Need more info !Does your furnace run continually, or is it cycling on
and off ?If you set the temperature lower (say, 66), does it maintain
that lower set point temp ?Does the current outside temp influence how
warm of an inside temp you can achieve...
I'd try isolating the problem to either the thermostat, furnace or
wiring.If you are qualified and comfortable working with low-voltage
wiring, with the nest thermostat removed from it's base, TEMPORARILY
shorting the red (R or Rh) and white (W) wire...