I don't think our situation is identical, but we are adding water every 1-2 weeks as the level drops about 2 inches a week. We use the spa daily, sometimes 2-3 times during the day. The top is off during soaks (obviously) and when chemicals are added. Our temperatures have been hovering mainly between 10 deg - 20 deg much of the winter. Most of the people who have responded to my similar questions have said this is pretty normal given the use and temperatures. I also understand that as a newer owner I'll have the top open more often dealing with the chemicals. That might be the same for you?