Replace A/C Evaporator When New Furnace Is Installed: Necessary ?

Hello:
Have a 25 year old 3 ton A/C unit (the split type) that has the evaporator section as part of my gas, forced hot air furnace. It does work, sort of, but would like to put off replacing it for another few years, if possible.
Will be replacing the furnace now however, with a new Trane unit.
The HVAC contractor suggested, strongly, that we replace the evaporator section now when he replaces the furnace. Have several questions, please:
a. is this in general a good idea ?
b. some time in the near future, we will, probably, be replacing the outside A/C condenser. Might be replaced witha another 3 ton unit, or perhaps a 3.5 ton unit-hard to know or guess at this point.
What potential problems would I be letting myself in for in trying to mesh a new condenser with an already installed evaporator coil in the new furnace ?
c. Best, in your opinions, to do the whole thing at once, and not replace the evaporator coil now ? The cooling produced now is marginal; would be nice if a new evaporator coil would improve the present situation; is it "likely" to ?
d. Also, would there likely be Brand incompatibilities, or is one evaporator coil like the next, etc.
What else should I be asking about this ?
Sort of a peripheral question: is it still a good idea to have the furnace, somehow, installed a few inched off the floor in case water comes into the basement for whatever reason ? Or, are the furnace guts high enough off of the floor, usually ? If a good idea, how is it usually done ? Do the mfg's make a frame to do this ? Bricks ?
Much thanks, Bob
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I would go ahead and replace the AC unit. A new system probably twice the efficiency as your old unit, thus half the power use. Summer's comin' on and I would not take the chance of a 25 year old system failing. It certainly should be cheaper to do it all at once.

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Have a 25 year old 3 ton A/C unit (the split type) that has the evaporator section as part of my gas, forced hot air furnace. It does work, sort of, but would like to put off replacing it for another few years, if possible. CY: Well, then, you may continue to pay high electric bills, sort of, until that time.
Will be replacing the furnace now however, with a new Trane unit.
The HVAC contractor suggested, strongly, that we replace the evaporator section now when he replaces the furnace. Have several questions, please:
a. is this in general a good idea ? CY: Yes, it's less expensive in the long run to do it now.
b. some time in the near future, we will, probably, be replacing the outside A/C condenser. Might be replaced witha another 3 ton unit, or perhaps a 3.5 ton unit-hard to know or guess at this point. CY: Just a wild guess, but perhaps your old tired inefficiant 3 ton system, if you put in a new coil with TXV, it might work closer to the real efficiency of a new one.
What potential problems would I be letting myself in for in trying to mesh a new condenser with an already installed evaporator coil in the new furnace ? CY: If it was only a couple years apart, none. If you're talking a 20 year old evaporator, it is likely to be dirty and low efficiency.
c. Best, in your opinions, to do the whole thing at once, and not replace the evaporator coil now ? Cy: Naah, go for the coil now. Space out the money a bit, and enjoy the new evaporator.
The cooling produced now is marginal; would be nice if a new evaporator coil would improve the present situation; is it "likely" to ? CY: There can be a whole bunch of things wrong. A new evaporator will fix many of them. A TXV costs more at first, but well worth it over the years.
d. Also, would there likely be Brand incompatibilities, or is one evaporator coil like the next, etc. CY: I'm sure there is some minor difference, but I'm not sure model for model.
What else should I be asking about this ? CY: Did the HVAC guy check the superheat? Subcooling? Air flow through the furnace, adn through the outdoor unit? Are both coils clean?
Sort of a peripheral question: is it still a good idea to have the furnace, somehow, installed a few inched off the floor in case water comes into the basement for whatever reason ? Or, are the furnace guts high enough off of the floor, usually ? CY: I think it's an OK idea.
If a good idea, how is it usually done ? Do the mfg's make a frame to do this ? Bricks ? CY: Bricks are good, or cinder blocks. Many furnaces are shorter than the ones that got taken out. YOur HVAC guy can help you figure this out.
Much thanks, Bob CY: Y'welcome.
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