As I promised, since the world did not end on 12/21/12 with the end of the Mayan calendar, here is my first post of the NEW YEAR.
Simple Scenario —– Here it is the middle of the night — it’s 10 degrees outside and it’s a weekend and you are on a “no heat call” and find the ECM motor module has failed. As you know from my previous posts on ECM motors, you need to get a correctly programmed motor for this unit and you don’t have it on your truck. What do you do? You need to keep the house from freezing (and your customer too).
Believe it or not, you can put in a PSC motor temporarily to give your customer heat until you get a new correctly programmed ECM motor from your supplier. (You can keep a “used” 1/2 HP motor in your truck stock just for this purpose). You need to pull the ECM motor out of the unit and unplug the 5 pin power plug and 16 pin control harness. Don’t cut any wires because you’ll need these when you put the correct ECM motor back into the unit once you get it from your supplier.
Mount the PSC motor into the cradle and attach the blower wheel. Now, you have one of 2 choices to make — do I run the fan continuous or do I create an intermittent fan operation to function like a standard furnace?
The first option is the easiest to accomplish. Simply run the power wires from the motor back to the power coming into the furnace. The fan will run 24/7 and the heat will cycle off and on from the thermostat.
But what do you do if your customer objects to having the fan run all the time? They feel it is too drafty or they don’t like to feel to “cold air” coming out of the registers. The easiest solution is to add a simple single pole single throw relay into the power wiring to the motor. Now, depending on the control board in the furnace, you power the coil off the EAC (air cleaner) terminals on the control board. Most control board use a 110 VAC power off these terminals but some may use 24 VAC so you need to check the output from these terminals. If these terminals are 110 VAC, you wire the coil of a 110 VAC relay to these terminal. DO NOT wire the motor directly to these terminal since most of these will only handle 1 amp and the motor draws a lot more than that. Use the relay as an isolating relay to protect the board (or you will be replacing the board on a “call back” the same night).
Since the EAC terminals are powered any time there is a call for either heat, cool, or continuous fan operation, the relay is energized with the call and de-energized when the call ends giving the home owner an intermittent fan operation. Here is a simple wiring diagram for accomplishing this:
The motor will now cycle with any call from the thermostat or control board.
Now, when you come back with the new ECM motor, you disconnect the relay, remove the PSC motor, and re-mount the ECM motor in the cradle and attach the blower wheel. Since you just un-plugged all the harness, now you just need to plug all the harnesses back into the new motor and you’re back in business and you look like a hero to your customer.
Keep in mind that this also works for cooling in the summer!