Hi,I installed a new trane air heat pump about 6 months ago.now outside temp is 6 and my pump is not working.aux heat is coming on automatically.when temp goes to 8 or 10,heat pump comes on again automatically.installer told me that heat pump will do job till -9.9,, when -10 heat pump will shut off automatically and furnace will kick on... I have brand new furnace from trane 96% high efficiency... so tell me why my pump stops working on plus temp instead of minus