HOW DO HOT WATER TANKS WORK?

Date Added: September 04, 2008 04:05:12 PM
Author: Nicholas Reume
Category: Energy Saving Improvements

INTRODUCTION TO DOMESTIC HOT WATER TANKS

 

Free standing water heaters are generally energized by electricity, natural gas, or fuel oil. The methods for the transfer of heat vary for water heaters, depending on their energy source. Each system will be covered in detail later.  First we will discuss those features which are common to all systems, regardless of fuel source.

 

The outer jackets of hot water tanks are usually made of sheet metal.  Between the outer jacket and the actual water containment tank is usually a layer of one to two inches of glass fiber or other types of mineral wool insulation. The cold water pipe is normally connected to the top of the tank and the cold water passes through a pipe or dip tube to the lower portion of the tank. Hot water is distributed from the upper portion of the tank and replaced with cold water from the dip tube near the bottom.

 

The water supply in some areas may contain corrosive element which could lead to deterioration of the containment tank in a process called galvanic corrosion.  The reduce the corrosion of the tank through electrolysis, magnesium rods are installed in the tank.  Because of the chemical properties of this metal, the rods will tend to corrode before the tank does.

 

A drain valve is usually located on the outside of the tank, near the bottom to permit drawing off sediment which may accumulate on the bottom of the tank.

 

To prevent overheating, free standing water heaters are usually equipped with a high limit thermostatic switch, which will stop the heating if internal water temperatures reach a dangerous level.  A pressure release valve, usually located on the top of the tank, serves as an additional safety measure.  If the temperature in the tank becomes too high, the pressure builds up and the relief valve will open.

 

Electric water heaters operate by resistence elements within the water containment tank.  These units can be equipped with either one or two elements.  Models with two elements usually have one located in the upper portion, and one in the lower portion.  Each element has an individual thermostat control found behind the access plates.  As previously described, hot water is drawn from the top of the tank and replaced by cold water entering the bottom through a tube. 

 

When the demand for hot water is low, the heating elemnt in the lower portion of the tank operates to maintain the water temperature. As the demand for hot water increases, the heating element at the upper portion of the tank switches on to maintain the temperature of the hot water.  The hot water is drawn from the top of the tank.  Once the water at the top of the tank has reached the desired temperature, the top element turns off and the lower element turns back on.

 

The heating surfaces for gas and oil fired water heaters are near the bottom of the tank.  Air enters the combustion chamber, combines with the fuel, and the mixture is ignited.  Heat is then transferred to the water through metal surfaces at the bottom of the tank.

 

The vent pipe, an exhaust pipe that carries the by-products of combustion to the outside, also serves as a heat exchanger.  This pipe is usually surrounded by the water containment tank, and often contains baffles to slow the escape of these gases through the vent, thus allowing more time for heat to be transferred to the water around the pipe.

 

Thermostatic controls regulating the temperature of the water are usually located near the mid-section of gas and oil-fired water heater tanks.

 

The proper mixure of additional air with combustion or exhaust gases is important in gas and oil-fired water heaters to assis in the safe passage of combustion products to the outside. If their escape is impeded or blocked, serious problems can develop.  To assure this proper mixture, gas-fired water heaters are equipped with a cone-shaped draft hood on the vent pipe, as it emerges from the tank.

 

Oil-fired water heaters are usually furnished with draft regulators attached to the vent pipe, between the tank and the chimney.  This is a hinged metal flap with a counterweight to allow for variations in the gas pressure.


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Comments:

Guest:
2009-02-08 21:12:58
What is the meaning of 'if the demand for hot water is low'? Does the heater have a flow measuring device? None was mentioned. We use the waste heat in our water heater to power a radiant floor in the winter, and were trying to figure what the controls would be doing as the floor does try to absorb the heat available in the water from the water heater. So far we just flip the water heater switch off (electric) when the floor's coils seem not to be spreading the heat evenly. Would have liked to have set the controls so that the heater turned on when the temperature dropped to 80*F and shut off when it reached 120*F. Demand should be quite constant when measured as flow because of the pump.

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