Key Conversion Factors
1 inWG = 249.089 Pa = 0.249 kPa = 0.0361 psi
1 bar = 100 000 Pa = 100 kPa = 14.504 psi
1 atm = 101 325 Pa = 101.325 kPa = 14.696 psi
1 mmHg = 133.322 Pa = 0.133 kPa = 0.01934 psi
Absolute = Gauge + 101.325 kPa (sea level)
Duct Static Pressure Reference
| Application | inWG | Pa | Notes |
|---|---|---|---|
| Residential furnace (rated TESP) | 0.5 | 125 | Typical nameplate limit |
| Residential system (target) | 0.3 - 0.5 | 75 - 125 | Good airflow range |
| Light commercial AHU | 0.5 - 1.5 | 125 - 375 | Fan rated for higher TESP |
| Commercial VAV system | 1.0 - 3.0 | 250 - 750 | Main duct at fan discharge |
| VAV box inlet pressure | 0.25 - 1.0 | 62 - 250 | Minimum for VAV control |
| Blower door test | 0.2 | 50 | Standard 50 Pa test condition |
| Return air grille drop | 0.05 - 0.10 | 12 - 25 | Sized for low velocity |
| HEPA filter (clean) | 0.5 - 1.0 | 125 - 250 | Rises to 1.5+ inWG dirty |
Refrigerant Operating Pressures (Gauge)
| Refrigerant | Low Side (psig) | Low Side (kPag) | High Side (psig) | High Side (kPag) | Canadian Status |
|---|---|---|---|---|---|
| R-410A | 60 - 80 | 414 - 552 | 350 - 450 | 2413 - 3103 | Phase-down per Kigali / SOR/2016-137 |
| R-32 | 75 - 105 | 517 - 724 | 400 - 520 | 2758 - 3585 | Lower GWP replacement |
| R-454B (Puron Advance) | 65 - 95 | 448 - 655 | 380 - 480 | 2620 - 3310 | ASHRAE A2L, growing adoption |
| R-22 | 55 - 70 | 379 - 483 | 200 - 280 | 1379 - 1931 | Banned in Canada since 2020 |
| R-134a | 15 - 30 | 103 - 207 | 120 - 175 | 827 - 1207 | Chillers, automotive A/C |
| R-744 (CO₂) | 500 - 800 | 3447 - 5516 | 1200 - 1800 | 8274 - 12411 | Transcritical CO₂ (food retail) |
Always verify against the P-T chart for specific refrigerant and system conditions. Use the Refrigerant Unit Converter for full P-T data.
Hydronic System Pressure Reference
| System / Component | Typical (kPag) | Typical (psig) |
|---|---|---|
| Residential closed-loop fill pressure | 100 - 150 | 14 - 22 |
| Expansion tank pre-charge | 100 - 125 | 14 - 18 |
| Pressure relief valve set point | 207 - 345 | 30 - 50 |
| Commercial chilled water system | 300 - 600 | 44 - 87 |
| Commercial hot water system | 400 - 800 | 58 - 116 |
| Hydronic loop pressure test | 690 - 1035 | 100 - 150 |
| City water supply (typical) | 275 - 550 | 40 - 80 |
How to Use the Pressure Converter
Choose whether your value is gauge pressure (relative to atmosphere) or absolute pressure (from true vacuum). Refrigerant manifold gauges read gauge; thermodynamic calculations need absolute.
Enter a value in any of the seven unit fields. All others update instantly. Use the quick-reference presets to load common HVAC values including R-410A operating pressures and standard test pressures.
The result panel identifies what HVAC application your pressure falls in: duct static range, refrigerant low or high side, hydronic system pressure, or test pressure. This helps catch unit errors before they cause problems.
Three reference tables cover duct static ranges, refrigerant operating pressures for all major Canadian refrigerants, and hydronic system pressures. Use these to verify converted values for duct design, refrigeration, and hydronic heating.
Pressure Units in HVAC -- Complete Guide
Pressure appears in every HVAC subsystem. Duct static pressure determines fan selection and airflow balance. Refrigerant pressures define the operating envelope of every refrigeration circuit. Hydronic system pressure affects pump sizing, expansion tank sizing, and safety relief settings. Each application uses different units, and Canadian HVAC work requires fluency across all of them.
