Canadian HVAC work happens in two unit systems at once. Codes and drawings are in SI, but much equipment data, manufacturer software, rules of thumb and field habits are still imperial. A single project can have airflows in litres per second on the drawings, cfm on the fan schedule and tons on the chiller nameplate.
Moving fluently between the two is a basic skill, and unit errors are a surprisingly common source of mistakes. This guide collects the conversions used most often, explains the core heat transfer equations in both systems, and highlights where errors tend to creep in. Factors are rounded to a practical number of significant figures.
Airflow and velocity
| Quantity | Conversion |
|---|---|
| Volumetric airflow | 1 L/s = 2.119 cfm; 1 cfm = 0.4719 L/s |
| Volumetric airflow | 1 m³/s = 1,000 L/s = 2,119 cfm |
| Volumetric airflow | 1 m³/h = 0.2778 L/s = 0.5886 cfm |
| Velocity | 1 m/s = 196.9 fpm; 1 fpm = 0.00508 m/s |
| Airflow per area | 1 L/(s·m²) = 0.1968 cfm/ft² |
| Typical example | 2.5 m/s is about 500 fpm |
A handy approximation: multiply L/s by about 2 to get a rough cfm, but use 2.119 for anything that matters.
Pressure
| Quantity | Conversion |
|---|---|
| Air system pressure | 1 in. w.c. (inch of water column) = 249 Pa |
| Air system pressure | 1 Pa = 0.004015 in. w.c. |
| Water system pressure | 1 psi = 6.895 kPa |
| Water head | 1 ft of water = 2.99 kPa; 1 m of water = 9.81 kPa |
| Water head | 1 psi = 2.31 ft of water |
| Atmospheric pressure | 101.325 kPa = 14.696 psi = 29.92 in. Hg at sea level |
Duct static pressures are often given in Pa on drawings and in inches of water on fan curves. Pump heads are typically in kPa or metres in SI and in feet of head in imperial.
Power, cooling and heating capacity
| Quantity | Conversion |
|---|---|
| Heat rate | 1 W = 3.412 Btu/h; 1 Btu/h = 0.2931 W |
| Heat rate | 1 kW = 3,412 Btu/h |
| Refrigeration | 1 ton of refrigeration = 12,000 Btu/h = 3.517 kW |
| Boiler output | 1 MBH = 1,000 Btu/h = 0.2931 kW |
| Mechanical power | 1 hp = 746 W = 0.746 kW |
The ton of refrigeration originated as the rate of heat absorbed by melting one short ton of ice over 24 hours. It remains the standard way chillers and rooftop units are described in North America.
Energy
| Quantity | Conversion |
|---|---|
| Electrical energy | 1 kWh = 3.6 MJ = 3,412 Btu |
| Natural gas | 1 GJ = 947,800 Btu = 277.8 kWh |
| Natural gas | 1 therm = 100,000 Btu = 105.5 MJ |
| Natural gas volume | 1 m³ of natural gas contains roughly 37 to 39 MJ, depending on supply composition |
| Energy intensity | 1 kWh/m² = 317 Btu/ft² |
Canadian gas utilities bill in gigajoules or cubic metres depending on the province. The heating value of natural gas varies, so use the utility's stated heating value for accurate conversions.
Water flow
| Quantity | Conversion |
|---|---|
| Volumetric flow | 1 L/s = 15.85 US gpm; 1 US gpm = 0.0631 L/s |
| Volumetric flow | 1 m³/h = 0.2778 L/s = 4.403 US gpm |
| Imperial gallon caution | 1 imperial gallon = 1.201 US gallons |
Equipment data in North America uses US gallons. Older Canadian documents sometimes used imperial gallons, which are about 20% larger. Always confirm which gallon is meant.
Temperature
- Celsius to Fahrenheit: °F = (°C x 1.8) + 32.
- Fahrenheit to Celsius: °C = (°F - 32) / 1.8.
