heater shuts off and back on 3 times

[rkfguard]

i have a day & night forced air heater unit 394jaz...for the last few months, when i turn the heater on, it does everything correct and reaches the appropriate temp, then shuts off like it should,then 2 seconds later the fan comes on for 2 seconds and shuts off,,then back on for 1 sec then off...then it stays off....i have a 2 story house with the same unit downstairs...so i switched the control board, no change..then changed the limit switch part 430-1441 stemco...that made no difference.....what else can i check...thanks

[heatseeker]

how old is this * Does it have an ignition control or standing *

[rkfguard]

its 20 years old and has an electronic control module

[Freon]

Check to see if the fan control mechanism isn't in a 3"x5" metal box mounted on the front of the furnace where the heat exchanger is. If you have that type of control, you can easily remove the box cover and you'll see a dial calibrated in degrees Fahrenheit with 3 pointers that slide along the dial. Those pointers are for starting and stopping the blower as well at the upper safety limit. Sometimes the blower will cool the heat exchanger to where the fan control will turn off the blower (the lowest slider setting on the dial). But residual heat in the firebox and heat exchanger can increase the temperature just enough to trigger the blower to start again (middle slider setting). It runs briefly cooling the heat exchanger down to the lower setting (blower off) and that's usually the end. By adjusting the lower setting, fan off, to a slightly lower temperature, you can usually stop that behavior.

[rkfguard]

i have no 3x5 metal box there....the only thing in the heat exchange area is the limit switch and the plastic box control module..which i have ruled out as defective...

[Freon]

"i have a 2 story house with the same unit downstairs...so i switched the control board, no change..then changed the limit switch part 430-1441 stemco...that made no difference.....what else can i check...thanks"

Since you have changed most of the best candidates without success, the last source for blower activation is the thermostat. Can you possibly swap * I know it's a pita but it will save useless waste of money if you can. Please be sure to turn off power to both furnaces. Hopefully the thermostats are the same and you can swap only the head units.

[rkfguard]

ive tried 2 different thermostats, with no sucess....i do have wiring that can change the blower fan speed...i dont know if that will do anything....

[rkfguard]

i do have another control board part number ceso 110018-18..when i looked it up it says its a fan control board...could that be bad and causing my problems

[rkfguard]

i do have another control board part number ceso 110018-18..when i looked it up it says its a fan control board...could that be bad and causing my problems

[Freon]

Since you are having fan issues and have ruled out everything else, the fan control board is a very good candidate. Can you swap it with the one at the other * Remember, always do any work with power-off.