Replacing furnace and AC
I had my AC serviced a month ago and it needed coolant. A month later it needed more coolant. It has been extremely hot during that time here in st. Louis, 100+ most days. The HVAC guy says I should consider replacing the system maybe next spring since it is 20 yrs old. His ballpark estimate for a 3 ton carrier unit, wich is what I have now, is $6,500. I replaced the upstairs 2 ton unit, also carrier, 5 years ago. My questions:
1) would you agree with the assessment I should replace because of its age? Service guys point was that the compressor useful life is only around 15 years and doesn't make sense to put money into it and that I can expect to have furnace problems.
2) do I have the right size units, 3 & 2 ton. I don't know the furnace specs? Service guys response was that the engineers who decided this 20 years ago knew what hey were doing. My 2 story house is 3,000 sf plus 1,000 sf finished lower level that may not have been in the calculation when the house was built.
3) is there a reason to stay with carrier just because the other unit is carrier?
4) what efficiency/spec levels should I be considering?