I have an in-ground spa that connects to the same equipment as my inground pool. This is the second winter I've had the house, and last year I simply closed the whole system at the same time. This year, I decided I wanted to keep the hot tub open for the winter. So 2 weeks ago I closed the pool and left the hot tub open. Since that time I've noticed that every few days the water level has dropped in the hot tub several inches below the skimmer intake, and I'm trying to determine the cause of the water loss. I measured today and we're talking about 3.5 inches over the last 2 days.
So here are some possibilities that I've considered:
1. The valves that turn off the pool intake/returns are bad and when I turned on the spa by itself after the pool was closed, water seeped into those closed pipes and filled them up, meaning I'll have to close the pool again, replace the valves, possibly close the hot tub for this winter as well.
2. The 'equalizer' line that connects the spa to the pool isn't sealed well. There is a single connection from the spa to the pool that's several inches below the spa return lines to connect the 2. During normal operation, you generally leave this open so that the water level stays equal in both. Because they use the same equipment, if you mess up, you could suck up all the water out of the hot tub and put it into the pool, or vice versa, and blow out your equipment.
3. Evaporation. I have a cover that mostly covers the hot tub. There is a hand rail that goes in, and my cover has 2 pieces to it (one for 1/3 and one for 2/3 of the spa). They connect using a velcro flap, and this flap covers part of the gap, but because the hand rail is in the way, the two are not flush. Possibly water is evaporating out of this gap.
So here is what I've done so far, and where I have the questions. To point out, this past summer i had an algae issue in the pool, so i closed the lines in and out of the hot tub while i fixed it. When i got back to it 2 weeks later, the water level had dropped about 12 inches (maybe 2 weeks of time). I assumed at that time it was also evaporation. Concrete in-ground offered no where to leak out of, though maybe a bad pipe but haven't noticed anything odd or leaks last year.
1. I heated the spa up and felt the pipes to see if they warmed to the touch. Only the spa intakes/return lines heated up. I then double-checked by opening the pool return lines and blowing them again (empty), and taking the return valve apart to check the seals (it was fine). i then checked the skimmer lines as well and found them still clear. It seemed that the pool is closed properly, so no worries there.
2. I tightened this as much as it will go. It has a 2" pvp cap that plugs into it. I could try putting teflon tape around it and reclosing it to make sure it has the best seal. I don't recall any issue during the summer with water getting into/out of this pipe when I had it closed. Trying teflon tape is a little problematic as I would have to do it while water is sucking out from the hot tub and into the much emptier pool. Because I have a solid pool cover, I could also maybe check the water level and see if it rises, but I feel that the amount of water would be too small to measure since the pool is so large in comparison to the spa (3" of loss in the spa would be like .01" rise in the pool).
3. This one intrigues me the most. Normal water evaporation is supposedly like .25" a day for pools. Not sure if that's surface area based or what. My water temp in the spa isn't that warm (maybe 70-75), air temp has been around 50-70 the past 2 weeks. But because i have a cover on, maybe the air trapped inside is warming from the sun, sucking the water up, and then escaping out of the gap. Essentially a mini green house effect with a vent to let the vapor out. I know when I took my solid pool cover off to check the return lines, there was a ton of steam escaping out from underneath. The underside of the pool cover was covered with water droplets due to all of the condensation caused by the hot air trapped in between the cover and the pool, so maybe the same is true for the hot tub.
So before I go much further, I wanted to see if anyone else has a hot tub they keep open during the winter. It could be i have a bad pipe somewhere that's leaking. It could be #2. Or hopefully it's just an overevaporation of the water due to the cover not completely covering the pool. Any thoughts or math-based guesses on what could be happening?