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Air Conditioning Heat Load


The Air Conditioning System Heat Load is comprised of two components, Sensible Heat and Latent Heat.


  • Sensible Heat - Causes a change in the temperature and is measurable by a thermometer.
  • Latent Heat – The amount of heat (absorbed or released) required to change the State of a substance with no change in temperature of the substance.


In a building that is Air Conditioned and plants are growing in that space the Sensible Heat Load and the Latent Heat Load are about equal. The Lights and other Equipment in the room produce most of the Sensible Heat Load. The Plants produce most of the Latent Heat Load.

Air conditioning unit

Through a process known as Transpiration the plants release water vapor into the room through the plant stomatal apertures. Water is passively transported into the Roots and then into the Xylem. The forces of cohesion and adhesion cause the water molecules to form a column in the Xylem. Water moves from the Xylem into the Mesophyll Cells, evaporates from their surfaces and leaves the plant by diffusion through the Stomata as water vapor.

Between 97 % to 99.5% of the water taken up by the plant roots is released by the plant Stomata as water vapor into the room. A change in state of the water has occurred. Water has gone from a liquid to a water vapor (Latent Heat). If a more Efficient Watering System is used and reduces the amount of water to the plants 50% to 60%, this will reduce the Latent Heat Load about the same. Reducing the Latent Heat Load 50% will reduce the Total Load on the Air Conditioning System 25%.

Water dripping on seedling

Water weighs 8.33 pounds per gallon. It takes 970 BTU’S of latent heat to convert One (1) pound of water vapor back to water. Reducing the water to the plants by only One (1) gallon will reduce the heat load on the Air Conditioning System by 8080.1 BTU’S. This is 0.673 of a Ton, a little over 5/8 of a Ton reduction on the Air Condition System Load. If you reduce the amount of water to the plants by Ten (10) gallons. This will reduce the heat load on the Air Conditioning System by 80801 BTU’S, a reduction of 6.73 Tons.

The rate of Plant Transpiration is affected by the following factors:

  • Temperature
  • Humidity
  • Wind or Air Movement
  • Soil Moisture

The above four factors will have an effect on the amount of water vapor released into the room by the plants. This will have a direct effect on the Latent Heat Load and also effect the total Air Conditioning Load.

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