I want to install a 24v/110 relay to control the two duct fans that I installed in my ducts. The idea is that the fans will come on when the blower comes on. To test this, I ran an 18/2 thermostat wire, and connected the red wire to the G thermostat terminal(green terminal) in the furnace and the white wire to the C/Common terminal(blue terminal)in the furnace. On the other side of the relay(the load side) I connected a box fan. The problem I’m having is that the box fan runs when the igniter turns on and turns off when the igniter turns off. It does not stay on with the blower being on to cool the furnace and to push the hot air out to the vents. Any suggestions? Thanks!!
PS: The box fan seems to stay on in the fan-only and AC mode, but not in heat mode.
Installing a relay to control duct fans!
hvac
Related Solutions
The cooling test sequence for this board, says to jumper Y & G to R. Make sure the fan comes on, and spins up to the proper speed. Then remove the jumpers, and check the fan off delay.
If the blower does not come on, verify that the COOL terminal is energized. Use a voltmeter between COOL and neutral, and verify 120 volts (with jumpers in place).
If the COOL terminal is energized, check the motor and verify all speeds are working properly.
If the COOL terminal is not energized, the board my be bad.
According to the schematic you've provided, you have a multispeed blower (which is quite common). The speed at which the motor runs, is determined by which wire is energized. Energizing the red wire; for example, will run the motor at low speed. Energizing the black wire, runs the motor at high speed.
Before you disconnected the board, one of the motor wires was connected to the HEAT terminal on the board, and one was connected to the COOL terminal. Looking at the ladder diagram, it looks like the blue (MED) wire was connected to the HEAT terminal, and the black (HI) wire is connected to the COOL terminal.
If you've run the tests described above, and found that the COOL terminal is energized when the jumpers are in place. You can try connecting a different speed wire to the COOL terminal. If the motor spins up with the other wire connected, it means that the original speed is dead or something's wrong between the board and the motor.
It looks like the LO speed red wire is attached to the M1 terminal on the board. I'd swap that to the COOL terminal (just temporarily for the test), and see if the motor spins up when the jumpers are in place.
WARNING: DO NOT run the A/C for long with the motor at a lower speed..
Your high limit switch may be bad, and need to be replaced.
Your furnace may be overheating, and the limit is not reacting. So the fusible link is doing just what it's supposed to.
There are a few things that can cause the furnace to overheat, a dirty filter being the number one cause.
Best Answer
Your G terminal isn't meant for that
On gas furnaces, the G terminal is basically an "override" input to allow the end user to turn the fan on without a call for heat or cooling. As a result, your results with your current fan relay aren't totally surprising; in fact, it shouldn't turn on at all unless you set the fan switch on your thermostat to ON.
But, your goal is still achievable
However, you can still get where you want to go. You'll simply need a different relay with a 120VAC capable coil (a RIBU1C mounted to a junction box will work just fine for this) instead of your current, 24VAC-coiled fan relay. The coil of said relay connects to the EAC-H and EAC-N terminals on your furnace's control board using crimp-on female quick-disconnect terminals, and the contacts switch the fan on and off just like you are doing with your existing relay. Note that you'll need to use mains-rated wiring methods for the connection from the furnace to this relay; a piece of 14-2 AC or MC that uses stranded wire is your best bet, or you can use 3 14AWG THHNs (or 2, in a metal conduit) in your choice of conduit flavors if that's your preference.