It doesn't do this when you leave it on all the time or off all the time. Only when it cycles. When I increase the "on" time (30on/30off) it loses 4" of water over about a week. The cycle timer cuts/applies power to the outlet that the hot tub is plugged into. I have no cover on the hot tub. I can not find any evidend of a leak.
It doesn't do this when you leave it on all the time or off all the time. Only when it cycles. When I increase the "on" time (30on/30off) it loses 4" of water over about a week. The cycle timer cuts/applies power to the outlet that the hot tub is plugged into. I have no cover on the hot tub. I can not find any evidence of a leak.

So a couple of people have answered "evaporation", but 4" from a 28" deep hot tub is like 17% volume loss. Could this really happen in 10 hours and only when the hot tub is cycling? It seems this would happen more when it's running or off than when it is cycling...