The heat is not going on when I set it. I have noticed a small lock icon in the screen. How do I unlock the thermostat?

You will need to give us the brand and the model number. It would be best to find the instruction manual. I cannot find it online without knowing more. DrTom4444

One thing you may try is to turn off power to furnace and remove battery in t-stat for about 10 minutes. It may reset. I read somewhere to also press next three times and then hold. I am not familiar with that model, however. DrTom4444

I have been in contact with the manufacturer and the man I talked with needs the model number to give the proper manual. If I can get a response from you I can help you. DrTom4444