Student Handout Packet – Page A7 GAS CONTROLS WORKSHEET
1. What is a glow coil? It is a device that when it receives 2.5 v it glows ‘red hot’ and when placed in the vicinity of gas, it will ignite the gas. It is used on automatic ignition systems with an automatic BASO switch, standing pilot and main burner only shut-off. A thermocouple is used as the flame proving device on this type of system.
2. What is a spark ignitor? It is a device that when it receives a very high voltage (10,000 +) it will create a spark by jumping a gap. When placed in the vicinity of gas, it will ignite the gas. It is used on automatic ignition systems with an automatic BASO switch, standing pilot and main burner only shut-off. A ‘Pilot Heat Switch’ is used as the flame proving device on this type of system.
3. Draw the thermocouple and pilot flame? Please refer to page 735 in your textbook. The flame should ‘impinge’ or strike 3/8 to 1/2 inch of the tip of the thermocouple, thus the term, hot junction.
4. What is a BASO switch? A BASO switch is a relay which has three parts: coil, contacts and mechanical linkage. It is unique in that its coil is rated at 30mv and is used with a thermocouple.
5. What are the setting(s) for the fan and limit switch? FAN Switch: generally on at 130 degrees and off at 100 degrees, but could be set lower if customer complains about ‘cold’ air blowing at the beginning of the furnace cycle.
5. What are the setting(s) for the fan and limit switch? LIMIT Switch: High Efficiency (90+) gas furnaces is usually 180 degrees, and may even be lower. Mid and Low Efficiency (70% and 80%) gas furnaces is usually 200 degrees. Always use what the manufacturer recommends.
6. What kind of threads are on a gas valve? Pipe threads as used in the plumbing industry.
7. How do you check for a gas valve? With a match (NOT) Check for leaks with an approved leak detection solution designed for the type of ‘gas’ being tested.
8. How do you test a limit? Disconnect the L2 wire of the fan motor. This will stop the blower from operating, no matter what speed it is on. This will elevate the temperature in the heat exchanger to a temperature that will ‘force’ the limit to open. When it opens, the gas valve will be de-energized and close. If the gas valve does not close, the limit is ‘broken’ closed. Replace it.