What Are They Used for?
The majority of all equipment that uses water to generate hot water or steam utilizes some type of water level control. One type is called a low water cutoff control. This control is used to simply shut down the heating circuit if the water drops below a safe level. On electrically heated units, if the water drops below a safe level, the emersion elements would be exposed to the air and burn out (dry fire). When you have a gas-heated unit and the water drops below a safe level, you risk damaging the tank that holds the water.
A water level control has two functions: it allows the tank to automatically fill and maintain water at a set level, and it includes the low water cutoff feature described above. When the unit is turned on, the control senses if there is water in the tank and turns on a fill solenoid if needed. Once the control senses that the safe water level has been reached, it allows the heat circuit to start while it is completing the water fill process.
How Do They Work?
These controls use a transformer, two relays, and two probes to operate. The transformer serves two purposes: it is a step up transformer and an isolation transformer. The primary side (120V), or supply voltage side, has a potential to ground. If you were to read from the hot wire to ground, you would read 120 volts AC. If you were to touch the hot wire to ground, you would experience a dead short. The secondary side (300V), or output side, of the transformer has no potential to ground because it is isolated. In this case, the voltage is created by a magnetic field, so if you were to read either wire to ground, you would read zero volts. If you were to touch either wire to ground, nothing would happen.
One of the wires on the secondary side of the transformer is connected to the body of the tank or grounded to the tank. The other wire from the secondary side is connected to one side of the fill relay coil. The other side of the fill relay coil is connected to a water level probe (short probe) screwed into the tank. The probe is isolated from the tank’s body by plastic or a Teflon sleeve.
When voltage is applied to the water level control, the fill relay’s normally closed contacts supply voltage to the fill solenoid, opening the valve to start filling the tank. When the water reaches the bottom of the water probe, the water AC current flows from the wire connected to the tank’s body through the water to the probe, completing the circuit. The completed circuit supplies voltage to the fill relay coil, energizing the coil and opening the closed contacts and dropping voltage from the fill solenoid. The low water cutoff portion of the control operates the same way, except the heat relay’s contacts are normally open and they close when water reaches the bottom of the long probe. The closed contacts then apply voltage to the heating circuit.
What Will Happen When?
The ground wire to the tank is broken:
If the secondary side wire going from the transformer to the tank body is broken or loose, there would be no continuity through the water to the probes to complete the circuit. The water solenoid would stay energized and overfill the tank with water, and the heat circuit would never energize.
The short probe wire breaks (water level):
If the wire from the short probe to the fill relay coil is broken or loose, there would be no continuity through the water to stop the water fill solenoid. The tank would overfill with water. The heat circuit would still work.
The short probe is grounded (water level):
If the wire or short probe is grounded to the tank body, there would be continuity all of the time in the fill circuit. The tank would never fill with water or the heat circuit would never energize. The water level control acts as if there is water in the tank completing the circuit.
The long probe wire breaks (heat circuit):
If the wire from the long probe to the heat relay coil is broken, there would be no continuity through the water to energize the heat circuit. The tank would fill with water to the proper level. The heat circuit would never activate.
The long probe is grounded (heat circuit):
If the wire or long probe is grounded to the tank body, there would be continuity all of the time in the heat circuit. The tank would start to fill with water and the heat would turn on immediately and could damage the tank or elements.