This is on Trane model YCC042F1M0BB gas heat and electric A/C. I have a LUX programmable t-stat. This unit is not heating up to the set temperature. Sometimes turning the switch off then back on will make the heat come on. However, the one time that I timed how long it stayed on it was about 12 minutes and it still wasn't up to temperature. What are the symptoms of a bad heat exchanger (heaven forbid)? I seem to remember hearing there was a particular sound you could hear coming from the unit if this was bad? In the event the unit has to be replaced (which we will probably move in with my in-laws because we can't afford a new unit) what brand/model would you recommend to replace this? The house is about 1880 sq feet all one level.
Thanks for any and all opinions!