AC Question For Doug

There are some RVP systems that utilize 8 volts to the tstat from the upper control board and THAT is normal. BUT only for the older mechanical wall tstats and not for more current LCD display. What I would do in your case is remove the Tstat and add 2 wires from the POS and NEG and connect to a Volt meter and see if the voltage drops from 12 volts when you experiance the DIMMING. IF NOT, then I would say the tstat is bad. If it does drop you have a bad ground or POS connection somewhere. Even the minimal milliamp draw of the tstat LCD causing a volt draw would be amazing to cause a bad connection to cause the volt drop. But, I guess it can happen. Maybe your GROUND to the tstat is NOT a real ground. Another way, is to run a 2 wire connection from your 12 volt fuse center and connect 12 volt POS and NEG at the Fuse center and run to the tstat to verify a GOOD voltage connection. You stated YOU did not run the tstat wires. Doug