Q (kW) = ṁ (kg/s dry air) × (hentering − hleaving) in kJ/kg
This is total (sensible + latent) cooling capacity. For dry air mass flow from volumetric airflow, use the Airflow Unit Converter with the Air Density Converter.
Conversion Factors
1 kJ/kg = 0.4299 BTU/lb = 0.23885 kcal/kg
1 kcal/kg = 1.8 BTU/lb = 4.1868 kJ/kg
Q (kW) = ṁ (kg/s dry air) × Δh (kJ/kg)
Moist Air Enthalpy Reference (at Sea Level)
| Dry Bulb Temp | Relative Humidity | Enthalpy (kJ/kg) | Enthalpy (BTU/lb) | HVAC Context |
|---|---|---|---|---|
| 0°C (32°F) | 50% | 3.7 | 1.6 | Cold outdoor air, winter |
| 10°C (50°F) | 50% | 25.8 | 11.1 | Cool shoulder-season air |
| 21°C (70°F) | 50% | 40.0 | 17.2 | Standard indoor comfort setpoint |
| 24°C (75°F) | 50% | 47.3 | 20.3 | Typical cooling setpoint |
| 24°C (75°F) | 60% | 50.9 | 21.9 | Higher humidity indoor condition |
| 30°C (86°F) | 50% | 63.9 | 27.5 | Hot summer outdoor air |
| 32°C (90°F) | 60% | 78.9 | 33.9 | Hot humid summer design condition |
| 35°C (95°F) | 50% | 78.1 | 33.6 | Very hot dry design condition |
| 35°C (95°F) | 70% | 104.6 | 45.0 | Extreme humid heat design condition |
Enthalpy of moist air is referenced to 0°C dry air and includes both sensible heat and the latent heat of moisture content. Values shown are approximate; exact figures depend on precise psychrometric calculations at actual barometric pressure.
Typical Enthalpy Difference for Coil Sizing
| Application | Entering (kJ/kg) | Leaving (kJ/kg) | Δh (kJ/kg) | Notes |
|---|---|---|---|---|
| Residential A/C, mild summer day | 50 | 32 | 18 | Return air mixed with some outdoor air |
| Residential A/C, hot summer day | 65 | 32 | 33 | Common Canadian summer design condition |
| 100% outdoor air unit, hot humid | 79 | 32 | 47 | Full fresh-air makeup unit; large latent load |
| Dedicated OA unit, extreme humid | 105 | 32 | 73 | Very high latent load design condition |
How to Use the Enthalpy Converter
Type a value in BTU/lb, kJ/kg, or kcal/kg. All other units update instantly. Use the quick presets to load common HVAC psychrometric reference points: standard indoor comfort conditions and typical cooling setpoints.
With dry air mass flow rate and entering/leaving enthalpy values, the calculator computes total (sensible plus latent) cooling capacity in kW and BTU/hr. This is the correct method for sizing cooling coils, since it accounts for both temperature reduction and moisture removal.
The result panel identifies what typical air condition your enthalpy value represents, from cold winter air through extreme hot humid design conditions. This helps confirm a converted value is reasonable for the psychrometric state you're working with.
The moist air enthalpy table shows values across common temperature and humidity combinations. The coil sizing table shows typical entering and leaving enthalpy differences for residential and commercial applications. For related psychrometric calculations, see the Psychrometrics category and the Specific Heat Converter.
Enthalpy in HVAC Psychrometric Work -- Complete Guide
Enthalpy is the total heat content of moist air, combining sensible heat (related to temperature) and latent heat (related to moisture content) into a single property. It's the key value used in psychrometric calculations for cooling coil sizing, total load calculations, and energy recovery analysis, since temperature alone doesn't capture the moisture-related energy that dominates cooling loads in humid conditions.
