Furnace blower not turning on with thermostat

Old 09-27-13, 12:34 PM
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Furnace blower not turning on with thermostat


I'm having a problem with my gas furnace which I suspect is a minor problem, but I'm having trouble diagnosing it.

My furnace and thermostat both worked fine last winter. Nothing changed over the summer, but this winter the furnace wouldn't kick on. I figured the problem was likely the thermostat, so I checked the settings were configured properly, I replaced the batteries, and then I even replaced the whole thermostat. No change. I also checked that the thermostat was getting power with a voltage tester. It is (and the breaker is switched on, correctly).

What I tried
So, I went down to the furnace. The pilot light is on and burning, and the doors were set in place correctly (I specifically checked the door switch and made sure it was pushed into place even when the doors were off, to be sure). Nothing else looked out of place or damaged.

Next, I adjusted the dial for the fan (area C in the pictures). The blower turned on, and hot air came out in the house as long as I held the dial.

At this point, I figured the problem must be the wiring between the thermostat and the furnace, so I tested that it was getting power.

Testing wires with a voltage tester
The last picture shows what I found with the voltage tester. All the wires in the top (area D) except one red wire have a current. There is no current for the wires in the bottom (area E) except the black wire for the door switch, and no current in any wire in (area F). The last picture shows this more clearly; in that photo, all wires numbered in red have a current, and all wires numbered in green do not.

The tan wire in the top is strange (wire #1). There is a break (with something inserted in the middle) at which point the current stops; area A is carrying a current, area B is not. Maybe this is all normal. I'm not really sure.

Am I correct that the problem is most likely the wiring? Is there any way for me to figure out the wiring issue, replace a wire, etc and have the furnace operational again? It's an old furnace from the 70s, so I figure if I need to call a professional to repair it, that would be a good time to just replace the whole thing.

I saw advice online that I could isolate if the thermostat was a problem by connecting the Rh and W wires manually. I also read that was a really bad idea and could damage my equipment. I'm not sure which is correct.

Lastly, while testing the thermostat I carelessly knocked the labels off the wires. There are only two (1: yellow or brown, 2: black, obviously non-standard colors. A third, red, is unused), and I'm fairly certain I'm using each wire for the correct purpose. However, is there a way I can test to confirm that without fear of damaging my furnace? I'm cautious about simply switching them to see. Should I not worry about that?

Any advice would be appreciated, even if quick or small. I'd love to figure out what's going on so I have a better understanding of the system for the future, and I really appreciate all the work you guys do to help out homeowners like me.
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Thanks so much!

Last edited by jmeasns; 09-27-13 at 01:37 PM. Reason: Add picture, add sections, clarify
Old 09-27-13, 11:56 PM
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Welcome to the forums.

WOW... I had that same exact furnace. It was costing a fortune in gas to heat. That unit is like 40 years old. You should check with your gas provider and see if they have any deals available to upgrade it. It's definitely on borrowed time.

There is a series of tubes inside the burner cavity that the air blows thru. This is called the heat exchanger. With age these tubes rust and wear thin. If they rust thru completely exhaust gases can be drawn in and circulated thru the house. That's not a good thing. At the very least make sure you have a carbon monoxide sensor near the sleeping areas.

On to the repair. There are only two wires on the thermostat. It doesn't matter if the wires are reversed on the terminals. You weren't clear on your diagnostics but I'm assuming that the burner does light on the thermostat's call for heat but that the blower doesn't come on.

You said you turned the silver disc in the blower thermostat and the blower came on. That unit is the burner limit and blower thermostat. It is sticking and needs to be replaced.
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Old 09-28-13, 02:54 PM
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The t-stat only tells the furnace when to come on; blower operation is controlled internally.

If the fan/limit switch has a white plastic peg, push it in so the fan runs continuously if heat is needed for the time being.

One thing to note is that your furnace over the entire heating season is less than 70% efficient and it may not even be safe any more.

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