TOOL GUIDE · EPISODE 3Calculation Tools

An Air Handler in Winter: Air Mixing and Heating

In winter, cold fresh air mixes with return air in an air handling unit, and the mix is then heated in the heating coil. This episode calculates both steps in turn with the air mixing and heating coil tools, showing mixing by mass, the heating capacity, the hot water flow and why relative humidity falls when air is heated.

Published: 13 October 2026Narration: Turkish and English2:56

In this video

  • When 2,000 m³/h of fresh air (0 °C, 80%, 3.03 g/kg) mixes with 8,000 m³/h of return air (22 °C, 40%, 6.59 g/kg), the mix comes out at 17.31 °C and 5.83 g/kg.
  • The tool weights the streams by mass, not by volume; the cold, dense fresh air makes up 21% of the total mass. On the chart the mixing point lies on the line joining the two streams, closer to the return air.
  • Heating the mix to 30 °C at 10,000 m³/h needs 42.98 kW; with a hot water heater this means a water flow of 1.85 m³/h.
  • Heating adds no water to the air: the humidity ratio stays the same while relative humidity falls to 21.9%, and on the chart the process is a horizontal line.
  • When relative humidity drops below 30%, the tool recommends adding a humidification stage after the coil; the amount of moisture to add is found with the humidification load calculation.

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