I am a veteran owner of an in-ground, vinyl lined, 25000 gallon pool for 17 years.

Pool pros or other veteran owners: have you experienced a temporarily leak in a pool or pool plumbing during unusually cold weather events?

Here is why I ask:

Here in Charlotte, Tuesday and Wednesday, we had a very unusual cold snap. On Tuesday morning the low got down to 6 degrees, well below normal, and the lowest in at least four years. The record all-time low for Charlotte is MINUS 5 degrees last hit in 1986. (Yes, I realize that is balmy compared to what you northern folks hit on Tuesday!)

Anyway, I keep my pool open and clean during the winter. Obviously, when temps drop below freezing, I always turn on the pool pump to keep water moving (freeze protection). During the "normal" winters, running the pump when temps are below freezing is never a problem. And the water level never has dropped during normal "cold snaps." The last leak I had was in the winter of 2010. This was due to holes in my previous 10 year old liner. I replaced the liner. My current liner is only four years old.

I did the bucket test from 11 PM Wednesday night to 11 PM Thursday night, with the pump running all 24 hours. The pool level dropped by almost 2 1/4" as marked by the bucket on my middle step. I went in to finding the leak mode!

So, I started to do my next test, "resetting" the water level to as before, then turning the pump off for 24 hours, as the temps Thursday and beyond rose quickly to above freezing, allowing me to keep the pump off. Within 9 hours, it had still dropped about an inch, but, rain, then heavy rain moved in Friday and Saturday (3 inches total).

So, I have not been able to complete the second test yet - keeping the pool pump off for a full 24 hours due to rain. But, overnight Saturday night in to Sunday, the pool water level has dropped ZERO. Temps hit 66 degrees on Saturday. Strange, because even with the pool pump off just less than 12 hours, before the rain got going, I still lost about an inch.

I also used pool dye penetrant in the bottom of the skimmer box, around the steps, and around the skimmer face plate Friday night. None of it got sucked in anywhere. It is becoming apparent I have no leak at all in the pool. But, if a leak were now still in the plumbing, I should have at lost at least some water.

My tests are not over, and I have some calls out to leak detection companies, but, these events got me thinking, is it possible that during really cold weather, a fitting could shrink just enough to allow some water to escape; but when it warms enough, expand and stop leaking?

On Tuesday and Wednesday the ground surface froze rock hard; not seen hear in years, and likely down to a depth of at least a couple inches, but maybe not down to the depth of the pool plumbing. Undoubtedly, the temperature of the water dipped a few degrees colder than it does during its normal lowest point of the winter. Normally, the pool water temp falls to the low 40's. But, with this past unusually cold, colder than a bad cold snap weather, the water temps probably fell in to the upper 30's.

I have Schedule 40 PVC. What little research I have done suggests, PVC is tough, rigid and lasts a long time, but is very rigid.

Thanks for any input.