Physics of Buildings

Sources and simplifications

Every reference cited on this site, and every place a figure idealizes the real system.

References

  1. TruTech Tools (2012). A/C-Heat Pump System Troubleshooting Guide for the Testo 550. TruTech Tools, Akron, OH. trade training Tables of suction pressure, liquid pressure, superheat and subcooling direction for common faults on fixed-orifice and TXV systems (overcharge, undercharge, liquid restriction, low evaporator airflow, dirty condenser, low ambient, inefficient compressor, non-condensables, TXV bulb faults); typical TXV subcooling 8–12 °F, evaporator superheat 6–12 °F; fixed-orifice target superheat 5–40 °F depending on indoor wet bulb and outdoor dry bulb; DTD 35 °F, condenser +20 to +30 °F; check airflow first; measure both superheat and subcooling. (Superheat and subcooling)
  2. 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. (Superheat and subcooling)
  3. 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). (Superheat and subcooling)
  4. 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. (Superheat and subcooling)
  5. 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. (Superheat and subcooling)
  6. 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. (Superheat and subcooling)
  7. Thompson, A. & Taylor, B. N. (2008). Guide for the Use of the International System of Units (SI), NIST SP 811, Appendix B.8 Factors for units listed alphabetically. National Institute of Standards and Technology. standard Standard atmosphere = 1.01325 × 10⁵ Pa; 1 psi = 6.894757 × 10³ Pa (so 1 atm = 14.696 psi); T/K = (t/°F + 459.67)/1.8; 1 Btu_IT/lb = 2326 J/kg. (Superheat and subcooling)
  8. 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. (About)
  9. 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. (About)
  10. 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. (How does a heat pump or air conditioner move heat?)
  11. 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. (How does a heat pump or air conditioner move heat?)
  12. 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. (How does a heat pump or air conditioner move heat?)
  13. 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. (How does a heat pump or air conditioner move heat?)
  14. 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). (How does a heat pump or air conditioner move heat?)
  15. 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. (How does a heat pump or air conditioner move heat?)
  16. 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. (How does a heat pump or air conditioner move heat?)
  17. 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. (How does a heat pump or air conditioner move heat?)

Simplifications