I have a ~ 15 yr old Furnace system. A month or so back, it just stopped heating. (There is separate heating system for the 2nd floor and that one works fine). I called Lennox (the OEM) to have a look at the system. Of course, as not unexpected, they wanted me to replace the gas control valve and Pressure switch... $1700 only. I thanked this chap. Called $50 / hour HVAC experts. The first chap looked the system and said that the Thermostat was broken. He hardwired (i.e. Bypassed the thermostat) the Red, Green and White cables together. That seemed to get the system working again. I bought another Thermostat from Lowe's. And it worked fine... For a few weeks. Then the same problem.. I.e. No heating even if I set the temperature at 80 (inside thermometer showed 65 or so). I bought another thermostat (this time a mechanical, non programmable one). It worked fine. For a couple of weeks. Now I went and bought another mechanical thermostat. And this time the new one is not working... Any insights would be greatly appreciated. Thank you for investing your time to help...

Can`t help but to wonder, if all these thermostats from Lowe's are Hunter brand?

I have used Honeywell, Hunter and another brand that I cannot remember.