Looking for a simple temp control for a water heater. (Sonoff/Mqtt)

Setup, Configuration and Use Beginners
8

https://www.openhab.org/docs/configuration/sitemaps.html#element-type-setpoint
add to your items and sitemap

Setpoint item=WunschTemp minValue=18 maxValue=24 step=0.5

and rule

rule "xxx"
when
   Item SonoffTemp changed or
   Item WunschTemp changed
then
   if ( SonoffTemp.state < (WunschTemp.state -1) && Schalter.state != ON) {Schalter.sendCommand(ON)}
   else if ( SonoffTemp.state > (WunschTemp.state +1) && Schalter.state != OFF) {Schalter.sendCommand(OFF)}
end