what happens if the boost times out at a temperature of 48? Wouldn’t the condition of below 40 mean it would stay off and not reach the 50 target?
You have to make up your mind on what’s happening between 40 and 50: something or nothing 
Sorry, I thought my intention was clear. Once the water temperature cools below 40c, it should heat until it reaches 50c - even if the 1 hr boost timer resets during that time. Once it’s heated to 50c then no further heating is applied until it hits the 40c lower limit again.
My response to @tom_l was meant to point out that if the condition is ‘below: 40’ then this won’t turn on the boost if it times out before getting to 50c. Leaving the condition at 50c means that the trigger at 40c still turns on the boost and if the boost times out before getting to 50c it gets switched back on. This is the behaviour I was shooting for. 
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