unit of compressor work. An electric resistance heater delivers one.

No machine can beat the reversible (Carnot) limit, which depends only on the two absolute temperatures: for heating, Thot/(Thot − Tcold) [2]. The smaller the lift from cold to hot, the more heat each unit of work can move. Real cycles fall well short, because each coil needs a temperature difference to push heat through it and the compressor and metering device waste some work. But the trend holds, and it is the reason heat pumps get worse as it gets colder.

Figure 3 Heating COP and heating capacity of the simulated single-speed heat pump as the outdoor temperature falls (house at 70°F; capacity relative to the same machine at 47°F, the standard rating point). Drag the marker or use the slider. Grey band: the 20–40% that Oak Ridge National Laboratory gives as typical for single-speed heat pumps at −13°F; open squares: ORNL’s laboratory cold-climate prototype, which brings in a second compressor when it is cold. Simulated curves assume an ideal compressor that fills completely on every stroke, and the simulated COP counts compressor work only.#

Two things fall at once. The COP falls because the lift grows: in the simulation it drops from 4.4 at 47°F to 3.2 at 17°F and 2.4 at −13°F. The capacity falls faster, because the vapour reaching the compressor gets thinner: a fixed-speed compressor that delivers 100% at 47°F manages about 62% at 17°F and 35% at −13°F in this idealized model, and real ones lose more, since their volumetric efficiency also drops as the pressure ratio climbs. ORNL’s report puts typical single-speed units at 20 to 40% of their 47°F capacity at −13°F, and notes the discharge temperature can exceed what many compressors allow [7].

Cold-climate heat pumps answer with variable-speed or staged compressors that run faster when it is cold. The prototype in the same ORNL report, built with two single-speed compressors, measured a COP of 4.24 at 47°F, 2.9 at 17°F while giving 110% of its nominal capacity, and 1.9 at −13°F while still giving 76% [7]. Below the point where the heat pump can no longer keep up, a backup heater, often electric resistance, covers the rest.

The same loop in refrigerators, stores and office buildings

Most refrigeration systems are this same vapour-compression cycle: the refrigerator in a kitchen, a window air conditioner, the freezer cases in a grocery store [13]. In a refrigerator the evaporator is inside the cabinet and the condenser is the warm grille at the back. Commercial rooftop units are packaged versions of the split system above, and most of the chillers that cool large buildings run the same four parts to chill water instead of air; the rest use an absorption cycle driven by heat [14]. Only the sizes, the compressors and the refrigerants change.

Refrigerants: R-410A, R-32 and R-454B

For two decades most American air conditioners and heat pumps used R-410A, a blend of R-32 and R-125 that behaves almost like a single substance: its bubble and dew points differ by about 0.1 K (0.2°F) [5]. Under the EPA’s Technology Transitions rule, new residential and light commercial systems from 2025 must use a refrigerant with a global warming potential of 700 or less (split-system equipment made before 2025 could still be installed until 2026), which rules out R-410A at 2,088; manufacturers have moved to R-32 and R-454B [15]. Existing equipment does not have to be replaced [15].

The figures here can run R-410A or R-32 from published equations of state. R-32 runs at slightly higher pressures than R-410A for the same temperatures (121 against 118 psig at 40°F) and a much hotter compressor discharge. R-454B is not modelled: there is no equation of state for it in the open property library used here, and as a blend with a temperature glide it needs separate dew and bubble tables that should come from its maker’s published data.

Go deeper

The essay What superheat and subcooling tell a technician explains the two numbers technicians read at the outdoor unit: what they mean physically, why both are measured, and what high and low combinations commonly point to. The superheat and subcooling calculator turns gauge pressures and line temperatures into saturation temperatures, superheat and subcooling for R-410A or R-32, with the working shown.

Physics of Buildings explains the systems inside homes and commercial buildings through their physics. Every model is checked by an independent verifier against the sources cited; see about this page and the sources. Another piece: why a well pump keeps cycling.

