pilot or both. Any reading falling below the shaded area is an indication of the need for a new thermocouple. It is also encouraged that, while taking these readings, a combustion analyzer is being used to diagnose possible combustion problems that may be contributing to the pilot going out. The pilot may also need adjusting, keeping in mind all pilots on heating equipment are on line pressure and are not regulated. The pilot flame should be a soft blue stable flame, not roaring, lifting or floating around. The flame should envelope the upper 1/2 to 3/8 of the thermocouple. and should cause the tip to glow “dull red”. A “cherry red” thermocouple tip is an indication of the pilot being too sharp and it should be softened up by adjusting the pilot gas flow adjustment screw on the gas valve. The issue of too sharp a pilot flame would be indicated many times by the tip of the thermocouple being welded or burned off. This will cause a breakdown in the millivolt production of the thermocouple. A poor combustion issue in the combustion chamber can cause the lead on the thermocouple to become burned and scorched, causing the thermocouple to break down. Excessive draft can cause the pilot flame to be pulled away from the thermocouple and would show up during the closed circuit reading with a full load (the burner running) as a gradual drop off in millivolts during the closed circuit analysis. Let’s take a look at Chart 1 and Chart 2 in Figure 1. The Chart 1 drop out range is different than Chart 2 due to the added resistance of the ECO into the circuit. The Chart 1 range is .75 to 6 millivolts—anything in excess of six millivolts means a replacement of the magnet assembly, again this is actually a gas valve replacement. The drop out range on Chart 2 is 1.5 to 11.5. As a rule of thumb, I like to see a spread of 10 or more millivolts between the drop out reading and the closed circuit reading. A normal set of readings is shown in Figure 2 Chart 1. Each line is one millivolt. We are using Chart 1 because the readings shown are for a non-ECO type system. It is important to understand that most heating systems do not have an ECO. They may, however, have a blocked vent switch installed in the system. That switch could be tied into the millivolt circuit and would therefore function like an ECO, adding the resistance of the blocked vent switch into the milllivolt circuit and giving a three to five millivolt drop across the blocked vent switch. In that case, Chart 2 would be used to chart out the readings. The reading on Chart 1 from Figure 3 above would be a typically good set of readings on a non-ECO type of system. In Figure 3, you need to replace the thermocouple; it is breaking down and the magnet will probably not hold. You would have been called for a pilot outage and the customer has attempted to light the pilot but it will not stay lit when the button is released. This could have been caused by the pilot being too sharp and welding the tip of the thermocouple, so you need to soften the pilot up a bit. The tip should glow a dull red, not a “cherry red.” This also could have been caused by the main burner flames hitting the cold junction. This is a combustion issue and may require some combustion testing and adjustment of the gas and air for better overall combustion operation. An indicator would be the thermocouple lead being Figure 2. Testing a 30 mV circuit Chart 1 Figure 3. Testing a 30 mV circuit Chart 1 ICM/January/February 2017 23
ICM January-February 2017
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