As a veteran of many "dry" darkrooms, the first question to ask is how much water do you want to bring up to (or maybe down to) 20-22 C? (Are you washing prints, with this water, or just need it to fill the holding tray"? Second question is the temperature of this water and your darkroom. These are the two key questions you need to know before you can decide on (if) how you want to heat the water in your darkroom.

For many years the wife let me wash prints in the kitchen sink after I processed in my dry darkroom. I re-used gallon milk jugs filled with tap water for the dark room. Kept the darkroom at a reasonable temperature and never had to worry about temperature.

When I did have to adjust for small temperature ranges, I relied a the plastic ziplock back filled with either hot water or ice cubes.