Do Grow Lights Produce Heat?

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Not all heat is created equal. We have all experienced heat in three forms. There is radiation where the heat falls directly on your skin, conduction heat when you touch something hot, and convection heat when the air around us is heated up.  

Light is yet to be created without some heat being lost. All grow lights produce heat,  through one or more of the heat transfer processes known as radiation, convection and conduction. The amount of heat produced depends on the type and size of the light.    

Understanding how the heat is generated can help you to control the temperature in your grow area. Depending on the prevailing conditions you can either choose to preserve this heat or to get rid of it. 

Heat transfer and evaporation 

Heat is energy that is transferred from one object to another as the result of a difference in temperature, which is the measure of heat. The energy will flow from a warmer substance to a cooler one. 

There are four ways energy is converted to heat. It is useful to understand these processes in order to use them to the advantage of your plants. Evaporation is included here, although the end result is a cooling of the environment. It is a form of heat transfer that is useful when growing plants indoors. 

Radiation 

Radiant heat is transferred directly from the source to the object through electromagnetic radiation. The way energy is transferred from the sun is an example of radiant heat. 

The sun’s energy is emitted in light waves. The waves on the red portion of the spectrum, particularly, the infra red rays are responsible for transferring energy in the form of heat onto anything they fall. 

If you stand in the sun for a length of time, you can feel it burn your skin. If you get under cover you can no longer feel that heat. The same is true when you stand near a fireplace. If you move away from the fire, you can no longer feel the direct heat. 

Conduction 

Heat transfer through conduction takes place in solids. The heat can move from one molecule to another within a substance, and it can be conducted from one type of solid to another. 

For example, the sun will heat the surface of a rock by radiation. The heat will then move by conduction to the portion of the rock that is below the surface. If you touch the rock, some of the heat will be transferred to your skin.  

The speed of the heat transfer depends on the substance through which it is being conducted. For example, metal will heat up faster than wood or stone. 

Convection 

When heat is transferred through a gas or fluid in motion, it is called convection. The heat will move from a higher temperature to a lower temperature. Effectively, the hot air is being carried away from the source.  

The heated rock mentioned above would transfer some of its accumulated heat to the cooler air around it, heating it up.  Heat causes the air molecules to expand, become less dense and rise. This leaves a vacuum for cooler air to move in. 

As the air rises away from the direct heat source, the molecules become less dense, the air cools, and then falls. This vertical cycle of heating and cooling is how convectional rain is produced. 

If the rock was next to the sea, the cooler air would move in from the sea at ground level and the heated air would move towards the sea at an elevated level, causing a horizontal cycle of moving air. 

Evaporation 

When sufficient heat energy is applied to a water molecule (H₂O), it will force the bond holding the atoms in the molecule together to break into the gases hydrogen and oxygen, and whatever minerals, e.g. salts, are trapped in the molecules. 

The gases are cooler than the water because of the energy lost, and the effect is that the air is cooled.  

Heat from various light sources 

In order to calculate the amount of heat generated by your grow light, you need to multiply the number of Watts used by 3.41 to get the number of British Thermal Units (BTUs). A BTU is roughly the amount of heat produced by a 4 inch match.  

While the heat produced remains constant per Watt, the number of lumens per Watt varies according to the type of lighting used. The efficiency of light produced as a ratio to heat produced also varies, although it is low for all forms of artificial light.   

Heat from Incandescent lights 

Incandescent lights work by blasting electromagnetic waves through a thin filament, usually made of strong metal wire. The heat from the filament causes the bulb to heat up. Depending on the amount of power used by the light, the bulb could reach temperatures in excess of 200ºF. 

Less than 5% of the power consumed by an incandescent bulb produces visible light. More than 80% is lost through radiation in the form of infra red rays, the rest is lost to convectional heat. 

Heat from HID – High Intensity Discharge – Lamps 

HID lamps produce light by arcing electricity through various gasses. They become hot enough to make the gasses glow which means the bulbs become extremely hot to the touch. Much like incandescent lights, much of the heat emitted is in the form of infra red rays. 

These lights have a very high light output and are generally used in commercial ventures. They are hung high above the plants’ surface so that the plants are not burnt by excessive radiation.  

Heat from Fluorescent lighting 

Fluorescent light is produced as a result of two reactions. Mercury vapour is agitated by an electric current which produces ultraviolet rays. These rays react with a phosphor coating which produces visible light. This process is more efficient as it produces more lumens per Watt and loses less energy to heat. 

Therefore, fluorescent lights are more efficient than incandescent lights. A 15 Watt fluorescent light will produce the same amount of light as a 60 Watt incandescent bulb, and about 10% of the power used turns into heat. Also, the surface of the bulb does not get as hot, reaching temperatures of around 100ºF.

Heat from LED lights

LEDs use a process called electro luminescence to produce light. This is more efficient than the processes mentioned above, as it produces even more lumens per watt. Claims vary on the ratio of light to heat emitted. On average 15-20% of the power input is harnessed to provide visible light. 

The percentage of heat lost by LED lights is still significant. Most of the heat lost is by conduction through the coatings of the semiconductors used in the process. Insignificant amounts are lost through radiation which results in the LEDs being relatively cool to the touch.

The larger and more powerful the light is, the more heat it will emit. The heat generated needs to be drawn away from the light quickly in order to save the semiconductors from harm. This is done by rapid conduction heat transfer to heat sinks attached to the light. These then lose heat more slowly, by convection to the environment. 

Managing the heat in your grow area

Managing Radiation 

Infra red radiation is useful for heating up your plants and promoting stem growth. If too much is aimed directly at the canopy, it could burn the leaves. 

Incandescent and fluorescent grow lights should not be positioned too close to the leaves of your plants for this reason. If your light is too strong, either move it further away or dim the lights. Remember to life the light as the plant gains height. 

Managing Convection 

Convection will only work if there is a difference in temperature.  

If the weather is warm, and the lights are producing too much ambient heat, create an outlet for the warm air at the top of your grow area, and an inlet to suck cool air in at the bottom.

If you introduce a fan to circulate the air, make sure that inlet and outlet are far from one another so that you are not just circulating the same warm air. 

If the temperature outside is significantly cooler than the air inside the grow area, you want to keep circulating the air, i.e. you do not want to introduce a continuous stream of fresh air from outside.

Caveat: If your carbon dioxide levels drop too low, photosynthesis will be compromised so you do need to replace the circulating air with fresh air regularly.  

Managing Evaporation 

If the ambient temperature in your grow area is high, it will cause evaporation and dry out your roots. If you wish to reduce the ambient temperature, you need to raise the relative humidity by supplying additional water in the area. This will increase the rate of evaporation but not affect your roots.

Bowls of water at ground level will provide a horizontal source, You can drape a moist towel on a peg to provide a vertical source of additional moisture.  

Conclusion 

Both light and heat are necessary for plant growth. Getting the balance right is a skill that comes with knowledge and experience. It is worth the while to get the balance right, to the benefit of your plants.