I have a 15 year old Trane 10 seer a/c system that has been leaking freon for some time now and have been told by different a/c techs after several freon recharges, that I need to replace both up and downstairs units (both have leaked freon at different times).
One company tells me I need to go with a 13 seer 2-1/2 ton system downstairs with outside unit, inside coil AND furnace replacement, since the furnace is that old too. Also says I need to go with a 2-1/2 ton system and furnace upstairs to replace the 2 ton system there, as I now have had a large bonus room finished over the garage since the house was first built (15yrs). I can understand increasing the size unit there.
Another company says pretty much the same thing, except they tell me the furnaces look to be in pretty good shape to them and I could probably not have to replace them.
My question is can you just install a new 13 seer coils on top of a 15 year old furnaces plus the new outside 13 seer units and be good to go? Prices seem to be around $2700 difference for replacing the furnaces or not. That's a lot of money, if furnaces are not needed. But on the other hand, he says the furnaces could last another month or last another 5 years.
Just trying to do the right thing without spending unnecessary dollars.
Sorry for this long post, but it's about as short as I could explain it.
Thanks for any info.

The only true way to know for sure what size units you need is to do a manual J. This takes all the guess work out of it.

You can put the new A/C units on the old furnaces but I would spend the extra $$$ now. The newer furnaces should be 90 percent + units and over time you will get your $$$ back on utility savings plus they will be good for years to come. Your other option is to roll the dice with the old units and pay higher fuel prices.

When you say "furnace" I'll assume that you mean gas fired equipment. It the best case scenario, your heat exchanger can give you 20 years... but many fail in 12 to 15. It will be cheaper to replace the furnace now while it's being dismantled. And as usual HVAC1000 is 100% correct. If you can afford it, make the change. The energy saved will be a plus and something you can see immediately. Plus you have the peace of mind knowing that this won't become an issue in the near future for you to worry about.

Thanks guys for your replies. I was kind of leaning toward replacing the furnaces also, and your replies confirmed my thinking. Thanks again.