We are expanding our house by 50% and planning to re-install an 8 yr old 13 SEER 3 ton AC with 92% efficient 80 kBtu/h furnace. The house envelope will be upgraded, so the system should be sufficient, at least one contractor said it would be oversized.
We live near Los Angeles where it doesn't get very cold. It has only frozen a couple times in the 15 yrs I've lived here. So far this year (Nov 1 ~ Dec 11)we've accumulated ~180 hrs with the temp below 45F, which is the most chill hours for this period in at least the last 15 yrs.
The remodel will also bring an oversized PV system (so electricity is free) and solar DHW with 90% efficient gas backup.
So I have 3 questions
1) Is it worth it to re-install an 8 yr old system. It is not a high end system and not from a large brand, but it worked fine previously and all ducts will be new. How much longer should the system work?
2) If we go with a new system, what are the pros and cons of the following options
a) AC with hydronic heat
b) heat pump with electric back up
c) heat pump with hydronic back up
3) We will have to run some ducts between 1st and 2nd floor. If we re-install the old system and have to replace it with a similarly sized or smaller system, would those ducts have to be replaced?