The size of your UTH will depend on what you are getting for readings (warm side, cool side, ambient). The factors that are working together here are insulation (or lack thereof) on the tank, UTH size, and room temp. If you cant get the temps where they need to be, I would go with insulation first. If that doesn't work you probably would need to go with a larger UTH. Just make sure you have it on a thermostat.