I wonder if anyone can advise -
Whenever we have a power loss (which happens every time we have wind and rain - long story and down to ENEDIS taking too long to replace a cable) the pool filtration obviously stops.
If it happens late evening by morning the pool water level has dropped considerably, below the bottom of the skimmers and just above the top of the inlets.
That’s a lot of water - it takes about a day to replace.
Anyone any ideas as to why ??
Very difficult to say without knowing a lot more. Does it still happen when the pump is off, or do you run the pump constantly? If you still lose water when the pump is off (can’t imagine why you would only lose water in a power cut), could be a leak on the low pressure side, I.e. between skimmer and pump. If you have inaccessible pipes, maybe some sort of endoscopic camera would help.
We got stuck in the UK when their lockdown started and with Scotland operating a more vigorous lockdown have been away for 4 months.
Normally both pumps are on a timer - depending on the weather.
The power went off a few times while we were away. A friend kept topping up the water and tried running the pumps full time to clear the algae build up.
We’ve kept them running non stop since we’ve been back but another power cut overnight last week saw the water levels drop again to the inlet valves. Below that is only the lights and main drains
Everything drains down to the pool house which sits below the pool and about 50’ away
If the pumps run the water level seems more or less ok
George Topp.
If I switch off the pumps and close all valves etc in the pump house then again water levels remain ok
George Topp.
As Mark said tricky without more details but I suspect a leak at the multiport valve, leaking to waste. Undo the waste line fitting and plug with a pool bung. Switch off the power and observe.
Just a shot in the dark. Does the circulation keep the water temperature down, like a car’s cooling system and thus reduce evaporation?
No idea but I don’t think so.
George Topp.
It’d be interesting to test the temp on the way out of the pool and on the way back and see if there’s a drop. Plus I guess with the circulating there’s flow within the pool. If the water is just sitting there in the sun then wouldn’t the top layer become “super” heated and evaporate more?
I dunno 
Is it possible for a joint or break in the pipework to be forced closed when under pressure - when the circulation is on - but open up when there is no flow?
But since most of the power cuts are overnight (rain and wind is shorting an overhead cable) and water level is ok before then that doesn’t give you evaporation.
And it is a v large pool so there’s a lot of water
George Topp.
Ah ha. Another theory bites the dust.
I know the square root of very little about pools but that doesn’t seem logical.
On the output side of the pump the pressure will be higher with the pump running and water will be forced out of any leak, with the pump off then the leak will slow & might even stop depending on the exact location & how big it is.
i guess what Geoff describes might happen on the inlet side of the pump - once the pump is running the pressure there will go down, a small leak might just suck air into the system, once the pump is off the inlet side pressure from gravity might be enough to allow water out of whatever is leaking.
But the comment was that it was “a lot” of water - surely it should be possible to figure out where it is going, if not where it is coming from.
I was thinking more of the switching on and off causing micro-movements of the pipe and opening/closing a break - but I agree it seems unlikely to account for a big loss.
I do know though that it’s quite possible to lose a lot of water without seeing where it has gone - it just finds its way deep underground without any evidence on the surface.