How would I be able to tell if it will hurt my furnace or not? Would I just have to get a model #. The current thermostat is digital but not programmable if that helps any. I think I could set the lower limit around 50 for night during the night and day time. My biggest problem is my wife is not very happy in the morning when it is 50 degrees and she gets out of the shower lol. My goal for this year is to make the house as efficient as possible. I have three more windows to change out (one is a huge picture window) and then need to put some new shades up.
Don't know if this would help your wife (and I am with her on the cold room after the shower) but...
electricity is really expensive here, so I am very careful. The one place I have incandescent bulbs (winter only) is the bathroom. With the door closed and the lights on, the room is definitely warmer when I get out of the shower, even with the warm air lost with the exhaust fan running. In general, it is more effective to spot heat, so if you can't do this, maybe a small heater?
For windows, I find bubble wrap against the glass does help some for heat loss. It does nothing for heat lost from air movement at the frame, so that is always a good place to put some effort in the fall (or spring if you have AC and want that cool air to stay inside in the summer).
I also have a programmable thermostat and it can be set manually as well, so when I am going out for any length of time I pop the temperature way down and only turn it back up when I am home. With how cold it is now, even a cold house seems warm for the first half hour, while the heat comes back up ;-)
For background, right now the house is set at 20C (68F) so I am being a bit wussy, but I am doing a lot of things that I need functional fingers for at the moment. Outside it is -9C (16F), wind chill makes it feel like -19C(-2F) because we are having gusts up to 55km/h (34miles/hour), so although it looks pretty out, I am staying inside ;-)