Hot Water Heater – How It Works
The hot water heater is an excellent conductor of heat. It only needs 40 minutes to heat 40 gallons of cold water but, the hot water heater is also a terrible conductor of heat. It retains heat in its hot water rather than transfer it to the outside air.
Thanks to its split personality, the hot water heater is a master of one-way heat transfer. From the flame to the water, heat flows; from the water to outside air, heat does not flow. Hmmm! But how does it work?
The heat source in this gas water heater is the burner flame. A chimney extends from the bottom to the top of the reservoir to efficiently transfer heat. Cold water comes in through the top, descends through a pipe, absorbs heat and leaves through the exit pipe.
Now, how can the hot water heater be a conductor when it’s time to heat the water and an insulator when it’s time to keep heat in? In order to understand this split personality, we have no choice, time to open her up! The bottom of the water heater and the chimney are both made of metal. That’s not for nothing.
Metals do a fantastic job of conducting heat from one place to another. That’s because speaking on an atomic scale, they are free electrons- tiny particles that move from atom to atom spreading heat. Okay, the flame heats the air, which heats the bottom of the water heater and enters the chimney. Thanks to the conductive properties of metal, the heat is transferred to the water.
But there’s more. A baffle in the chimney slows the air. It must work its way through the zigzags, which allows more time to transfer more heat to the water, a clever design which optimises heat conduction.
But, if conduction is so great at transferring heat from the burner to water, then it can also conduct heat from the hot water to air outside the tank. To prevent that, the water heater is also a perfect insulator, thanks to this polyurethane foam.
It’s the exact opposite of metal. Instead of transferring heat, it traps it. How? The foam is made up of billions of tiny gas bubbles imprisoned in plastic.
The gas bubbles are terrible at conducting heat. In a gas, molecules are extremely spread out. This makes transferring heat from one to the other much more difficult than in a solid.
Also, the plastic surrounding each bubble is itself a bad heat conductor. It doesn’t have any of those famous free electrons. The heat is trapped and stays inside the reservoir. Thanks to insulation, the water heater keeps water hot a long time.
The water heater really does have a split personality, there’s no possible doubt it’s an excellent conductor; transferring maximum heat from the burner to water in the reservoir but it’s also a champion of insulation when it comes time to keep heat inside.