Why Does the Inside of Cars Get so Hot?

We’ve all heard the warnings: don’t leave your children or pets in your car (or truck) during this time of year.  The temperatures inside the passenger compartment can reach over 100 degrees easily and quickly causing heat stroke and death.  This sounds like common sense but, every year, during the summer, reports of a death of a child or animal left in a hot car, are not uncommon.  The reason that passenger compartments get so hot is because of the way heat is transferred into and out of the compartment.  First, in order for heat to flow, there has to be a temperature difference.  As long as the inside is cooler than the outside ambient temperature, heat will flow from outside to inside.  More specifically, heat transfer will occur by convection from the ambient air to the outer surfaces of the passenger compartment, including glass.  Heat will then be transferred by conduction through the roof, insulation and headliner.  Heat transfer by conduction will also occur through windows.  There is also a radiation component that serves to heat the interior.  The sun’s rays will heat the solid parts of the interior such as dashboards and steering wheels.  The heat absorbed by these objects is then radiated to the air inside the passenger compartment.  The real clincher to this process is that the heat entering the vehicle enters at a rate faster than it is dissipated.  As a result, the temperature inside the passenger compartment can only increase.  It is not until the rate of heat transfer entering the vehicle is reduced below the rate of heat transfer out of the vehicle that the temperature in the passenger compartment is reduced.  This point usually starts to occur at dusk. 

Remember, we still have approximately one and a half months of summer left.  Please be mindful of your young passengers and pets and don’t leave them locked in a death trap!

The photos below illustrate how hot a passenger compartment can get. The photographs were taken inside a Chevrolet extended cab pick up truck.

Temperature in cab after 7 minutes with windshield shaded

Temperature after 30 minutes, windshield uncovered

Temperature after 1 hour

Temperature after 2 hours

Danger: Using Lights for Space Heating

During this past week, hundreds of thousands of people experienced the polar vortex that swept across our country. In the process, many have had to figure out a way to heat spaces where there was no heat in order to keep pipes and pets from freezing. One of the things being done is using lights with high wattage bulbs. Clamp lights as well as any type of spot or work light are rated for use with a bulb of specific wattage. It is never a good idea to use a light rated for a specific wattage with a bulb rated for a higher wattage. For example, if your light is rated for 100 watts, do NOT use a bulb rated with a wattage greater than 100 watts. A higher wattage bulb will produce more heat, always in excess of the rating of the light itself. The problem lies in the fact that more heat results in a higher current draw. If the wiring cannot handle the current draw, it will melt and burn resulting in a fire, particularly if an extension cord is also used. A fire hazard will also arise if the light bulb produces enough heat to ignite combustible material in close proximity to the light. How far away should a light be kept from combustible material depends on the heat output of the light bulb. Recently, we were asked to investigate a fire involving a clamp type worklight and its use in heating a dog house. As it turned out, the light was rated for 150 watts and the homeowner installed a 250 watt infrared bulb to provide the necessary heat for their pet. At 120 volts, a 150 watt bulb will draw 1.25 amps whereas a 250 watt bulb will draw a little over 2 amps. These current levels are very low in comparison to other heat producing devices and were not a factor in this incident. On the other hand, the amount of heat produced at 150 watts is about 511 BTU/hr. At 250 watts, the amount of heat produced is 852 BTU/hr, a difference of 341 BTU/hr. This difference was enough to cause the plastic material in the dog house to melt and eventually ignite. The fire went on to cause damage to a deck and vinyl siding material used to cover the soffitt. Fortunately, the owner’s dog survived the fire and was unharmed.