Is it allowed to turn on the hot water on Shabbat if my dishwasher is already running on Friday night, since it is already causing the water heater to work?
Nope
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Would it be different if another shower or sink was already running from before Shabbat (so you know for sure the flame under the boiler was already on, as opposed to a dishwasher which engages the boiler more sporadically).
No, It has nothing to do with the flame on the boiler. It has to do with the fact that the second you open the hot water faucet new cold water is being added to the boiler and being cooked.
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Same thing for tankless?
Same. Also prohibited.
In all cases you are cooking water.
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