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Calculating HVAC coil loads from psychrometric conditions
The most accurate way to calculate coil capacity is from the psychrometric state points of the entering and leaving air, combined with the air mass flow rate. The total coil load equals mass flow rate times the enthalpy difference: Qtotal = ṁ × (h1 − h2). Since enthalpy (h = 1.006 × T + W × (2501 + 1.86 × T) in kJ/kg) captures both sensible and latent heat content, this formula gives the complete picture regardless of whether the process is sensible-only, latent-only, or a combination of both. The mass flow rate uses the specific volume of the entering air: ṁ = Qvol / v1, where v1 = 0.28705 × TK × (1 + 1.6078 × W) / P. This is more accurate than the standard-air shorthand (4.5 × CFM × Δh for BTU/hr) because it corrects for actual temperature, humidity, and elevation.
The sensible portion equals mass flow times moist-air specific heat times the dry-bulb temperature difference: Qs = ṁ × cpm × (T1 − T2), where cpm = 1.006 + 1.86 × Wmean kJ/(kg·K). The latent portion is the remainder: Ql = Qtotal − Qs. The split between these two, expressed as the sensible heat ratio, drives coil selection, and it is exactly this split that manufacturers must report under the test procedures in AHRI Standard 410. Use the SHR calculator if you don't need absolute loads but only the ratio, or the apparatus dew point calculator to convert the SHR into a coil ADP for coil selection.
Process identification: cooling, heating, and mixed processes
The sign of the enthalpy difference identifies the process. When h1 > h2 the coil removes heat from the air (cooling or dehumidification). When h1 < h2 the coil adds heat (heating or humidification). Within a cooling process, if W1 = W2 it is pure sensible cooling. If W1 > W2 moisture is condensing on the coil (cooling and dehumidification). Rarely, W1 < W2 during cooling indicates evaporative or adiabatic cooling, where moisture is added as dry-bulb drops. This tool identifies the process automatically from the state point signs and labels the result accordingly. For Canadian summer design, the typical process is cooling and dehumidification, with SHR ranging from 0.70 in Toronto to 0.87 in Calgary for standard office applications. Since the entering air state is usually a mix of outdoor and return air, confirm the outdoor air fraction with the outdoor air calculator before locking in the entering condition used here.
Unit conversion: kW, BTU/hr, MBH, and tons of refrigeration
One ton of refrigeration equals 3.517 kW or 12,000 BTU/hr, a conversion factor documented in NIST Special Publication 811. MBH (thousand BTU/hr) is commonly used in Canada for equipment ratings: 1 MBH = 0.2931 kW. This calculator outputs all three simultaneously. For reference: a typical 100 L/s (212 CFM) per person office space at Toronto summer design with a 0.75 SHR carries roughly 25 to 35 kW total cooling load (7 to 10 tons). At Calgary summer design (drier air), the same space might carry 18 to 28 kW (5 to 8 tons) with a higher SHR closer to 0.85. Use the specific volume calculator to verify the entering air density used in this tool's mass flow calculation, especially for higher sites above 500 m.
Frequently Asked Questions
Sensible load changes the dry-bulb temperature without changing moisture content: Qs = ṁ × cpm × ΔT. Latent load changes the moisture content without directly changing temperature: Ql = ṁ × Δhl where Δhl is the enthalpy change due to moisture alone. On a cooling coil, latent load represents condensation of water vapour onto the cold fin surface. The sensible heat ratio (SHR = Qs / Qtotal) tells you the split. A lower SHR means the coil is doing more dehumidification relative to cooling, which is typical for humid Canadian cities in summer. Accurate SHR is essential for selecting a coil that meets both the temperature setpoint and the indoor humidity setpoint simultaneously. Use the bypass factor calculator to verify the coil contact efficiency needed to achieve the target leaving condition from the calculated SHR.
One ton of refrigeration = 3.517 kW = 12,000 BTU/hr. Divide kW by 3.517 to get tons. Divide BTU/hr by 12,000 to get tons. For example, a 52 kW total cooling load equals 52 / 3.517 = 14.8 tons. MBH (thousand BTU/hr) is also common in Canada: 1 MBH = 0.2931 kW, so 52 kW = 177 MBH. This calculator displays all three units simultaneously. For chiller sizing, remember to add pump heat and any other heat gains between the chiller and the air handling unit. For direct expansion equipment, use the total load with the compressor sizing calculator to select compressor capacity. See the cooling coil calculator for full coil selection including face area, fin spacing, and refrigerant considerations.