The Conversion: BTU/lb to kJ/kg
Multiply BTU/lb by 2.326 to get kJ/kg. This factor comes from combining the BTU-to-kJ conversion (1.05506) with the pound-to-kilogram conversion (0.453592): 1.05506/0.453592 = 2.326, both set out precisely in the NIST Guide to the SI, Appendix B. Air at standard indoor comfort conditions (21°C, 50% RH) has an enthalpy of approximately 40 kJ/kg (17.2 BTU/lb). Both units are always expressed per unit mass of dry air, not total (dry plus moisture) air mass -- an important convention in psychrometric calculations.
Sensible vs. Latent Heat in Enthalpy
Total enthalpy change during cooling splits into sensible heat (temperature drop) and latent heat (moisture condensation). In a humid climate, latent load can represent 30-50% or more of total cooling load. Using dry-bulb temperature difference alone -- the simpler sensible-only approach -- significantly undersizes cooling equipment in humid conditions because it ignores the substantial energy required to condense moisture out of the air. Enthalpy-based calculations capture this total load correctly, following the psychrometric methods set out in the ASHRAE Handbook -- Fundamentals.
Enthalpy in Coil Selection
Cooling coil capacity is properly calculated as: Q (kW) = mass flow of dry air (kg/s) x (entering air enthalpy - leaving air enthalpy) in kJ/kg. For a system handling 0.5 kg/s of dry air, cooling from an entering condition of 65 kJ/kg (hot humid outdoor air mixed with return air) to a leaving condition of 32 kJ/kg (cold supply air), the coil load is 0.5 x (65-32) = 16.5 kW. This single calculation captures both the sensible cooling and the latent dehumidification the coil must perform. The Cooling Coil Calculator works through this same enthalpy method with full psychrometric state points.
Heat Recovery Ventilation and Enthalpy
Energy recovery ventilators (ERVs) transfer both sensible and latent energy between incoming and outgoing air streams, making enthalpy the relevant metric for evaluating their performance -- described as total enthalpy recovery efficiency. Heat recovery ventilators (HRVs) transfer sensible heat only, without moisture transfer, so their performance is evaluated using sensible temperature recovery efficiency alone. In humid Canadian summer conditions, or in winter when indoor humidity retention matters, ERVs generally capture more total energy savings than HRVs because they recover the latent component that HRVs simply exhaust. The ERV Calculator works through this total enthalpy recovery calculation directly.
Frequently Asked Questions
Multiply BTU/lb by 2.326 to get kJ/kg. So an enthalpy of 30 BTU/lb equals 69.8 kJ/kg. To convert kJ/kg back to BTU/lb, divide by 2.326. This is a common conversion in psychrometric work since Canadian and international engineering software typically uses kJ/kg, while some North American psychrometric charts and legacy coil selection software use BTU/lb.
At 21°C (70°F) and 50% relative humidity, moist air has an enthalpy of approximately 40 kJ/kg (17.2 BTU/lb) of dry air. This is a common reference point used in cooling load calculations, since it represents typical target indoor conditions during summer cooling operation. Enthalpy at 24°C and 50% RH -- a common cooling setpoint -- rises to about 47.3 kJ/kg.
Cooling coils remove both sensible heat (temperature) and latent heat (moisture) from air. Enthalpy captures both in a single value, making it the correct property for total cooling load calculations: Q (kW) = mass flow (kg/s) x enthalpy difference (kJ/kg). Using temperature alone would only account for sensible cooling and would significantly undersize a coil in humid climates where latent load can be 30-50% or more of the total. Use the enthalpy difference calculator above to compute total cooling load directly.
ERVs (energy recovery ventilators) transfer both sensible heat and moisture, so their performance is evaluated using total enthalpy recovery efficiency. HRVs (heat recovery ventilators) transfer sensible heat only, with no moisture transfer, evaluated using sensible recovery efficiency alone. In humid conditions or when indoor humidity control matters, ERVs typically deliver greater total energy savings because they recover latent energy that HRVs simply exhaust to the outdoors.