I recently had an XP16 installed, but don't think that the 2nd stage of the compressor is ever coming on. I know for a fact that my stat is 1 stage cool and 2 stage head. The installer swears that after a certain time period of being on then the 2nd stage auto engages, is that true? I've stood outside next to it for 20 minutes and never heard any changes. Is there anything other than a stat that'll control the 2nd compressor stage (both cool and heat)?