After reaching desired temp, my Carrier unit 48GS will not reignite unless the power to it is cut and then restored. We have replaced the gas valve and thermostat already. There was some rust inside the unit that burning off and was causing a foul odor. After removing it, the odor dissappeared. I was told that I may have a clogged heat exchanger... Can this too be cleaned out , or do you have to replace it ?