Furnace keeps shutting off... HELP?

[Chilly Willy]

My furnace is a 1990 model manufactured by Inter-City Products in Lavergne, TN. It's model #NUGE050CG02, with a newer (8 years old?) Honeywell digital thermostat.
It acts normal when I turn it on - inducer fan comes on, gas comes on, igniter lights, burners burn, but this only lasts for two or three minutes, then everything shuts off, including power to the thermostat. I can only get it back on by unplugging the furnace for a few minutes, then after plugging it back in it goes through the same short cycle again then shuts down.

Things I've done so -
1. Checked the flue for obstructions - all clear.
2. Put on a new ground wire for the control board (Robert Shaw brand without indicator lights).
3. Changed all the filters.
4. Put on a new vacuum line from pressure switch to inducer fan.
5. Verified that I'm getting 24 volts to my pressure switch.
6. Cleaned the flame sensor with steel-wool.

What now?

[Chilly Willy]

theduke03 -
> Is the blower not running? Cycle heat and check for 120v to the blower motor.
If you mean the blower motor to the flue, with the vacuum line going to the pressure switch, yes, it runs fine.

[Freon]

At the thermostat set the FAN to ON (not auto). Do you hear and feel air circulating in the house (out from supply registers, in to the returns)? If yes, now call for heat (leave the FAN set to ON) and see if the furnace behaves badly again.

"but this only lasts for two or three minutes". This is long enough so I doubt it's a flame sensing issue. It's also long enough for a high limit safety to shut the furnace down if it is a bad sensor(assuming it happens after the test above is tried). You may have the older dial limit switch and can read the temperature from the dial. It will be in a small, 3"x5" metal box mounted on the front at the heat exchanger. The probe of this safety extends into the heat exchanger. You could jumper (if it's not a dial high limit design) the high limit safety TEMPORARILY to see if things change. You could also TEMPORARILY jumper the draft inducer pressure switch to be sure it's not tripping as the inducer assembly warms up.

You have a hard lockout and could read what situations cause a hard lockout.

[Chilly Willy]

WOW! Thanks Freon, we're makin' heat again! I set thermostat to ON and now it's been running for about ten minutes. And yes, I do have the old dial type limit switch with four heavy gauge wires coming out (two red, one tan, one white). So, you think it's just a bad safety switch/probe, or might I still have a larger problem?

[Freon]

Carefully remove the cover to the limit switch. You should only have to squeeze it and gently pull it towards you. There will be pointers set at different temperatures. One will be the Fan-on (middle setting), the lowest setting will be Fan-off temperature setting and the highest pointer will indicate the High limit setting. With the cover removed you need to watch this dial as it moves. As it moves past the middle pointer, the main blower (circulates air in the house) should start to run. Once the main blower starts to run the dial should only rotate a little more toward the highest pointer (high limit) and then stop rotating as the flow of air through the heat exchanger stabilizes the temperature in the heat exchanger. Let us know what happens especially if the main blower does not start as it should and the dial keeps rotating to the high limit setting.

[Chilly Willy]

Do I start this process by setting the fan to AUTO and the thermostat to HEAT?

[Freon]

D0 not setting the FAN-ON at the thermostat. Leave it at AUTO. You want to see how the dial moves in a normal heating cycle.

[Chilly Willy]

Ok, with fan on AUTO and system on HEAT, everything comes on as it's supposed to. The dial slowly moves clockwise past the first pointer and when it gets to the second pointer, the fan comes on and the dial begins moving counter-clockwise and stops just above the lower pointer. The furnace then stays on until it reaches the temperature on the thermostat, then shuts off. At least it's not turning off all power like it used to. Also, I notice when the fan is running on AUTO and I switch it over to ON, the fan comes on even stronger.

[Freon]

You describe a perfectly working blower switch. That's a good diagnosis... great work. As long as that dial stays between the middle (blower motor start) and first pointer (Blower motor shutoff), your furnace should work fine

The FAN-ON setting will have the blower motor use the same speed tap as the AC which could be set to a higher speed than heating speed. The FAN-AUTO when in heating mode will use the blower motor speed set for heating and it's different, as you noticed.

If you can, observe the furnace operation for a few cycles making sure the dial never gets to the highest pointer, the high temperature cutoff. Out of curiosity, what it the high temperature cutoff set at?

[Chilly Willy]

The third pointer is at the end, right at 250, but there's also a little square tab at 200. For some reason, running it in the ON position, then running it through the AUTO/HEAT cycles, everything seems to be back to normal. Don't know why, but I'm real happy about it and my wife is happier yet! THANK YOU FREON!!!! One little thing that seems to have come back though, is a problem I had real bad before I did all the stuff I mentioned before. The ignitor and gas valve rapidly turn on and off now and then. It used to be almost constant, now it's just once in a while.