A few things:

Whenever a tech tells you your heat exchanger is bad, you should always have him show you proof, right then and there. If he can't and does not have a good explanation, I would be suspicious.

What is the model number of this unit? Before proceeding you should look into your warranty status. The heat exchanger is usually warranted well beyond the rest of the unit's parts, though it may have required you or the installer to register it to get that extended period.

Having said all that, I would not be surprised if the heat exchanger did indeed fail. The type of unit you have is known in our industry as a "gaspack" which is short for gas/electric package unit. Unlike split systems (with the furnace indoors), most of these are designed so that the heat exchanger comes after the cooling coil, meaning it sits in the cold air stream while the A/C is running and rots out quickly compared to a conventional furnace. Hence the rusting you've observed.

Overall, if the heat exchanger is indeed bad, yes I would replace the unit. There are some things you can look for when buying a new unit that will help the heat exchanger last longer. Either

-Heat exchanger placed before the evaporator coil (York to my knowledge is the only manufacturer that does this, no idea why the rest haven't copied them)

or

-Stainless steel heat exchanger - far more corrosion resistant (available on many brands)

Despite my agreement with the contractor to replace the unit if the HX is indeed bad, I would still like to call attention to a bogus statement they made:

>The gentleman from the HVAC company also said that my unit uses an older refrigerant that is no longer sold, so if the AC compressor ever goes out we'll need to replace the unit anyway. Is there truth in this statement?

That would be R22 refrigerant. Yes, it is true that as of 2010 manufacturers were no longer allowed to produce units with this refrigerant. Now the industry has switched to R410A. However, R22 compressors are still available as replacement parts, and the R22 refrigerant itself is still being manufactured, although it is far more expensive than before. The manufacture of the actual R22 refrigerant does not need to stop until 2020. Even then, the use of recovered refrigerant will still be allowed for servicing. Bottom line: units with the R22 are still capable of being serviced easily and that in itself should not be a motivator to buy a new unit.

Agree with hvtech

X2

TD

What happened with the 22/410a deal is the manuf is no longer allowed to ship pre-charged, from the factory, 22 units. This was to encourage the move to the better 410a. As ever, the manuf found a loophole in the regs and continue to sell 22 units. They just have to be field charged.

That is my understanding of it. Yes/no?

The dry R22 units are designed for retrofit purposes only, installing a new system and charging it from scratch with virgin R22 would not make economic sense compared to just putting an a new R410A system. None of that applies to the original poster because he has a package unit. Only builder model split systems are sold in dry R22 versions.

But could you still run the air conditioner but not run the heater ?