See Figure 2 for simple contact type ECOs with bi-metal. These were known for premature failure, opening at temperatures below the actual desired safety opening temperature. The problem with them was that when they cooled, they would remake (close after being open). This caused troubleshooting problems because you never knew if the water heater went off during over temperature, or if it was a thermocouple problem because the bi-metal cooled and would remake. These have been out of the field for some time. See Figure 3 for the disc type, they are typically found on commercial applications and anywhere a surface mounted type ECO could be used. They are used with a junction block type of thermocouple that will allow the pilot safety system to shut down in the event of an over temperature condition. Allowable length of lead wire to supplementary limit (ECO) An ECO is a method of providing added safety to a standing pilot burner control system. If the high limit fails, a secondary high limit, the ECO control, will open and interrupt the thermocouple circuit causing the safety shutoff valve to close. Most modern systems utilize a dual-seated gas control to provide this same added safety. However, there are some ECOs in service that need to be dealt with. Because the power source in the ECO circuit is only a 30 millivolt thermocouple, any added resistance tends to reduce reliability. Therefore, there are limits on Chart 1 Chart 2 ICM/May/June 2017 21
ICM May-June 2017
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