I think most of these "acid free" rust removers are formulated with phosphate esters which is why they are slower & need higher temps than the acid based ones.
It might help to put it in the sun protected from the wind and place a piece of black plastic over it.
If that doesn't work it might be easier/cheaper to get a small amount of the acid based material instead of buying a heater that might have a soldered seam that dissolves.
Another thought - re-read the directions carefully - do they mention anything about shelf life after dilution?
