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Jeff,

I am about to pull out my old control board and do this (for my bedroom ac unit). I had a couple questions, if you don't mind. I am doing one thing different, I am not changing my thermostat. So if you don't know the answers, I understand.  I checked and the old thermostat sends 12vdc signals to the AC unit.

1. Do you see any reason I can't use 20a contactors that have 12vdc coils controlled from the old bedroom t-stat? (I am assuming the coils can't use much power, and the old thermostat won't have a problem powering the coils on the contactors).
2. I think the bedroom ac unit is limited to only one compressor when the electric water heater switch is on. Did you have to deal with this? It appears this is just controlled from one of the signals sent on the same wires coming from the t-stat.
3. Someone made a comment in an earlier post, and I think it was just a typo. The compressors do NOT have two speeds, correct? The condensor fan (and evap fan too) has two speeds, but not the compressors. The compressors work with either one on, or both on, depending on what is called for from t-stat?

Thank you,
Alan Johnson

Alan,
No problem. 

1- Yes you can absolutely run 12v coil 20amp rated contactors with the existing thermostats. The thermostats already run coils on the contacts on the board so its going to be fine. 

2- So on my coach the switch that turns on the electric water heater, also turns on the relay to shut off the power to the air conditioner, so the thermostat has nothing to do with it. You will not have to worry about this, it will function as normal when you are done. 

3- Correct only single speed compressors, but there are two compressors in the large a/c units. You have a two speed outdoor fan and a two speed indoor fan. You will probably need to wire up the relay like I have it on my diagram to get the outdoor fan to go to high speed when the thermostat calls for the second compressor. The indoor fan will work as it always did, based on the selector switch position on your thermostat.