Should I replace my old mercury type thermostat with a digital or programmable thermostat?

Yes. The old style thermostats can ruin a system’s efficiency. Digital thermostats are more accurate than their mercury cousins, and Programmable ones are better than them both. A programmable thermostat can be set to maintain different building temperatures depending on whether or not the building is occupied. This can save you a ton of money in unneeded energy expenditures.