I am going to be installing the Z-wave thermostat and see that it needs a common "C". My present system is just the 4 wire hookup. I do have the blue wire already run just not used at this moment. On the HVAC Control board where the thermostat wires connect is a "C" and in the manule it shows it running to the AC unit. I also see right behind that there is a post connector with "COM" and under that is one listed as "24V" and thoes are running to a transformer. Where should I connect the Blue wire too on the control Board? The "C" on the board or to the 24v before the transformer? The Board is a Nordyne 624631-a if that helps.

Thanks in avance for all your help.

I just rather ask before hooking up an brakeing it due to hooking it up wrong.

If it also helps the thermostat that I am installing is an tz43 from RCS.

Connect the blue to the c on board were it shows wire to ac unit.

Should end up looking like this... red to r, green to g, white from furnace to w, yellow to y( and red from outside unit to y if you have central air) Blue to c( and white from outside unit to c if central air) Hope this helps and please let me know how it turns out.

That's what I was thinking but wanted to ask before doing it. The Thermostat will be here on Monday. I will try it right away on Monday.