Gas valve to main burn burner wont come on.


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Old 08-30-08, 04:50 PM
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Unhappy Gas valve to main burn burner wont come on.

My dad has a hot air furnace with a Robertshaw SP715A Ignition control Module and a Robertshaw 7100 DER Gas Valve. When the thermostat calls for heat the igniter clicks, the gas valve opens the pilot gas on the gas valve, the pilot lights, but the main burner will not light. There is no gas comming out of the gas valve to the burner. Can anyone give me step by step instructions on how to troubleshoot this, so I can get the right parts to fix it. There is no click to the gas valve. so I assume that the flame sensor is is not being sensed by the module, or I have a bad wire, a bad module, or a bad gas valve. Please HELP! I think the Furnace is a Century brand, it is about 20 years old.
 
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Old 08-30-08, 06:33 PM
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Step By Step

Sorry Jonnie but I can't give you step by step but can tell you a few things to check. If you have a multimeter & know how to use it, you can check for power going to the gas valve by putting the probes on the two wires connected to the valve. You are looking for aprox. 24 volts AC. You can also disconnect the wires & check for continuity by putting the probes on both terminals where the wires were connected. No continuity (open circuit) tells you the gas valve is bad. If you get both continuity & voltage but the valve still does not click, it is probably stuck & needs to be replaced. If the appliance has a flame sensing rod (usually located on the opposite end of the burner rack from the pilot) remove & clean it using a scotch brite pad or fine steel wool.
 
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Old 08-30-08, 06:57 PM
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There are 3 wires going to the gas valve. I think 2 go to the piolet and the other to the main burner. I have 24 volts to the first 2 but no voltage to the other.
 
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Old 08-30-08, 07:26 PM
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Some furnaces (Carrier and Payne at least) had a bimetal switch integral with the pilot burner. This pilot burner has three wires (in addition to the high-tension spark ignition) that are used in controlling the main gas valve. It requires the pilot to be of sufficient size to "warp" this bimetal switch first cutting off the ignition spark and then closing the power to the main gas valve. If the spark does not stop or the pilot flame itself is weak it will not allow the switch to make the main valve contact.

If the pilot flame is weak then you need to clean the orifice. The switch itself could be faulty.

A twenty-year-old furnace may have other problems including a leaking heat exchanger. It may be a good idea to have a qualified service person look this unit over carefully.
 
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Old 08-30-08, 07:27 PM
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Terminal Markings?

Could you please tell me how each terminal is marked & it's voltage reading? The pilot stays on after it lights?
 
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Old 08-30-08, 09:39 PM
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Originally Posted by furd View Post
Some furnaces (Carrier and Payne at least) had a bimetal switch integral with the pilot burner. This pilot burner has three wires (in addition to the high-tension spark ignition) that are used in controlling the main gas valve. It requires the pilot to be of sufficient size to "warp" this bimetal switch first cutting off the ignition spark and then closing the power to the main gas valve. If the spark does not stop or the pilot flame itself is weak it will not allow the switch to make the main valve contact.

If the pilot flame is weak then you need to clean the orifice. The switch itself could be faulty.

A twenty-year-old furnace may have other problems including a leaking heat exchanger. It may be a good idea to have a qualified service person look this unit over carefully.

It won't be a DNP product here...He said it had 3 wire valve. Sounds like it may be a Rheem with those controls?? First clean the pilot like furd said. Sounds like may be a rectification problem.
 
 

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