I also have a Trane XL15i, with electric strip heaters as my auxiliary heat. When it gets particulary cold, say less than 20 F, my thermostat shows that my auxiliary heat is supplementing, but I have my doubts. The system is not keeping up, with my inside temp dropping as soon as the auxiliary heat kicks on. I've measured my register temps when this happens, and it is reading 65 to 70 F. If I engage my emergency heat, meaning the heat pump is turned off and only strip heaters engaged, my house warms up within 20 minutes; however, I have to do this manually. My contractor says that he thinks the heat pump is getting stuck in defrost mode, so the strip heaters are being negated by the A/C during defrost mode. He's planning on installing an outside temp sensor to force the heat pump "off" when the temp gets below a certain level. Why is this necessary if the system is operating correctly? Why would my system be in a prolonged defrost mode? I've also noticed a massive of ice pooling around the base of my heat pump. Any assistance would be greatly appreciated.