What it does. Drag two points across a psychrometric chart and read live dry-bulb, wet-bulb, RH, dew point, humidity ratio, enthalpy, and specific volume. Free, mobile-friendly, no login.
Interactive Psychrometric Chart
Detailed Overview
The Interactive Psychrometric Chart is HVAC Know It All’s flagship calculator for understanding how air-side properties relate to each other in a single conditioned space. It uses the ASHRAE Hyland-Wexler equations and I-P (inch-pound) units, calibrated to sea-level atmospheric pressure (14.696 psia).
Purpose
Psychrometric math is the foundation of every cooling-mode diagnostic, every humidity complaint, every duct-dump argument, and every Manual J revision. The chart turns the math into something you can move around with your thumb on a phone in the truck.
- Plot Point A (room condition) and Point B (coil leaving condition) to read the process line between them.
- See sensible heat ratio (SHR) and the split between sensible and latent capacity in real time.
- Enter CFM to compute total BTU/hr, sensible BTU/hr, latent BTU/hr, and moisture removal in pints per hour and per day.
- Read room conditions backward from a sensor reading: drop one point and the chart computes every other psychrometric property.
When and Where to Use It
- Cooling-mode commissioning. Verify the SHR of an evaporator matches the load it is conditioning. Mismatched SHR is the most common reason a “properly charged” system still leaves a humid room.
- Humidity complaints. Compare what the customer is feeling (RH) against what the coil is delivering. The chart will tell you whether the problem is sensible capacity, latent capacity, or airflow.
- Heat-pump heating math. Read enthalpy of return air vs supply air to confirm capacity in cold weather. Particularly useful with R-454B and R-32 equipment where the published rating is at a single condition.
- ERV / dehumidifier sizing. Use the moisture-removal output to compare ventilation strategies in tight homes.
- Apprentice training. Drag a point across the 50% RH curve and watch dew point change. Two minutes of doing this beats an hour of static-chart explanation.
How It Differs From a Wall Chart
A printed psychrometric chart freezes a single atmospheric pressure and forces you to read intersections with a straight-edge. This interactive version lets you:
- See every property at once without measuring distances on the chart.
- Compute capacity on a process line without leaving the chart to do the math separately.
- Save a screenshot of the points + properties for the customer file.
- Work from a phone in the field rather than at a desk.
Limitations
- Sea-level pressure only. For elevations above ~2,000 ft, the chart’s enthalpy readings will be slightly off the actual pressure-corrected values. Field error is typically below 2%.
- Single-process (Point A to Point B). For multi-process flows (cooling then reheat, for example), plot each stage separately.
- Does not model the coil itself. The chart shows where the air starts and ends; it does not predict what a specific coil will do with a given charge or airflow.
Sources
- ASHRAE Handbook of Fundamentals (2021), Chapter 1: Psychrometrics.
- ASHRAE Hyland-Wexler saturation pressure equations for I-P units.