First I would ask for more details because if the idea is to do something simple like eat up a bowl of soup or even a nice warm bathtub, that's one thing. If you're trying to use it to create an elaborate steam trap or some steampunk engine or something, that's another.
Yes produce flames can kill a peasant. So can falling face first into a firepit. Doesn't mean that it can heat up water that fast. The thing to keep in mind about heating water is that it steams and evaporates and loses heat very quickly.
So time for some math. Human skin is "destroyed" at around 160 degrees farenheit but I also want produce flames to set paper and clothes on fire. Paper combusts at around 480 degrees and cloth at much lower so lets say that Produce flame heats up a spot to 500 degrees for a second.
It takes 0.24 BTU of heat to change the temperature of one pound of air by one degree F.
So to go from 60 degrees to 500 degrees = 105 BTUs
Plugging that information into this:
https://bloglocation.com/art/w...or-time-energy-power
Means it would take 3 hours for a spell that can ignite paper and severely burn flesh to heat up 1 kg of water to boiling. If you assume say 5kg in a bathtub and even if you double or triple the btus, you're still standing around for hours trying to heat it up. Using that calculator at 315 BTUs you're at 5 hours for a bathtub.
On top of that trying to heat it up is going to be impaired by the fact that it isn't going to be constant heat. It is going to be a burst of heat and then a couple seconds of cooldown for the rest of the round and repeat.
Fireplaces and stoves operate in tens of thousands of BTUs mainly because of surface area. You're not heating up a singular point you're heating up an entire pot or fireplace worth of air.
Water is very slow to heat so you're better off getting a normal fire going. FYI boiling water is a big challenge for survival shows that can't build giant roaring fires. They will often have pots of water boiling/cooling all day and tending the firepit and water situation is basically a half-day affair for a camp of 2-3 people if they can't get a clean source of water. Once boiled the water takes almost an hour to cool back down to drinking I think....
Anyway this has been very crappy math brought to you by me, someone with no background in physics or math or anything :D