Nest’s products track not just a home’s temperature and the presence of smoke but also when people wake up, leave and return home. By incorporating hardware and software and using sensors and algorithms to learn behavior, they program themselves and can be controlled remotely with a smartphone.

Mr. Fadell said in an interview that he was aware of concerns among customers about Google having so much data about people, and that Google agreed that Nest’s privacy policy — which says that Nest will use customer information only to improve its products and services — would remain unchanged. “That was a major concern or question we had, and they have done an amazing job of convincing us that our privacy policies are going to be well-respected in their organization,” he said.

Still, Mr. Sullivan said, “History has shown that privacy policies do change.” He added, “They won’t hand over Nest data to Google, and Google mines it for whatever they want, but there could be incentives or reasons why it might make sense to tie it to a Google account.”

It is easy to see how Google products could be integrated into Nest. For instance, Nest users who log in to Google could theoretically someday see their home’s temperature or an alert about the presence of smoke in Google Now or in gmail, and information about a person’s home life could be used to target ads.

Google and Nest declined to comment on future plans because the deal has not yet closed.

Creating the so-called connected home through products like Nest’s is a major goal for the technology industry, even if it has not yet made its way into the mainstream. Apple, Philips, Bose, Dropcam and Lively, among others, make Internet-connected devices to dim lights, close garage doors, listen to music or keep an eye on family members or pets with sensors or live video. Still, just 1 or 2 percent of people have common connected devices, and while a third say they are interested in connecting their homes, almost half say they are not, according to Forrester, a technology research firm.