Welcome to Just Answer, I will do my best to help you solve your problem
You had the thermostat wired right. The problem may not be in the thermostat
what happens when the thermostat is set higher than the room temp?
it indicates heat on and the heat turns on
then what happens, does the temp go up to the set point ?
then I don't understand your question
what's the problem?
if thermostat is set below 73 temp. it maintains temp at 73
it will go hotter but not cooler
old white rodgers thermostat worked fine
Then the most likely cause is a defective thermostat. There are other system related issues that can make the temp override but you seem to be ruling that out
the only way to verify this is to check the thermostat continuity at the boiler when it's set for a lower temp then overrrides
if there still is continuity across the thermostat leads at the boiler when the stat should be satisfied then the stat is faulty
whole heating system is brand new and other zones work fine
then a bad stat is the most likely culprit
ok thanks i will replace it. thanks again