New 4th Gen Learning Thermostat wont power up to enable install

[SGM1]

I have a dual fuel system, oil boiler for on demand hot water & baseboard heating and forced air for AC. The Common wire is powered from the air handler. With the old thermostat I get 22.5V between C and R terminals and everything works fine. I powered everything off at the main breaker and swap in the Nest connector block - Power back on - no Voltage. 

I then reinstalled the old thermostat - everything works again.

The end result is I can't initiate the setup of the Nest.

Any suggestions welcome, thanks

[SGM1]

Update: if I leave the C wire disconnected the wiring block powers up (spirit level lights up). Is it OK to proceed with install and tape off the C wire?