You offer the capability to turn the heater on, and run a timer. But your app doesn't allow the heater to be turned OFF after a certain amount of time. It should. It should also be settable so that it can turn ON at a pre-set time (either some interval into the future, or at a time on the clock).
Last, but probably most challenging, please create a WEB APP. Everybody, regardless of smart phone, or the amount of memory available (for your unnecessarily-huge app), can use a web page on a laptop or phone.