Pathway Off/Medium/High is set in either the mobile or web-based app.
No, the Nest thermostat is not needed for use with the smoke/CO detectors. It is nice to have though, as the thermostat will automatically shut down the AC when smoke is detected to prevent spreading smoke throughout the house.
To those of you referencing the periodic table as a way to assert your answers, please note that you are wrong when it comes to ANY real world application. Tests have been performed in multiple sealed chambers by the NIH and are available from the US National Library of Medicine as one of the top search results from any engine. For those unwilling or unable to view the results, there was NO LAYERING FOUND. "RESULTS: Contrary to a significant amount of public opinion, CO did not layer on the floor, float at the middle of the chamber, or rise to the top. In each case, the levels of CO equalized throughout the test chamber. It took longer to equalize when CO was infused at the top of the chamber than the bottom, but levels always became identical with time. CONCLUSIONS: As would have been predicted by the Second Law of Thermodynamics, CO infused anywhere within the chamber diffused until it was of equal concentration throughout. Mixing would be even faster in the home environment, with drafts due to motion or temperature. It would be reasonable to place a residential CO alarm at any height within the room."
It works over Wifi and to an app on iPhone.
I have mix of both wired and battery operated units. All work the same through nest app. Very easy
Let's say 10 years up. Can I just replace only the CO detector part or the entire unit needs to be replaced? Is it wroth say to spend on replacing 10 devices distributed around my home, causing me to $1500 every 10 years. This is a poor and expensive design.
Yes. In fact, every night when you turn off the lights, the indicator ring makes a sweep in green to let you know that the battery will last the night.
The wire depends on what size circuit you are tapping off of 20a circuit 12 ga and 15a circuit 14 ga. Use 3 conductor w/gr if you are connecting more than one unit together per the NEC. They also should be on a local lighting circuit, ie bedroom light circuit. This ensures that you know the power is on all the time. There is next to zero load.
Nest Protect comes in both wired and battery powered models.