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How to Use the Compressor Sizing Calculator
Choose the refrigerant and the compressor type. The refrigerant sets the enthalpy, specific volume and pressure data used throughout, and the compressor type sets the volumetric efficiency model when you leave that field blank.
Enter the required cooling capacity in BTU/hr, then the saturated evaporating and condensing temperatures in °F. Use the design condition for your location, with air-cooled condensing about 20-30°F above the outdoor ambient, rather than a generic rating point.
The defaults of 20°F suction superheat and 10°F liquid subcooling are reasonable starting points. Replace them with manufacturer targets or field measurements, since superheat changes the suction volume and subcooling changes the refrigerating effect. The superheat calculator helps turn gauge readings into those values.
Leave volumetric efficiency blank to use the modelled value, or enter the manufacturer figure. Read the required displacement, then compare the 10% and 15% margin values with catalogue capacity at your real operating conditions, and use the compression ratio to judge cold-weather performance.
How compressor sizing works for Canadian HVAC systems
Compressor sizing starts with three inputs: the required cooling capacity, the evaporating temperature and the condensing temperature. The calculator divides the capacity by the refrigerating effect (the enthalpy gained across the evaporator) to get the mass of refrigerant that must circulate, multiplies by the suction specific volume to get the gas volume the compressor has to draw in, and divides by volumetric efficiency to get the swept displacement. Power then follows from the isentropic work and the isentropic efficiency you enter.
Pick the design condition first
Condensing temperature drives almost everything downstream, and for an air-cooled condenser it sits about 20-30°F (11-17 K) above the outdoor ambient. That makes the ambient you choose the most influential assumption on the page. Peak afternoon temperatures in Windsor or Winnipeg are far higher than on the coast of British Columbia, so use the design value for your own location, which the design temperature lookup provides, rather than a figure borrowed from another region.
Manufacturer capacity ratings are published at one fixed rating point, and a job site rarely sits on it. Size to the temperatures the system will really see, then check the catalogue at those temperatures.
From cooling load to displacement
The tool takes enthalpy, specific volume and saturation pressure from fits calibrated against the CoolProp implementation of the reference equations of state for each refrigerant, the same kind of data published in the NIST Chemistry WebBook fluid tables. Across the working range the fits land within about 3% for refrigerating effect and 4% for suction volume, which is closer than typical field measurement error.
The example (36,000 BTU/hr of R-410A, 40°F evaporating, 110°F condensing, 20°F suction superheat, 10°F subcooling) gives a refrigerating effect of about 74.1 BTU/lb, a suction specific volume of about 0.482 ft³/lb and a mass flow near 486 lb/hr. That is 3.90 CFM of suction gas before volumetric efficiency, which is why the efficiency matters.
Compression ratio and volumetric efficiency
Compression ratio is absolute discharge pressure divided by absolute suction pressure, and the calculator reads it from the fitted saturation curve for the refrigerant, within about 1-2% of the true value. The example runs at 2.86:1. When the volumetric efficiency field is blank, the tool lowers efficiency as that ratio climbs. A scroll loses about 2 points for each additional unit of ratio, a rotary about 2.5 points, and reciprocating and screw machines follow a clearance-volume relationship with a refrigerant-specific re-expansion exponent.
That model is consistent with the 85-95% usually quoted for scrolls, which corresponds to ratios of roughly 3.8:1 to 8.7:1. A reciprocating compressor on R-410A with 5% clearance drops from about 80% at 6:1 to about 65% at 10:1. These are modelled defaults, so enter the manufacturer's volumetric efficiency when you have it. The compression ratio calculator and the compressor efficiency calculator go deeper on both numbers.
Cold-weather capacity loss
Capacity tracks refrigerant mass flow, and mass flow collapses as evaporating temperature falls because the suction gas gets thinner. For an R-410A scroll, moving from 45°F evaporating and 113°F condensing (2.7:1) to -4°F and 104°F (6.1:1) cuts mass flow to about 37% of the summer value. Run the calculator at both conditions and the required displacement for the same load rises by roughly 2.7 times.
Heating output falls by less than mass flow, because compressor work adds to the heat delivered, and inverter units raise speed to compensate. That is why the NEEP cold climate heat pump specification has manufacturers report capacity and COP at 47°F, 17°F and 5°F rather than one rating point. For cold-climate sizing, use those published tables, not a summer rating.
Power, COP and EER
Isentropic work is estimated from the pressure ratio and an effective isentropic exponent fitted to each refrigerant, which keeps it within about 4% of real-fluid values. Dividing by the isentropic efficiency you enter gives power input. The default of 70% is an upper-end figure for a scroll near its best compression ratio, so try 55-65% for a worn machine or a high-ratio duty. The example lands at 2.69 kW, a COP of 3.92 and an EER of 13.4. Compare those with your system figures using the EER calculator; this one covers the compressor alone, without fan and pump power.
Choosing a compressor and a margin
Scroll compressors dominate residential and light commercial equipment because of their volumetric efficiency, low noise and few moving parts. Reciprocating compressors remain common in commercial refrigeration where cylinder unloading gives capacity control. Rotary compressors are typical in small mini-split systems, and screw compressors suit large commercial and industrial capacities.
The calculator shows the displacement with 10% and 15% added. That range is a common allowance for load uncertainty, not a standard, and extra margin costs part-load efficiency. Match the figure to catalogue capacity at your real conditions, then confirm the condenser with the heat of rejection calculator and cross-check pressures on the P-T chart.
Frequently Asked Questions
Start with the design cooling load, then pick the outdoor design temperature for your own location rather than a generic figure, since hot-afternoon peaks differ widely across the country. Air-cooled condensing temperature runs about 20-30°F (11-17 K) above ambient, so a 95°F day means roughly 115-125°F condensing. Evaporating temperature comes from the application: 35-45°F for comfort cooling, 15-30°F for medium-temperature refrigeration, and -20 to 0°F for low temperature. From those two temperatures the calculator finds the refrigerating effect, the mass flow, the suction volume and the displacement, and then you check the manufacturer's published capacity at your actual conditions.
Volumetric efficiency is the ratio of the gas a compressor actually draws in to its swept displacement. It falls as compression ratio rises, because gas trapped in the clearance volume re-expands and takes up part of each stroke. A compressor sized as if it were 100% efficient but running at 65% delivers only 65% of the flow you wanted, so the displacement comes out too small by about a third. Leave the field blank and this calculator models it from the compression ratio, compressor type and refrigerant, or enter the manufacturer's figure for your compressor to override it.
Capacity follows refrigerant mass flow, and mass flow depends on suction density and volumetric efficiency. For R-410A with a scroll compressor, 45°F evaporating and 113°F condensing is a compression ratio of about 2.7:1 at roughly 97% volumetric efficiency. At -4°F evaporating and 104°F condensing the ratio is about 6.1:1, volumetric efficiency is near 90%, and the thinner suction gas cuts mass flow to roughly 37% of the summer value. Put another way, the same load needs about 2.7 times the displacement. Inverter-driven units raise compressor speed to compensate, and heating output falls by less than mass flow because compressor work adds to condenser heat, so use manufacturer capacity tables at the actual outdoor temperature for cold-climate sizing.
Many designers add 10-15% to the calculated displacement to cover uncertainty in the load and in coil condition, but that figure is a judgement call and not a code requirement, and this calculator shows both values. Going beyond it brings short cycling, weaker humidity control in cooling mode and poorer part-load efficiency. For a replacement, compare capacity at your actual evaporating and condensing temperatures rather than nameplate tonnage, because nameplate figures are taken at one fixed rating point that may sit far from the installation.