Timmie McElwain Gas/Propane Basics... President, Gas Appliance Service gastc@cox.net Thermal Electricity & Electromagnetism Part 5 Figure 1 There are, basically, three types: 1. Disc 2. Contact 3. Fusible Link These are used on rod and tube type controls, found on under fired water heaters. They are contained in the rod and tube assembly. In most cases, when they open up, the control must be replaced. On warm air furnaces, they are typically the fusible link type that melt on an over temperature condition and must be replaced. On some commercial water heaters, the disc type is surface mounted at crucial locations on the tank. They are wired into the typical thermocouple and electromagnet circuit and add some resistance to that circuit. When troubleshooting, one must take into consideration the millivolt drop across the ECO. The charts used for non ECO systems will not properly diagnose the problem and will, in fact, sometimes lead you astray. You must use Chart 1 when charting out your readings. The closed circuit (CC) and drop out (DO) will be higher. The drop out range is 1.5 to 11.5 millivolts. On Sept. 1, 2012, standing pilots on new equipment were done away with. This only pertains to new equipment, not existing equipment. There are still many systems, both heating and hot water, that use a thermocouple as a pilot safety system. This series of articles adddress these systems as to how they operate, their many uses and how to diagnose and troubleshoot thermocouple systems. These systems appear on both propane and natural gas equipment. A large number of heating and water heating systems are still operating with a thermocouple pilot safety system. Being able to understand and service these systems is still vital to keeping the older systems, which still have some operating life in them. We have in Parts One through Four been presenting some in-depth information about thermocouples and the electromagnetic principle that works in conjunction with this very reliable and long standing system. The first chapter in my manual on Circuitry and Troubleshooting Volume II is, in fact, on thermocouples. Let’s continue our discussion on thermal electricity and electromagnetism. This discussion ties together several things, including my unbiased findings on some particular replacement parts for these systems. The recommendation for the thermocouples presented was determined after a 15 year survey of over 100 homes in which thermocouples were replaced and then tracked as to performance—Honeywell and Johnson controls were by far the most dependable and reliable. We will expand on this in later articles. One of the types of thermocouples we sometimes see is junction block thermocouples. These thermocouples allow the attachment of safety switches that, when activated, cut off the flow of millivolts, thus causing the electromagnet to snap shut and shut off the flow of gas. It is however important to understand what an energy cutoff (ECO) is and how it is incorporated into the safety system. So we will cover ECOs before junction block thermocouples. Energy cutoffs (ECOs) We will continue the discussion about ECOs used on various types of equipment and, in particular, the 30, 40 and 50 gallon storage gas water heaters. The easiest way to look at this is illustrated in Figure 1—a switch introduced into the millivolt circuit. The resistance of that ECO switch has to be considered when taking readings and diagnosing problems with Thermocouple systems using that control. On some installations, an ECO safety device is designed into the electromagnet circuit. It functions to open the circuit to the magnet in cases of excessively high temperatures. Figure 2 20 ICM/May/June 2017
ICM May-June 2017
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