I have 25 year old (approx.) 18' x 36' in-ground pool that is losing water while the pump is running. I am losing about 1/8" - 1/4" over 12 hours. I have compared this to a bucket of water, the bucket does not evaporate appreciably so I know it is not evaporation. Also, it loses water when covered and it is a new phenomena (i.e. hasn't been a problem in previous summers). I plugged all the returns and the skimmer basket and turned the pump off for 24 hours with no loss. This indicates that there is no liner leak. I ran the system in with my valve on recirculate which had no effect, i.e same water loss over 12 hours. I see no water anywhere so I am a bit stumped.
One thing to mention is that when the pump timer turns the pump off from filtering the pump basket drains back into the pool quickly. This does not seem right since the air, and alot of it, must be coming from somewhere, but I can't determine it.
A second thing to mention is that I don't know whether my bottom drain is working or not, I don't know whether the previous owner blocked it for fear of children being stuck to the bottom. Apparently this was common around here.
A third thing, there is a circular plastic piece screwed into the bottom of my skimmer basket that has a hinged piece over the drain line and a hole over the return line. The gasket on this is really bad. I don't know what this piece is for (can anyone help with what this is), but it is definitely sketchy. Could that be a problem? See attached photos.
Hayward 1HP Superpump (just installed by myself)
Hayward 220t sand filter (pretty old i imagine)
Watermaid Salt Water Chlorinator