I live in an older home (1928). The furnace thermostat is mounted to a wall which also carries a heat duct to the second floor. The wall surface is warm during the heating season and I think this radiant heat is affecting the thermostat. Is there a way to shield the stat to minimize the heat affect of the duct work?

Remove stat. Space thermostat off the wall with small blocks of wood. Allow air to circulate from bottom to top. In other words just put the wood spacers on the left and right side under the thermostat. This will room temp air to circulate behind stat. Where wires come through wall use putty and block off the hole to prevent heat migration from the hole to the thermostat.

This always works.

Thanks hvac1000. Good suggestion. I'll give it a try.