I have a Lennox whisper heat that is 12 years old. What it does is shut off during a call for heat and not come on until someone manually turns it off and back on via power switch or thermostat. It usually occurs overnight or when everyone is gone to school/work. As a result it gets really cold in my house. It did it again last night and the temp. outside was 13-15 degrees F. It was about 59 degrees F in my house this morning. This has been happening for 2 1/2 weeks now. I have a cold because of this. Can somebody please tell me what is wrong with it!?