Wawa:
Did you measure the 0-10volt signal input with a DMM.
Could be that 10volt is already on the input/ground of the device when nothing is connected to it.0-10volt has been covered before.
PWM to 0-10V standar signal - General Electronics - Arduino Forum
Leo..
what complicates his project is the floating AC.
he either needs to ground one leg of the secondary. it should be marked + and - even though it is AC.
or allow the ENTIRE control system to float.
Much of HVAC lives in 24VAC. being low voltage it does not have to be in conduit in commercial facilities like anything over about 48 volts does.
most of the european manufacturers that sell in to the US have sheets on how to work with 24VAC and control circuits.
http://www.kmccontrols.com/images/agiods_files/downloads/AN0604D-RevB.pdf