Day and Night furnace fan control problem


The thermostat is a simple mechanical one with the plastic lever all the way to the left is off and all the way to the right is maximum heat. There are no other adjustments. The problem is when the lever is all the way to the left, the fan kicks on when its cold but the gas for heating does not come on. Heat come on only the lever is move more toward the right. (Its been cold the last few days so the fan has been on constantly.)
Furnace belongs to my MIL and was installed by a contractor about 5 years ago. I suspect the control wires were hooked up wrong. Does this sound reasonable? What are the color codes for the thermostat? Anything else I should check out for her?
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wrote:

Is this a heat pump with gas backup?

There are no real firm wiring color codes. it also depends on what you have.
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<HVACTECH2> wrote in message > wrote:

No, just a natural gas furnace.

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# Fred # wrote:

So don't put the lever all the way to the left.
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