Furnace won't turn on. Power to thermostat?

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Old 12-22-07, 12:03 AM
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Furnace won't turn on. Power to thermostat?

There are two wires coming out of the wall. Shouldn't they have 24 volts? I can't get any reading at all with a voltage meter.
 
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Old 12-22-07, 04:41 AM
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Yes, you should be reading 24 volts.

Also make sure the power switch near the furnace is on, and breaker/fuse in the main box is OK.

Go back to the furnace, and take a reading at the transformer.

Should read 24 volts.. Nothing, then read other side of transformer, 120volts.
 
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Old 12-22-07, 08:44 AM
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OK, Thanks, I'll do that.

Why does there seem to be a black wire and a white wire coming out of one side of the transformer and a black and a white wire coming out of the other side of the transformer?
 
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Old 12-22-07, 09:07 AM
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I stand corrected. One one side there is a YELLOW wire and a black wire. On the other side, there is a white wire and a black wire. They all appear to be the same size. Which would be the input and which would be the output?
 
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Old 12-22-07, 09:11 AM
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One side of transformer should read around 24 volts. (Guessing yellow wire is low voltage)

Other side is your main power feeding the transformer, 120volts.
 
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Old 12-22-07, 11:28 AM
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Keep in mind that the voltages are all AC.
 
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Old 12-22-07, 03:20 PM
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Well, the output voltages should be 24vdc, right? I can't get any voltage on the output of the transformer. There is good AC voltage going in though.

Sometimes, the flame goes on. But after about a minute, it goes off. The fan never goes on. I'm wondering if there is some reason that the fan won't turn on. Then if something senses that the fan isn't going on, does it switch the flame off?
 
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Old 12-22-07, 04:55 PM
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Originally Posted by gregp1962 View Post
Well, the output voltages should be 24vdc, right?
No, should be 24 volts AC.

There is good AC voltage going in though.
120 volts?
 
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Old 12-22-07, 08:11 PM
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OK, you're right. There is 27v AC coming OUT of the transformer.

Now, why would the flame turn on occasionally, then turn off without turing the fan on? Should I try to connect the fan directly to see if it works?

The fan has four wires. Black, White, Yellow, Brown.

The black is connected to terminal #3 on what I think is the fan relay. (Not sure if that is really what it is)

The white is connected directly to the white wire from the power supply.

The yellow is not connected to anything.

the brown connects to a black wire that leads up higher on the furance to another switch box looking thing near the gas valve.

How would I connect the fan directly to power to see if the fan is ok? If I just connect the black fan wire to the black power supply and white to white, will that test the fan?
 

Last edited by gregp1962; 12-22-07 at 08:43 PM.
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Old 12-23-07, 05:52 AM
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Ssounds like your burner is going out on high limit.

Do you have a fan switch on the t-stat? Turn the fan switch to ON, and see if the fan comes on.

If the fan comes on, can be a couple of things.

-Bad fan control (Tell the fan to come on when the heat exchanger is warm)
-Bad winding that the power is hooked up for that speed.
 
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Old 12-23-07, 12:47 PM
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Nope, the fan won't go on by turning the t-stat fan switch to on.

So, when it is all connected normally, the flame goes on for about a minute then goes off before the fan is able to turn on. When I bypass the relay and connect the black fan wire to power, the fan stays on all the time. The flame cycles on and off about every five minutes while the fan is running.

Does this bevavior tell us anything? Should the burner be set to allow more heat before turning the flame on. Or should it turn the fan on sooner than it does?
 
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Old 12-23-07, 03:31 PM
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Originally Posted by gregp1962 View Post
Nope, the fan won't go on by turning the t-stat fan switch to on.
That's not right, then. I'd get that figured out first. There shouldn't be much in the circuit to turn the fan on and off from the switch.

Originally Posted by gregp1962 View Post
So, when it is all connected normally, the flame goes on for about a minute then goes off before the fan is able to turn on.
You don't, by chance, have an older Janitrol furnace, do you? It sounds like what mine does when the pilot light / flame sensor isn't working right. The pilot gets ignited ok, but the flame sensor shuts the furnace down soon after the main gas solenoid opens.

Originally Posted by gregp1962 View Post
When I bypass the relay and connect the black fan wire to power, the fan stays on all the time.
If this fan also works for A/C, are you running it at the same speed as your heat would normally use? A/C fan speed is different from heat fan speed.

Originally Posted by gregp1962 View Post
Does this bevavior tell us anything? Should the burner be set to allow more heat before turning the flame on. Or should it turn the fan on sooner than it does?
You don't say what make/model furnace you have. That might be helpful information here.

The way mine works:

1. The thermostat clicks on
2. The electronics in the furnace wake up
3. The pilot gas solenoid valve opens
4. The electric igniter starts sparking, and keeps sparking until the flame sensor confirms that the pilot is lit.
5. The main gas solenoid valve opens, lighting the burners
6. Once the temp sensor reaches the set temperature, the fan kicks on to start moving air through the furnace
7. Thermostat clicks off - target temperature reached
8. The gas valve solenoids shut off
9. After the temp sensor reaches the low set point, the fan motor turns off

As I mentioned, mine is an older Janitrol. Yours probably works similarly, but may be different.
 
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Old 12-23-07, 04:21 PM
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Mine works about the same as yours. But, mine has a pilot rather than an electronic ignition.

I'm pretty sure that the problem is that it is shutting off the flame before it reached the proper temp.

So, the question is is there a sensor that shuts off the flame before it reaches a certain temp? If so I think that is the problem. Where is it?
 
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Old 12-23-07, 05:27 PM
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Originally Posted by gregp1962 View Post
So, the question is is there a sensor that shuts off the flame before it reaches a certain temp? If so I think that is the problem. Where is it?
What's the furnace make and model?

Is there a flame sensor? Probably, yes. What's it look like and where is it? Don't know, but somebody here probably does if they know what furnace you have.
 
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Old 12-23-07, 06:06 PM
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LENNOX built in 1979.
 
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Old 12-23-07, 06:24 PM
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Originally Posted by gregp1962 View Post
So, the question is is there a sensor that shuts off the flame before it reaches a certain temp? If so I think that is the problem. Where is it?


The problem is that modern furnaces are relatively complex, and a 1979 furnace can have a number of things that would cause the symptoms you describe.


It sounds to me like you are going to need a good repairman to sort out the actual problem from all the possible ones.
 
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Old 12-23-07, 07:42 PM
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This description will allow anyone familiar with this design to diagnose the problem down to one, or at the most, two possibilities......


The flame goes on. Then goes off after about a minute before the fan ever gets a chance to turn on.

Bypassing the relay to the fan, connecting the black fan wire directly to the power supply turns on both the flame and fan. While the fan is on, the flame sometimes cycles off every five or ten minutes.

1979 LENNOX FURNACE
 
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Old 12-23-07, 08:17 PM
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[QUOTE=gregp1962;1281819]This description will allow anyone familiar with this design to diagnose the problem down to one, or at the most, two possibilities......


Interesting you can reach that conclusion when you didn't realize that the furnace voltages were typicall AC rather than the DC you thought they were.


I repeat the comment I made earlier.


Perhaps someone will come up with an answer for you. I'm willing to be surprised.
 
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Old 12-31-07, 01:55 AM
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Thanks all that posted info for this.. It helped me figured out why our 1958 Lennox GH5 stop working tonight.
It's hummming along once again.
 
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