Flows can have more than one trigger, and they can share the same 'only if' condition. Add more date time triggers above or below the existing one, and connect them all to the Heater Thing. Then draw lines from the weather Thing to each of the yellow dots on those connections from the date time things. Set each date time Thing for a different time.
But what happens if it gets cold enough to freeze the water during some time when your rates are high so you aren't running the heater? Also, keep in mind this stuff isn't foolproof, there may be a day here and there where the flows won't run because some server, or connection between servers, is down.