Heil Furnace, erratic operation


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Old 01-15-05, 05:55 AM
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Heil Furnace, erratic operation

I have a 12 year old Heil model NGUK075DG07, 75k BTU furnace. The problem is erratic operation. With the call for heat, the system relay will chatter and the pilot and main burners will fire erratically for 2 to 10 cycles sometimes with a very low height main burner flame. Over time, this apparently caused the ignition control module to fail and it was replaced. The system relay was arcing badly inside the clear plastic case so it was also replaced. Normal operation for about a week. Same problems again and I found that there were erratic voltage readings (4 to 15 volts) from the valve terminal on the fan control module leading to the gas control valve. (Honeywell) The fan control module was replaced and it didnít solve the problem. Same low voltage readings from the valve terminal. When I jumper 24 volts directly to the gas control valve terminal, everything seems to work normally. My logic was that the low voltages were not enough to reliably operate the gas control valve. Iíve just now noticed that when the main burner is burning normally, the pilot flame is still burning, the ignition cycle is still trying to ignite the pilot, and the system relay is chattering. Any thoughts? Canít put up with the cold and afraid that this condition will cause controllers and relays to fail again.
 
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Old 01-15-05, 08:32 AM
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You need to folow the 4-15 volt problem to it's source and that will be your problem.Just put one lead on a known ground and use the other lead to follow the low voltage wires untill you find the drop. I haven't ran into it with he Heil as much but the solder joints on the back of the control boards will occasionally crack and cause erratic voltages due to bad connections.
 
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Old 01-15-05, 09:56 PM
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Thanks for the reply. I probably wasn't clear enough in the original post. Every place in all the circuits has 24 to 26 volts. The fan control has 26 volts going in and only the eratic 4 to 15 volts coming out at the valve terminal. I've tried a pulled fan control module from a working furnace, a new replacement, and of course the original one on my furnace. The fan control module is the only place where the voltage drops. Am I safe in assuming that the gas control valve needs reliable and steady voltage in the 22 to 26 volt range. Can't figure why three modules would all produce the same arratic voltage results.
 
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Old 01-16-05, 07:23 AM
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Yes, you need at least 24v to the gas vlave for it to work properly. With the power off remove the wire from the terminal. Turn it back on and check voltage again and see if you still have the erratic reading. If you do and you are sure have 24v going into it then the module is bad. If you have good voltage, reattach the wire and check voltage at the gas valve if it's erratic, make a new wire with new conections and see if that fixes it.
 
 

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