Thermostat will not raise heat above 66 degree's, replaced thermostat, still won't go above 66 degree's and house is cold, what could be the problem?

Jump terminals R and W OR RH and W to lock the furnace on. Then see if the temp goes up. If it does it is in the tstat if not the problem is in the furnace.

If a furnace problem list EXACT brand and model number.