I2C ADC ADS7830 and Digital Isolator MAX14850 - reading central heating system thermistors
A few pointers: -


The ADC has an internal reference voltage that is quite suitable for reasonably accurate measurements so please consider ditching the external reference unless you have some cunning plan in mind that is not apparent from your question.
Thermistors are just resistors so you need a pull-up resistor to the local supply in order to generate a signal voltage that can be measured by the ADC. 
If you buffer the reference voltage output you can use this as your pull-up point for the pull-up resistors then you gain accuracy in that the measurements are ratiometric.



  They talk about GND and not COM. Not connecting GND to the supply and
  voltage reference? Do i link GND and COM?


The common pin has to be within +/- 0.2 volts of the ground pin - see "Absolute Input Range" on pin 3 and note that "Negative Input" refers to the common pin.

You do need an isolated power supply for the isolated side of the circuit - consider using an isolating dc-dc converter or find an isolator that generates isolated power. ADI's ADuM540x range does this and also provide the isolated IO for the device: -



Be aware that the isolated power than can be provided is limited so, whatever you choose for pull-up resistors be economical in taking current.

My biggest worry is the commoning up of all the thermistors located around your house to the common point on the ADC. There could be a lot of induced voltage due to these components sharing pathways with regular AC power signals. It's something you need to consider and consider it strongly.