[letmetellu]

If the burners turn off while the fan is running could be caused by two things I can think of, one is that the limit is set too low, therefore the limit breaks before the house is warm. Another things is about the fan switch spinning when the burners turn off, Could some outside air be getting into the return air duct, if so this would make the fan cut off and 90 degrees F and then come back on when the burners get the temp back to the temp set on fan on setting. The 90 degree setting is a little low for the fan off setting, I would rather see something like 105 degrees.

[letmetellu]

Could it be possible that some outside air is getting into the return air duct, this would make the fan switch drop very fast making the fan cycle on and off at the one minute rate you mentioned.
The 90 degree fan off is a little low I would rather see something like 105 degrees.

[L_to_the_W]

This was occurring before the thermostat change.

The point is... the burners turn off exactly when the 'fan on' switches on, regardless of the limit set points. They turn off cleanly and abruptly, as if the limit had been tripped... but the limit is not tripping. Every once in a while, the burners will actually stay lit for a complete cycle.

I am now thinking the gas valve must be faulting. Any thoughts on that?

[KC13]

Put a voltmeter across the high limit switch, run furnace, and see if voltage changes when blower starts.

[L_to_the_W]

Good idea KC... why didn't I think of that. :)

Won't get to it until tomorrow... but I'll let you know what happens.

[KC13]

Believe it or not, (believe it), I have replaced many a bad part with... a bad part. New, but bad. Most common? Sequencers... and combination fan/limit controls.

[MarkwithaK]

I highly doubt that your valve is the cause of your problem.

[L_to_the_W]

Mark - I came to the same conclusion after I thought about it some more.

Anyway... I tested the limit side of the fan/limit control with a voltmeter to make sure it wasn't closing when the fan side closed. And this was confirmed... it's not closing. So, it is working as it should.

The only thing left that I can think to replace is the transformer/relay (White-Rodgers 90-112), which I should be able to do tonight if the part arrives as scheduled. I'll post an update.

[KC13]

I reviewed your whole thread, and a very unusual possibility comes to mind. Check the blower compartment for a current-sensing relay clamped to the heating speed blower motor lead. If one is present and the load is shorted, it could overload the transformer and deprive the gas valve of power. Unusual, but it explains why the burners go out as soon as the blower starts.

[L_to_the_W]

Thanks KC, I'll give that a look as well.
What would the current-sensing relay look like (size,shape,etc.)?

[KC13]

Typically, looks like a rectangular box, two wires leaving, and clamped to one of the blower leads. Usually found in the blower compartment, but could be anywhere along the length of the lead wire. This is a bit of a reach, but it does fit the "profile".

[hvac1000]

Thanks KC, I'll give that a look as well.
What would the current-sensing relay look like (size,shape,etc.)?

Here you go

[L_to_the_W]

Ok... so after some digging around and closer examination, I have discovered that my furnace has an auxiliary limit control (150 deg. Open). I didn't notice it before because I have little documentation for the unit, and it was tucked behind the blower in the blower compartment.

Anyway... can someone tell me what this aux. limit is doing? Does 150 deg. Open mean that it is turning the gas off if it senses >150 deg. And if so, how do I coordinate that with the fan/limit control (assuming the aux. limit is functional)?

*EDIT*
I forgot to mention that I didn't see anything resembling a current sensing relay. I also replaced the transformer/relay assembly. But the problem persists.