Lennox Mod. GS18Q3 / 4 - 100 has No fan, no burner, no AC...

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Old 01-05-12, 12:25 PM
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Lennox Mod. GS18Q3 / 4 - 100 has No fan, no burner, no AC...

General description: Nothing works, AC, burner, fan, but the pilot light is on. I have tested to the unit itself.

I have power to the unit via a light switch and screw in fuse. I have power to one side of the fan door switch but not the other; therefore, no 120VAC to the transformer and no 24 VAC.

Here is what bothers me: the fan does not work with the door closed but when I open the door the fan comes on. This seems backwards to me unless it’s setup that way for testing.

Let’s start here before I continue.

I greatly appreciate any assistance.

Thanks Terry
 
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Old 01-05-12, 12:31 PM
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It's clear you have 120 VAC to the furnace because the fan will operate.

Very likely, something in the 24 VAC low voltage circuit is preventing the furnace from turning on. You will need a multimeter to measure that voltage and you need to understand how to use it.

Start by measuring the voltage from the C to the W terminal. When the thermostat is turned down, you should find zero volts and 24 VAC or thereabouts when the thermostat is calling for heat.


Then you need to trace that low voltage circuit through the limit switch(es) and other parts until you get to the gas valve.

Post the results of your tests.
 
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Old 01-05-12, 12:51 PM
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Thanks for your fast responce.

Thanks for your fast response.

Sorry I do not have a C. I have the following: W, Y, G, & R/Rc. R & Rc are jumped. I have checked the voltage at R-G, and R-W with thermostat down and calling for heat all read 0 volts.

I disconnected the thermostat and then checked the same at the unit, still 0 volts. I check for 120VAC at the transformer which still checked 0 volts. The lines to the fan appear to bypass the transformer. When I open the fan door it closes the switch and turns the fan on. I have pictures if that would help.

Thanks
 
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Old 01-05-12, 02:28 PM
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Pictures are always good.

Well, you need 120 VAC across the primary of the transformer to operate the furnace. Were you checking for that when the power switch was on and the fan compartment door switch closed?

Since there is no C terminal, use the sheet metal on the furnace as ground.
 
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Old 01-05-12, 04:12 PM
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I will reconnect the thermostat and check tonight when I get home. One leg of the transformer is hot the other is not and that is the one that goes throught the door switch. I will try to attach the pictures. Let me know if they come through. I used Imageshack to upload the pictures.

Is there is any thing else you would like me to check if no on voltage above? If the thermostat is disconnected from the 4 wires shouldn't there always be 24VAC at the R terminal on the unit?

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Thanks
 
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Old 01-05-12, 04:48 PM
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The transformer should have two or more wires for the primary, and two of those should be seeing 120VAC when the power is on and the door switch depressed.

The other side of the transformer are the secondary, which should have two or more wires, with two of them seeing 24 VAC across them when the power is on and the door switch depressed.


Yes, there should be 24 VAC at R when the power switch is on and the door switch depressed.
 
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Old 01-05-12, 05:32 PM
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Thanks, I will post my findings in the morning.

Thanks again for you assistance.
 
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Old 01-06-12, 09:42 AM
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Good Morning

Sorry the format is not quite the same as Word.

I performed 4 tests as follows with the thermostat connected to the unit via the 4 wires:

1. Thermostat off & unit fan door closed.
2. Thermostat off & unit fan door open.
3. Thermostat fan to on & unit fan door (closed & open).
4. Thermostat fan and burner to on with unit fan door open. *see below.

1.
R – B/C ≈ 6 VAC
R – G ≈ 6 VAC
R – W ≈ 6 VAC
B/C – G ≈ 6 VAC
G – W ≈ 6 VAC

2.
R – B/C ≈ 0 VAC
R – G ≈ 0 VAC
R – W ≈ 0 VAC
B/C – G ≈ 0 VAC
G – W ≈ 0 VAC

3.Same as above

4.
R – B/C ≈ 29 VAC (transformer working)
R – G ≈ .3 VAC
R – W ≈ 29 VAC
B/C – G ≈ 29 VAC
G – W ≈ 29 VAC

*The burner ignites and the fan comes on only when the fan door is opened allowing 120VAC to the transformer. There is no delay between the burner igniting and the fan coming on.

Regardless of the thermostat setting the follow is always true:
1. With the fan door closed the fan is off.
2. With the fan door open the fan turns on.

Please forgive my limited knowledge but the fan door switch seems to be backwards. Well only to the point of allowing 120VAC to the transformer.

There is a wire coming from the relay to the other side of the fan door switch which goes to the transformer. Assuming the switch is operating correctly could it be the relay?

Thanks
 
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Old 01-09-12, 11:36 AM
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Fixed

Thanks for your assistance.

I found the schematics for the unit which answered most of my questions.

I had to replace the door disconnect switch which was testing closed when it was a normally open switch. I had to replace the relay because the 24V side was bad, and the thermostat was not working properly.

What are the odds of all three going out? I guess one failed and set off a chain reaction.

Thanks again.
 
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