There are a number of different smart thermostats available on the market but the two most popular ones are the Nest and Hive.
The Nest brand is owned by Google and along with their thermostat there is a range including video doorbells, cameras and sensors.
Both of these thermostats give you access via your smart phone and allow you to set up timers or simply just put the heating up or down.
They can be controlled from outside the house so you can put the heating on whenever you're heading back to the house or if you leave without remembering to turn it off itis simple to be from your mobile.
It also means if you have access to the app you can control the heating for someone else.
Once connected to Alexa it can be as simple as saying, “Alexa set the temperature to 23"
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As always it is advisable to have professional installation