We have a 15 year old premier water furnace. Mostly very trouble free. Setting the air conditioner lately to 72 and waking up in the middle of the night to find the temp has gone down to 63. The contractor put in a new 200. Dollar thermostat. Same problem occurs. Everything looks good inside, contactors, wiring etc. I am a licensed electrician and this is out of my field. I could set up some tests. Any suggestions? The hvac contractor is stumped too and the charges are adding up.