Fan runs constantly in "Auto" but not at all in "On" on Thermostat?

[stoneattic]

Does anyone have an idea as to why the fan on my heater/AC would run constantly when the thermostat is set to have the fan in "Auto" mode but not run at all when in "On" mode?

The whole story:

My fan was running all the time regardless of thermostat setting, including heat and AC off. I disconnected the green wire from the thermostat and it kept running. From everything I read that indicated a stuck fan relay. When I looked at the fan really it was cooked, burned plastic, etc. So I replaced it with the same model. I'm pretty sure I wired it exactly the same, I took notes, etc. So now I got the weird situation described above. I picked up a new thermostat thinking that maybe the burned relay took out the thermostat but it does the same thing. That makes me believe I did screw up the wiring but after much reading and checking and rechecking the wiring looks right. Does any know what could cause this?

thanks

[mm]

Well, it would have been best if you had figured out what ruined the first relay. I don't know much about how these things are wired these days, but it's very very unlikely the relay ruined itself.

I'll bet if you check the new relay, its contract points are fused too.

What would ruin a relay? Maybe it's a 24 volt relay but somehow 110 volts it getting to the coil?

[RBM]

That would fry the coil, not the contacts

[stoneattic]

Thanks for the reply.

I don't think the new relay is fused because if the relay were fused wouldn't you expect it to run in both "auto" and "on" like it did before?

I checked and the relay is rated for 13.8FL @ 120 and 82.8LR @ 120. The 24v side of it is hard wired so I can't screw that part up. :)

[RBM]

Depending upon the make and model of the unit, something other than the thermostat is sending power to the fan relay. Some units use a delay timer, which could be faulty, or shunted wires at the blower could cause it

[Tony Hwang]

If 'stat fan control does not have an effect, logic control board in the furnace went bad. Try to measure 24V on that relay primary(coil) if there is 24V all the time with G lead removed at the furnace end not at the 'stat to rule out shorted wire. then, well?

[Zyp]

You're right. You messed up on the wiring. Check the units blower door for the mfg.'s wiring diagram, and compare notes. I think you'll find you've mis-wired the relay. And BTW, if you're not careful, you will let ALL THE FACTORY SMOKE out of the motor and it won't run at all.

[stoneattic]

I'm pretty sure something is not wired right, despite my best effort. There is no wiring diagram in the blower units door unfortunately. Assuming I have the thermostat connected correctly...

What I've got is.. power coming in (white and black and bare "ground") red and black on the N/O side of the fan relay, a black and a white on the coil of the relay a blue and a white going to the oil burner motor a red, an orange, and a black going to the fan limit control

any ideas?

[RBM]

The colors mean little. You need a diagram. Your fan/limit control may be defective

[stoneattic]

If I had a diagram I'd be all set :) There in lies the problem. There's no diagram to be found. I'm thinking that the fan limit control may be bad as well, but until I get it wired correctly I won't know.

[Bubba]

I hope you are not holding your breath while all this is going on. Intelligent people know when to say when. I think you pass that point quite a while back. Certainly your time is worth something? Its time to break open the piggy bank and get it done right, get it done quickly and get it done with a warranty. Bubba

[RBM]

If you can identify the make and model, the manufacturer may have a diagram on their website

[stoneattic]

I figured it out. :) It was a wiring mistake on my end. I just started from scratch. My mistake was trying to figure out the problem with the way it was currently wired. I just disconnected everything and work the wiring out logically.