@Geo126 

Thank you for reaching out,

The Smart Action in the tapo app should be able to meet your needs:

In Tapo App> Smart tab>  click   + to add an Automation,  here is related FAQ that can help 

How to create Smart Action (automation or a shortcut) of my Tapo devices

1.Add an Automation #1

When:

T315 / temperature below 21°，

Then 

P100 plug/ON 

Effective time:  between 17:00 and 21:00

Then you will need to add an Automation #2 to turn plug off when the T315 / temperature is over 22°C.

Feel free to let us know if this works for you and you may share some screenshots if this does not work. 

  @Santimandu 

Hi, 
Currently, the Tapo T315 will trigger the action only when the actual temperature crosses the set temperature, for example, it will only trigger the plug turn on when the temperature drops from a higher one to below 21°. 
In the scenario that the initial temperature is lower than 21°, you can turn on the plug manually to heat the room until the actual temperature is higher than 21°.

And I'll log the usage scenario and pass it along to the team to evaluate and improve the Tapo T315, however, we don't have any specific details that I can share about if the related feature can come to the product or when it might.

Your understanding is highly appreciated!
Best Regards

  @Murdo900 

Hi,
Thank you for your feedback. I have forwarded the inquiry to the related team to evaluate, and you can go to this thread to vote and leave your comment if you have the same request.
Best Regards