Want to change 110v thermostat to low voltage programmable. Is it possible and how.

They may make a 120 volt digital?
If you want to use a Low Voltage (24 Volts AC)Programmable, You will need a 24 Volt Transformer/120 Volt Primary(40 VoltAmp will handle a couple of contactors), Then the 24 volt AC Coil Contactor, With the Contacts Rated for the Voltage and Current of the heater.

I had this issues in 1996 when I bought my house and wanted to have a programmable thermostat installed.

They were all 24volts so I had went to home depot and looked in there and inquired with no success. I know what I wanted from doing electronics in college.

Sure enough I found a device that I believe is called an adapt-a-stat which took line voltage (110v) at the original thermostat, converted a 24V fed to the thermostat, and then send a 110v feed to the boiler when thermostat called for the boiler to run.

I have searched all the posts and have not seen anyone mention it. I had an electrician by my house who was shocked to see the gadget in the wall behind the new digital thermostat.