I recently purchased a Honeywell wifi thermostat and tried to hook it up as a replacement for an existing battery powered unit. I have a typical forced air HVAC setup with gas furnace (no heat pump), and I have replaced several thermostats in the past at other houses on very similar systems and never had any issues.
In this case, when I removed the original battery powered unit, the C wire was tucked behind the wall, so I manually connected it back up to the wall plate and inserted the new thermostat, but it would not power on. Then I tried putting the battery powered unit back in (removing its batteries, the manual says this will work), and still that one would not power up. I got out my volt meter and measured 20 volts across R and C, and 27 volts across R and G. I put the old thermostat back in with the batteries and that unit is still working fine.
Any idea why I am not able to power either thermostat when using the C wire, but I can measure voltage across it?