Understanding and Servicing the Heating Circuit
/by RICH LEESON
As a beverage equipment technician, we can see many different types of machines that heat water. These are espresso machines, drip coffee machines, tea brewers, hot chocolate machines, hot water machines and others. Most common in today’s equipment, the heat source is electrical, and this is what we will be discussing in this article.
The heating circuit is usually powered with the highest voltage that the machine is wired to, and caution is very important. Servicing a machine with a heating issue begins with understanding the heating circuit, its path, and the parts in the path. It is a must to be able to read electrical wiring schematics and have working knowledge of how to use a multimeter.
Most multimeters have settings to measure AC voltage, amperage, Ohm’s, and continuity. Using Ohm’s Law and your meter on the proper setting should allow you to diagnose any heating issues.
Heating Parts and Function
Heater Element: Creates heat
High Limit: Bi metal safety link
Thermal Cut Off (TCO): Resistance-controlled safety link
Pressure Stat: Pressure-controlled contactor
Contactor: Bridge transfers electricity from supply to load
Solid State Relay: Controls power from supply to load
Relay: Controls power from supply to load
Triac: Controls power from supply to load
Thermostat: Controls voltage to element to control water temp
Temp Probe / Thermistor: Controls voltage to element to control water temp
Breaker: Amperage-controlled switch
Fuse: Amperage-controlled link
Terminal Block: Connection where power supply enters machine
Power Switch: Controls all electricity after the switch
Not all these parts are used in every machine, but some combination of these parts will certainly be there.
Diagnoses
There are just three heating circuit issues
1. Too Hot
2. Too Cold
3. No Heat
Our first step is to not over “tech “ the situation, which means check and understand settings, expectations, and machine capabilities. These could be programming adjustments and not part issues.
1. Begin with identifying the machine’s voltage and amperage requirements.
2. Check outlet for required voltage with multimeter.
3. Check outlet completely with meter to confirm function, by checking each power leg as well as neutral and ground.
4. If there is an issue here, check breaker panel for tripped breaker.
5. Check and confirm expected voltage, same as outlet is available on both sides of the terminal block.
6. If not, check cord set for damage / broken wire.
Now is when you want to understand the complaint. We should use the three potential issues to guide us though the diagnosis and repair.
Examples are:
1. Too Hot, means we have power getting to the heater element, but the components that control that power or temperature are the parts we need to test.
2. Too Cold, means there could be a lack of voltage reaching the element or the parts that control the temperature are not working correctly.
3. No Heat, means the element has failed or that a part on the voltage path has failed, stopping voltage to reach the element.
When it’s a Too Hot issue, it is suggested to check any parts that are monitoring the tank temp for limescale, as the part could simply need to be cleaned so it can read properly. If this doesn’t correct the issue, move on the part that is controlling the power/temp. These would be a contactor, relay, triac, thermostat, or thermistor that is probably stuck in a closed position. These can be tested by disconnecting them and testing for continuity. These are usually a normally open part so if there is continuity when disconnected, it is bad and must be replaced.
With either a Too Cold or No Heat issue, it is best to test for voltage the heater element first. Identify if the required voltage is reaching heater element. If the required voltage is not there work backwards and test parts on that path. If proper voltage is present, use the amp meter to test the load. Using Ohm’s Law or machine data plate to determine what the proper amperage should be. Continuity testing of the element can be done by disconnecting from the power and removing, isolating the element. Switch meter to Ohm setting and test between element terminals, also test each terminal to tank.
The continuity / Ohm value is a good test between the element terminals, but not wanted between a terminal and the tank, which can indicate the element is open or broken.
Testing, Multimeters & Ohm’s Law
When servicing the heating circuit, it is critical that you have solid working skills of how to test each component, use of a multimeter and how to reference the component’s value using Ohm’s Law.
The design of a heater element is simple, it’s an element wire in a protective casing to keep water away from the element wire. The gauge of element wire creates resistance, which translates to wattage, that creates heat based on the applied voltage. The working test would be amperage.
Testing an element on a live machine, we would use our meter to measure amperage draw.
When we look at Ohm’s Law we find that if we have two values, we can figure out the third. With our multimeter we can test voltage to the machine, we can get an amp reading, and we can then calculate what the resistance should be. In This example the applied voltage, divided by the measured amperage equals the resistance. If we then test the resistance of the element, if it’s a good part, the Ohms should be what is calculated.