You have a few options when replacing your mercury thermostat. Since this type of thermostat is quite outdated, you can opt to change it to a digital thermostat, which is easier to use and more economical. You can further break down your options on whether you should get a manual or programmable model. Manual digital thermostats need to be set every time you use the heater, but programmable thermostats can be set in advance to automatically set the temperature at certain pre-determined points of the day. Many digital programmable models even allow you you choose which days and how many times a day each temperature setting will occur.