[bbro62]

Originally Posted by where2

You're sure you're getting changes in current checking B and G in different modes? Can you measure resistance and check something...

The troubleshooting (Pg 14) indicated using a resistance jumper 3.9K Ohm between Black (J3-9)and Green (J3-8) to trigger LOW speed fan.

Troubleshooting (Pg14) indicated using a dead short between Black (J3-9) and Green (J3-8) to trigger HIGH speed fan.

Another tech forum indicated other brands of HRV's use different resistances between Black (Ground; J3-9) and Green (Control; J3-8) to switch modes. That would be consistent with the two carrier troubleshooting/installation guides I've been reading.

Early on I thought the black and green wires were the key to a solution. A jumper between the two terminals does initiate a high speed fresh air exchange and jumpering with a 3.9K resistor does initiate a low speed exchange. So I set up a test circuit with relays that either made that connection as a dead short between terminals, or with 3.9 K Ohms between. It works except that it throws the wall controller into an error state. A closer read of the troubleshooting showed that you are supposed to disconnect the wall controller before jumpering the terminals. The error state is only temporary and the controller returned to normal after the relay contacts were open, but still not ideal. I thought maybe I could insert diodes to prevent current from flowing from the relay controlled circuit into the controller. But I didn't have any luck.

A momentary short of J3-4 to J3-5 is the same effect as the 20 minute override switch closure. The override feature runs for 20 minutes after the final moment those two contacts were closed. If you close them for 60 minutes, you'll get 80 minutes of High Speed override.

This was the approach I took with my earlier timer. I built a timing circuit with a 555 IC and decade counter to allow me to select 1 to 6 hour timing intervals. At the end of each interval, a relay would close contacts between J3-4 and J3-5 for 1 second to trigger a 20 minute low speed exchange. It works, but the Nest offers more sophisticated control of time duration and lets you set "quiet hours" when exchanges aren't triggered. That would work well to prevent exchanges during cold Montana nights when heat loss would be greatest.

Wiring: Black = ground; Yellow = +12V (DC); Green = control; Red = ???

I've done a lot of poking around with a multimeter and learned that both the green and red wires are control/signal wires. They both read 5v which I've learned is a typical voltage for serial communication. When a control state is changed on the controller (e.g. starting the fan), voltage fluctuations can be seen on both red and green wires but the pulses are too fast to read accurately with the meter. Again, I've learned this is typical of logic signalling rather than basic on/off switch control that I'm used to.

Now the question is how to determine if the unit is in defrost before making either of these two calls for fan. It's explicit that the unit NOT be in defrost mode when calling for either fan speed.

If you relayed the green wire attached to J3-8, open circuit appears to STOP the fan and potentially energize K5. Closed circuit appears to run low speed fan.

I believe the defrost cycle is initiated by closing the damper while running the fan which recirculates warm interior air across the core to thaw it out. But yes, the trick is to not defeat the defrost cycle.