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How to use the psychrometric chart in Canadian HVAC design
A psychrometric chart is a graphical representation of all possible moist-air states at a given atmospheric pressure. The horizontal axis shows dry-bulb temperature and the vertical axis shows humidity ratio (W) in g/kg or gr/lb. Every point on the chart represents a unique combination of air conditions, with all other properties readable directly from the surrounding curves and lines.
The curved lines sloping upward from lower-left to upper-right are constant relative humidity lines. The outermost curved boundary is the saturation line (100% RH). Any air state to the right of the saturation line doesn't exist as stable single-phase air. Diagonal lines sloping downward from the saturation line toward the lower right are constant wet-bulb temperature lines. Lines of constant enthalpy run nearly parallel to the wet-bulb lines. Lines of constant specific volume run similarly but with a slightly different slope.
Six core HVAC processes on the chart
You can identify any HVAC air-handling process as a line segment between two state points. Sensible heating and cooling are horizontal lines (W stays constant, T changes). Humidification and dehumidification are approximately vertical lines (T stays approximately constant, W changes). Cooling and dehumidification (the most common summer air-conditioning process) follows a line from the entering state toward the apparatus dew point on the saturation curve. Use the apparatus dew point calculator to find where this line intersects the saturation curve for coil selection. Mixing of two airstreams plots as a straight line between the two state points, with the mixed state at a proportional distance based on mass flow ratios.
Canadian winter and summer design conditions
Canadian summer design conditions span a wide range: Vancouver might use 25°C DB / 50% RH while Calgary uses 29°C DB / 30% RH and Ottawa uses 30°C DB / 60% RH. Winter conditions are even more variable, with outdoor design temperatures from -20°C in Vancouver to -33°C in Winnipeg. On the psychrometric chart, this translates to very low W values in winter (often below 1 g/kg outdoors) requiring significant humidification to reach indoor comfort. Use the mixing air calculator to find the mixed state when blending outdoor and return air, then plot that state point here to verify it clears any condensation or freeze risk before the coil.
Reading state points at non-sea-level elevations
Standard psychrometric charts are drawn for sea-level pressure (101.325 kPa). In Calgary or Edmonton, atmospheric pressure is about 89-94 kPa, which shifts the saturation curve and all RH lines. This chart recalculates all curves at your entered elevation. At lower pressure, the same W and T_db give a higher RH than at sea level, and the saturation line shifts slightly. This matters for coil sizing at altitude, where air leaves the coil with a different state point than sea-level charts would predict. See the specific volume calculator for mass flow corrections at altitude.
Frequently Asked Questions
A psychrometric chart plots moist-air state points using dry-bulb temperature on the horizontal axis and humidity ratio on the vertical axis. Any point on the chart represents a unique air condition. Curved lines from lower-left to upper-right are constant relative-humidity lines, with the outermost curve being the 100% RH saturation line. Diagonal lines sloping downward from left to right are constant wet-bulb temperature lines. To read a state point, locate the dry-bulb temperature on the bottom axis, move vertically to the RH curve, and read all other properties from the intersecting lines. This calculator plots the state point and reads all values automatically from any two input properties. Use the enthalpy calculator to verify enthalpy values for coil load analysis.
Six core HVAC processes show up as line segments between state points: sensible heating (horizontal right, T up, W constant), sensible cooling (horizontal left), humidification (vertical up, W rises), dehumidification (vertical down), cooling and dehumidification (line toward apparatus dew point on the saturation curve), and mixing of two airstreams (straight line between the two states). Canadian heat recovery ventilator (HRV) analysis also uses the chart extensively to show the process from outdoor air to the supply state. Use the mixing air calculator to find mixed-air state points before plotting them here.