I know this is a bit of an odd question, but how warm does it need to be to replace the liner in an inground pool. I know many of the threads on this subject talk about liners being easier to handle after being warmed by the sun, etc. Well I have an indoor pool so don't have that luxury, and I would ideally like to change out the liner in the off winter season as it is showing its age and starting to wear through the print pattern in places on the floor (either this year or next). My current liner is about 12 or 13 years old, and the 2 liners before it were both changed out in the warmer months after catastrophic failures. (seam split open on one, ATV (Kawasaki Mule) ended up in the pool for the other.
p.s. I have a ducted 120,000 btu portable (ish) indirect fired diesel heater so I can heat the air up reasonably warm on demand, but at a cost of about 1 gallon per hour of diesel. (It will heat the air in the pool house air from near freezing to 70 degrees when it is 25 degrees outside in about an 1.5 hours) The pool house usually does double duty as a green house for freeze sensitive plants in the winter and I use said diesel heater to keep things warm when we have hard freezes (or for much of the winter a few years ago during the polar vortex mess), did not need it at all here last winter. My only concern about the heater is it has a safety thermostat that turns off the heat if the return air is over 86F, could probably trick it to heat the room warmer than 86 by bleeding in some cold air on the return though.