Pascals and Inches of Water Column
The Pascal (Pa) is the SI unit of pressure. In Canadian duct design and building science, Pa is standard. Inches of water gauge (inWG) is the dominant Imperial unit for duct static pressure and fan ratings. The conversion: 1 inWG = 249.089 Pa. A residential furnace rated for 0.5 inWG TESP equals 124.5 Pa. For quick field work, 250 Pa is a close approximation for 1 inWG. Use the dedicated inH2O to Pa Converter for just this conversion, or the Duct Sizing Calculator to work directly in duct design.
psi and kPa for Refrigerant Work
Refrigerant technicians in Canada work with psi daily because manifold gauge sets read in psig. Engineering drawings and Canadian refrigerant handling regulations under SOR/2016-137 use kPa. The conversion is 1 psi = 6.895 kPa. R-410A operates at 60-80 psig (414-552 kPag) on the low side and 350-450 psig (2413-3103 kPag) on the high side. As the Kigali Amendment HFC phase-down proceeds, newer refrigerants like R-32 and R-454B operate at similar pressures, so the same conversion factors apply regardless of refrigerant.
Gauge vs. Absolute Pressure
Gauge pressure measures pressure relative to local atmospheric pressure. Absolute pressure adds atmospheric pressure on top. At sea level, atmospheric pressure is 101.325 kPa (14.696 psi). A refrigerant manifold reading of 70 psig is 84.696 psia absolute, or 584.3 kPaa. Thermodynamic calculations like COP and enthalpy use absolute pressure. Field measurements and equipment datasheets use gauge. Always confirm which convention a datasheet uses before entering values into calculations.
Bar and Atmosphere
Bar is a metric pressure unit common in European equipment documentation. 1 bar = 100 000 Pa = 100 kPa. One atmosphere (atm) is the standard atmospheric pressure at sea level: 101.325 kPa or 1.01325 bar. European chiller and heat pump datasheets often specify pressures in bar(g). Converting to kPag: multiply bar(g) by 100. Converting to psig: multiply bar(g) by 14.504.
Altitude Effects on Pressure in Canada
Atmospheric pressure drops with altitude. Calgary sits at 1045 m elevation where standard atmospheric pressure is approximately 89.9 kPa rather than the sea-level 101.325 kPa. This affects the gauge-to-absolute offset, fan performance (less dense air), and refrigerant saturation temperatures at a given gauge pressure. For altitude-corrected calculations, see the Air Density Converter.
Frequently Asked Questions
Multiply inWG by 249.089 to get Pascals. So 0.5 inWG = 124.5 Pa and 1 inWG = 249 Pa. To reverse, divide Pascals by 249.089. This is the most common duct static pressure conversion in Canadian HVAC, since US fan ratings use inWG while Canadian engineering standards use Pascals. For just this conversion, the dedicated inH2O to Pa Converter is faster.
Residential systems should run at 0.3 to 0.5 inWG (75 to 125 Pa) total external static pressure. If your furnace is rated for 0.5 inWG TESP but your actual system resistance is 0.7 inWG due to undersized returns or a dirty filter, airflow drops significantly. Light commercial AHUs handle 0.5 to 1.5 inWG. Commercial VAV systems can see 1 to 3 inWG at the fan. Always verify against the fan curve, not just the nameplate TESP.
Gauge pressure is measured relative to local atmospheric pressure. Absolute pressure adds atmospheric pressure on top. At sea level, 1 atm = 101.325 kPa, so a refrigerant low-side reading of 483 kPag is 584 kPaa in absolute terms. Refrigerant manifold gauges and most field instruments read gauge. Thermodynamic calculations and saturation property tables require absolute pressure. The gauge/absolute toggle in this converter handles both modes.
Multiply psi by 6.89476 to get kPa. To go the other way, multiply kPa by 0.14504. Quick reference: 100 psig = 689.5 kPag, 400 psig = 2758 kPag, 150 psig = 1034 kPag. For refrigerant work, the same factor applies in gauge or absolute since you're just converting the scale. Use the Refrigerant Unit Converter to pair pressure conversion with saturation temperature lookup.