Sources

  1. Ling, S. J., Sanny, J. & Moebs, W. (2016). University Physics Volume 2, §4.3 Refrigerators and Heat Pumps. OpenStax, Rice University. textbook Coefficients of performance K_R = Q_c/W and K_P = Q_h/W; the coolant vaporizes in the evaporator taking heat from the cold side and exhausts heat to the outside in the condenser.
  2. Yan, C. Y. (2022). Introduction to Engineering Thermodynamics, §6.3 Refrigerator and heat pump; §6.5 Carnot cycles. BCcampus / Engineering LibreTexts (CC BY-NC-SA 4.0). textbook The four devices of the vapour-compression cycle (evaporator, compressor, condenser, expansion valve); COP_R = Q_L/W, COP_HP = Q_H/W, COP_HP = COP_R + 1; adiabatic expansion valve with h4 = h3; reversible (Carnot) heat pump COP = T_H/(T_H − T_L), the highest possible between the two temperatures.
  3. Carrier (2025). What Is a Reversing Valve in a Heat Pump?. Carrier Corporation. manufacturer guide The reversing valve switches the system between heating and cooling by changing the direction of refrigerant flow; energized position depends on the system design; reversing valve issues involve refrigerant handling, which requires professional certification.
  4. Saleeby, K. (HVAC School) (2021). “The Pressures Look Good.” How Do You Know?. HVAC School. trade training Evaporator design temperature difference (DTD) about 35 °F ± 3 below return air; condensing temperature over ambient (CTOA) 30 °F (6–9 SEER), 25 °F (10–12), 20 °F (13–16), 15 °F (17+); worked example for R-410A: 74 °F return, 88 °F outdoor → 39 °F evaporating (116.1 psig) and 108 °F condensing (356.5 psig).
  5. Lemmon, E. W. (2003). Pseudo-Pure Fluid Equations of State for the Refrigerant Blends R-410A, R-404A, R-507A, and R-407C. International Journal of Thermophysics 24(4): 991–1006 (copy hosted by NIST). journal article R-410A = 50% R-32 / 50% R-125 by mass; Helmholtz-energy equation fitted to the full mixture model (densities within 0.1% except near the critical point; heat capacities 0.1% average, 0.5% max); dew and bubble pressures from ancillary Eq. 13 (Table VI), within 0.02–0.05% of the mixture model; valid 200–450 K; Table V check values (300 K: bubble 1.7404 MPa, dew 1.7351 MPa); dew and bubble curves of R-410A nearly coincident.
  6. HVAC School (2018). Charging a Heat Pump in Heat Mode. HVAC School. trade training Heat mode rules of thumb: suction saturation 20–25 °F below outdoor ambient; condensing temperature 30–40 °F above indoor dry bulb; manufacturer charts are the better reference.
  7. Munk, J., Shen, B. & Gehl, T. (2021). Cost-Optimized Cold Climate Heat Pump Development and Field Test, ORNL/TM-2021/2044. Oak Ridge National Laboratory, for the U.S. Department of Energy. government report §1: heating capacity of a single-speed ASHP decreases with ambient temperature, typically to 20–40% of the 47 °F rating at −13 °F; low suction pressure and high pressure ratio cause high discharge temperatures; heating COP degrades at low ambient because of the large source–sink temperature difference; prototype CCHP: COP 4.24 at 47 °F, 2.9 at 17 °F, 1.9 at 76% of nominal capacity at −13 °F.
  8. U.S. Environmental Protection Agency (2020). EPA’s Refrigerant Management Requirements: What Technicians Need to Know (Section 608 fact sheet). U.S. EPA. government guidance Section 608 prohibits knowingly releasing refrigerant during maintenance, service, repair or disposal; requirements cover HCFCs and non-exempt substitutes such as HFCs and HFOs; only certified technicians can use non-exempt refrigerants to service AC and refrigeration equipment; refrigerant sales restricted to certified technicians; technicians must pass an approved certification exam.
  9. Carrier (2015). 25HBB5 Base Series Heat Pump with Puron Refrigerant: wiring diagrams and charging label. Carrier Corporation. manufacturer manual Required subcooling is given on the unit rating plate, with a ±3 °F tolerance; reversing valve solenoid and defrost control in the ladder diagram; relieve pressure and recover all refrigerant before system repair or final disposal.
  10. Tillner-Roth, R. & Yokozeki, A. (1997). An International Standard Equation of State for Difluoromethane (R-32) for Temperatures from the Triple Point at 136.34 K to 435 K and Pressures up to 70 MPa. Journal of Physical and Chemical Reference Data 26(6): 1273–1328. doi:10.1063/1.556002. journal article Read via the NIST Chemistry WebBook fluid page for R-32, which uses this equation and states its estimated uncertainties: 0.05% for density, 0.02% for vapor pressure, 0.5–1% for liquid heat capacity and speed of sound.
  11. Bell, I. H., Wronski, J., Quoilin, S. & Lemort, V. (2014). Pure and Pseudo-pure Fluid Thermophysical Property Evaluation and the Open-Source Thermophysical Property Library CoolProp. Industrial & Engineering Chemistry Research 53(6): 2498–2508. doi:10.1021/ie4033999. software CoolProp (MIT licence), version 8.0.0, implements the Lemmon (2003) R-410A and Tillner-Roth & Yokozeki (1997) R-32 equations used to build this site's property tables.
  12. Lemmon, E. W., Bell, I. H., Huber, M. L. & McLinden, M. O. (2025). Thermophysical Properties of Fluid Systems: Difluoromethane (R-32), saturation properties (NIST Chemistry WebBook, SRD 69). National Institute of Standards and Technology. government data R-32 saturation table (e.g. 0 °C: 0.8131 MPa, h_L 200.00, h_V 515.30 kJ/kg; 30 °C: 1.9275 MPa); IIR reference state (h = 200 kJ/kg, s = 1 kJ/kg·K for saturated liquid at 0 °C); critical point 78.105 °C, 5.782 MPa; equation of state Tillner-Roth & Yokozeki (1997).
  13. Richards, D. E. (LibreTexts edition) (2023). Basic Engineering Science: A Systems, Accounting, and Modeling Approach, §7.9 Thermodynamic Cycles. Engineering LibreTexts. textbook Most refrigeration systems, whether cooling the air in a house, the food in a refrigerator or the freezer display cases in a grocery store, are mechanical vapour-compression cycles; a window air conditioner is modelled as one.
  14. Sullivan, G. P., Pugh, R., Melendez, A. P. & Hunt, W. D. (Pacific Northwest National Laboratory) (2010). Operations & Maintenance Best Practices Guide, Release 3.0, §9.4 Chillers. U.S. Department of Energy, Federal Energy Management Program. government guidance §9.4.1: a chiller is a refrigeration system that cools water and, like an air conditioner, uses either a vapor-compression or an absorption cycle; §9.4.2.1: the mechanical compression cycle has four basic components (evaporator, compressor, condenser, expansion valve); diagnostic tools listed for 'packaged cooling systems including roof-top units' alongside split systems.
  15. Rheem Manufacturing (2024). 2025 A2L Regulatory Guide (RHM5848A). Rheem Manufacturing Company. manufacturer guide EPA Technology Transitions Rule (AIM Act): 700 GWP limit for new residential air conditioners and heat pumps from 2025, which rules out R-410A (GWP 2,088); R-454B and R-32 are the A2L alternatives; existing equipment need not be replaced.