Mismatched heating and AC

[Brad Smith]

House we just bought has problems with it's gas heater heat exchanger being rusted and possibly a source of CO.  When the tech came out to look at it, he said the AC compressor, heater and "coil" were mismatched.  The air handler/coil is a 4 ton and the AC and heater are 3 1/2 ton.  For 1700 sq ft, we need it all to be 4 ton... I'm told.

I asked them to just replace the heater and we'll do the AC later.  Nope, won't do it in parts.

Whats up with that?  Was told it creates problems but I thought they were separate units simply using the same duct work.

Would love to know more!