If you look at the lid on the auto fill there should be a brand on it. Because the are many types of auto fills I would pull up the manual online and follow there procedure. But basically you should be able to turn off the supply line to the auto fill and place a plug in the outlet port in the pool and then pour some pool or rv antifreeze into the auto fill.

Drain and fill line ports are separate. Autofill to the sprinkler system as a source is something I can see but there are special valves to prevent back flow required in case the float valve fails. The drain is just a port that is an inch or two higher than the correct water level. That needs to be connected to a piece of pipe that is pitched down slightly to the sump. 15 feet is too far to tunnel. The drain port in the autfill tank must be plugged or water will get behind the pool wall, creating potential washouts and loss of support for the pool wall and surrounding area. A way to manually lower the pool will need to be used. That is the faucet at your cartridge filter. It works but is slow and doesn't give you the option to vacuum to waste. A 3way Jandy valve over the pump's discharge can direct water to the filter or the other port. Attaching a pipe and barbed fitting to that port allows you to connect a backwash hose, giving you a fast way to lower the pool and the vacuum to waste option you would use to get rid of an algae bloom. You don't want dead algae vacuumed into the cartridges. Its a PITA to clean them after. Scott

If you haven't done so already, I suggest you purchase a hygrometer. Buy a good one that can be calibrated. There are YouTube videos on how to do the calibration. Does your apartment have steam radiators? Steam radiators will release a good amount of moisture into the air. Perhaps your apartment isn't as dry as you think it is. What humidity level are you trying to achieve? Randy makes a good point about using a humidifier designed to be installed inside the piano. Your goal is to keep the inside wooden components of the piano from drying out. a humidifier running inside with the lid closed should not have to filled too often especially if your apartment is not too dry.

I too have searched for an auto-fill that would work in a northern climate. Unlike yours my tub is on a deck that is located well above my basement so to clear dedicated fill line is as simple as opening a solenoid valve to let the water line empty by gravity into a drain. In your installation it might require the brief operation of a water pump to immediately pull the water out of the line to prevent freezing. This is a work in progress and at this time I have completed the component selection and have moved onto system integration. Current requirement: This solution will be best served if you have a cat5 network cable ran into the tub pump control are since this would allow remote management of the water level controller and facilitate email notifications from the water level controller. Elements that can be monitored by the water level controller are: water temperature, control area temperature, outside temperature, control area humidity, tub water level, jet pump activity.