Residential Water Heater

Posted by PITHOCRATES - April 10th, 2013

Technology 101

When you Heat Water in a Sealed System that Prevents it from Expanding it produces Great Pressure

Most of us start our days with a hot shower.  If we’ve had a rough day we may relax in a hot bath with a glass of wine after work.  We don’t think twice about enjoying these things.  Because we take them for granted.  All we have to do is open a hot water tap.  And soothing hot water flows out of a faucet.  It wasn’t always like this.  Once upon a time we had to heat water over an outdoor fire and carry it to a tub.  Fully clothed.  Wearing shoes or boots.  Perhaps with someone helping us.  Today a lady can simply turn on the hot water after work.  Pour in some bath oils.  Undress.  Pour a glass of wine.  And settle into a soothing hot bath.  And let the stress of the day melt away.

This convenience is brought to us by our water heater.  That round tank in our basement.  Or in our utility room.  Which works similarly to heating water over an open fire.  And carrying it inside to a tub.  The difference is that we keep a tank full of heated water in our home to use at any time.  And instead of carrying it to where we want to use it we deliver it in a pipe.  That terminates at a faucet.  In our kitchen.  At our laundry tub.  And in our bathrooms.  Giving us hot water to wash our hands.  To use in our dishwasher.  To shave with.  And, of course, to shower or bathe in.

If you look at your water heater in your home you will notice something conspicuous by its absence.  A water pump.  Yet that hot water tank can supply water to an upstairs bathroom.  How?  Because of something that happens when you heat water.  It expands.  And when it tries to expand in a sealed system but can’t pressure builds up.  Like in a car’s cooling system.  If you squeeze the radiator hose when the engine is cool you will be able to compress the hose with your fingers.  Something we do before removing the radiator cap.  If the engine is hot and you squeeze the radiator hose you are not going to be able to compress it.  Because of the high pressure inside the cooling system.  Created from the water trying to boil inside a sealed system but can’t.  If you remove the radiator cap when it’s under pressure the water will explode out of the radiator in a scalding geyser.  Sending you to the hospital with severe third-degree burns.  So never, ever remove a radiator cap when there is pressure in the cooling system.

A Hot Water Tank does Two Things: it Heats Water and it Stores Hot Water

Hot water tanks come in two basic styles.  Gas-fired.  And electric.  There are others (oil-burner, solar power, etc.) but they are not as common.  The gas-fired and electric have their own benefits and disadvantages.  Burning gas requires air to aid with combustion.  And we have to vent the products of combustion outside of the house.  An electric water heater doesn’t need either of these.  And can therefore be packed away into a small utility closet in the middle of a house or apartment.  The advantage of the gas water heater is that it can heat water more quickly than an electric water heater.  So gas water heaters tend to be smaller than electric water heaters.  Electric water heaters have bigger tanks because it takes longer to heat the water.  So it heats and stores a larger volume of water to keep someone from using all of the hot water when taking a shower.  Whereas a gas-fired heater will be able to heat new water in time before someone else takes a shower.

A hot water tank does two things.  It heats water.  And it stores hot water.  An electric heater has one or two heating elements inside the tank to heat the water.  Something similar to the heating elements in an electric stove or toaster oven.  A gas-fired heater has a burner below the bottom of the tank.  The fire heats the bottom of the tank while the exhaust gas vents up through a pipe running through the middle of the water tank.  This hot exhaust gas heats the water in the center of the tank.  When the temperature of the water falls a heating element turns on.  Or a gas valve opens and a pilot light or an igniter ignites this gas.  After the temperature rises to the setting on the thermostat the heating device shuts down.

We make these tanks in layers.  The inner most layer is a glass lining.  Under this glass lining is a metal tank.  The glass lining is to prevent the metal tank from rusting.  (Also inside the tank is something we call a sacrificial anode rod.  Its purpose is to rust so the tank doesn’t.)  Around the metal tank is an insulating layer.  The final layer is an exterior decorative shell.  What you see when you look at your water heater.  Think of the tank as a coffee thermos that you pack with your lunch.  It keeps the coffee hot for an extended period of time.  Just like the insulating layer does on the water tank.

The Pressure inside a Hot Water Tank will push Water out of the Tank up to an Upstairs Bathroom

When we heat water it expands.  If it can’t expand it builds up pressure.  If it builds up too much pressure the tank can explode.  The escaping boiling water/steam turning the tank into a missile.  To prevent this from happening there is a temperature and pressure-relief valve installed at the top of the tank.  Running from this valve will be a pipe down the side of the tank to about 6 inches above the floor.  If the temperature rises too high building up an unsafe pressure the temperature and pressure-relief valve opens to allow this expanding water out of the tank.  Which brings us to how a hot water tank can pump water throughout a house without a water pump.

At the top of the tank you will see two pipes.  One bringing cold water into the tank.  And one taking hot water out of the tank.  To an open faucet somewhere in the house.  The cold water pipe enters the tank and extends to the bottom of the tank.  Bringing the cold water to the bottom of the tank.  As it heats it rises.  Bringing the hottest water to the top of the tank.  The hot water pipe extends down into the tank only a short way.  The bottom of this pipe is near the top of the tank.  In the hottest water.

A hot water tank is a sealed system with heated hot water under pressure.  Like a car’s cooling system.  Though not at quite at the same temperature or pressure.  But if you turn on the hot water to run a soothing bath after a trying day in an upstairs bathroom the pressure inside the hot water tank will push that water out of the tank and all the way up to that faucet.  And when this water leaves the hot water tank the city water pressure pushes more cold water into the tank.  Bringing the temperature of the water down.  Turning on the heating device.  Heating the new cold water inside the tank.  Causing more hot water to rise near the hot water pipe at the top of the tank.  Having hot water ready the next time someone opens a faucet.  Simplicity at its finest.  Using the physics of water and thermal dynamics to put hot water at our fingertips whenever we want it.  Without ever having to heat it over an outside fire and trudging it inside.

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