I have a M# 580DPV090180ACAA that has rusted through on the right 4 tubes. The left 3 are ok and actually look newer. This would lead me to believe that the two sections can be replaced separately. Do I have this correct? I have not changed out a heat exchanger on a Carrier RTU in some years. Does it just slide out the front burner area? Approximately how many labor hours should be allowed? Thanks for any input.