- A temperature difference converts by the factor 1.8 only, with no offset. A 10 K (10 °C) difference is an 18 °F difference.
- Kelvin: K = °C + 273.15.
Confusing a temperature with a temperature difference is a classic error. A 5.6 K chilled water delta T is a 10 °F delta T, not 42 °F.
Envelope and heat transfer
| Quantity | Conversion |
|---|---|
| Thermal resistance | RSI 1.0 = R-5.678; R-1 = RSI 0.1761 |
| Heat transfer coefficient | 1 W/(m²·K) = 0.1761 Btu/(h·ft²·°F) |
| Heat transfer coefficient | 1 Btu/(h·ft²·°F) = 5.678 W/(m²·K) |
| Area | 1 m² = 10.764 ft² |
| Length | 1 m = 3.281 ft; 1 in. = 25.4 mm |
See envelope heat loss, U-values, thermal bridging and infiltration for how these are used.
Efficiency metrics
| Metric | Relationship |
|---|---|
| COP to kW per ton | kW/ton = 3.517 / COP |
| COP to EER | EER = COP x 3.412 |
| Example | COP 6.0 is about 0.59 kW/ton or EER 20.5 |
Lower kW per ton is better; higher COP and EER are better. Seasonal metrics such as IPLV, IEER and SEER2 are weighted averages and do not convert directly to a single-point COP.
The core heat equations
These equations use standard air (about 1.2 kg/m³) and water near typical HVAC temperatures.
Air side
| Load | SI (W, L/s, K) | Imperial (Btu/h, cfm, °F) |
|---|---|---|
| Sensible | 1.23 x L/s x delta t | 1.08 x cfm x delta t |
| Latent | 3010 x L/s x delta W (kg/kg) | 0.68 x cfm x delta W (grains/lb) |
| Total | 1.2 x L/s x delta h (kJ/kg) | 4.5 x cfm x delta h (Btu/lb) |
Water side
| Fluid | SI (kW, L/s, K) | Imperial (Btu/h, US gpm, °F) |
|---|---|---|
| Water | 4.19 x L/s x delta t | 500 x gpm x delta t |
For glycol solutions, specific heat and density are lower than water and the constants must be adjusted for the concentration. See glycol systems. At altitude, the air constants fall in proportion to density; see air density, altitude and HVAC design.
Worked example
A chilled water coil has a flow of 6.3 L/s and a 6 K temperature rise. Capacity is 4.19 x 6.3 x 6, about 158 kW. In imperial, 6.3 L/s is about 100 US gpm and 6 K is 10.8 °F, so capacity is 500 x 100 x 10.8, which is 540,000 Btu/h or about 45 tons. Dividing 158 kW by 3.517 gives about 45 tons, which confirms the result.
Avoiding unit errors
- State units on every number in calculations, schedules and reports.
- Keep each calculation in one unit system and convert only at the end.
- Distinguish temperatures from temperature differences.
- Confirm US or imperial gallons, and actual or standard airflow.
- Check results against an independent rough calculation in the other unit system.
- Watch for software that defaults to imperial units even on SI projects.
Frequently asked questions
Why is the sensible heat constant 1.23 in SI?
It is the product of air density (about 1.2 kg/m³) and specific heat (about 1.006 kJ/(kg·K)), adjusted for units. It changes with altitude and air temperature.
Is a ton the same as a tonne?
No. A ton of refrigeration is a rate of heat removal (3.517 kW). A tonne is a unit of mass (1,000 kg). They are unrelated.
Why does the water equation use 500?
It comes from water density (about 8.33 lb per US gallon), 60 minutes per hour and a specific heat of 1 Btu/(lb·°F).
Key takeaways
- Canadian projects routinely mix SI and imperial units, so fluency in both is essential.
- Temperature differences convert by 1.8 only, with no offset.
- Confirm gallon type and actual versus standard airflow.
- The air and water heat equations depend on fluid properties and must be adjusted for altitude and glycol.
- State units everywhere and cross-check with a rough calculation in the other system.
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