Someone had installed a Hunter Thermostate and had chosed electric heat and didn't work correctly.

I have corrected all the wiring and reinstalled and now the blower wouldn't trun off.

When first connected I put fan on auto and than turned to heat and the blower didn't start blowing till the gas furnace came on and heated the thermostate to sense the gas furnace. I then turned the heater off meaning put the cool/heat switch to off position and the blower didn't stop. Only way to turn off the blower was to go and turn the power switch off.

It is a lennox furnace/blower about 20 years old. How can I trace the fault? Could the thermostate have gone bad or the circuit board gone bad. It seems the circuit board is behind the wiring box and is attached behind the wiring box It seems to be a unit enlosed and wires coming out of it.

Thanks for all the help

Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. Many have RH and RC to allow separate transformers for heating and cooling. With just one red wire, jumper them. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.


To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.

OK, first disconnect the green wire at either the thermostat or the furnace. If the blower still runs, the problem is in the furnace, otherwise the thermostat.

If it is the thermostat, study the directions. There is a sticky on finding manuals for them.

If it is in the furnace look for a stuck relay or a bad limit switch.