Apart from these products, Nest also makes Internet-connected security cameras and a doorbell with a camera.
Nest co-founder Tony Fadell said in July 2015 that the data collected by its products would be kept separate from the massive amount of information that Google collects.
But those promises on data privacy have now gone up in smoke with the two teams working together as one. The change came after two years of poor earnings by Nest, a bid by Google to hawk the unit, and poor earnings by Google's parent company Alphabet this month.
Nest chief executive Marwan Fawaz said on the company's blog that the aim was to "supercharge Nest’s mission", which was to “create a more thoughtful home, one that takes care of the people inside it and the world around it".
The BBC asked Google about the integration of the two teams and got the following reply: "Nest users' data will continue to be used for the limited purposes described in our privacy statement like providing, developing, and improving Nest services and products.
"As we develop future plans and future product integrations, we will be transparent with users about the benefits of those integrations, any changes to the handling of data, and the choices available to consumers in connection with those changes."
But the Big Brother Watch campaign group expressed scepticism about the reaction. "Google already harvests an incredible amount of detailed information about millions of Internet users around the globe," director Silkie Carlo told the BBC.
"Now, Google is becoming embedded in the home, through 'smart' soft surveillance products.
"Adding data from Nest's home sensors and security cameras will significantly expand Google's monopoly on personal data. Many customers will be justifiably anxious about Google's growing, centralised trove, especially given that its business model relies on data exploitation."