I have a tri-level condo that is now 20 years old. The heater portion of my H&A unit has gone out. My question then is since I've not gone on the roof to actually see my own unit but I'm being told I have to replace the ENTIRE ac/heater unit in order to fix the heater. (Problem on heater is that a certain important part - block? - is cracked.) Is this true? Is the heater even replaceable? And if I do need a whole new unit is $2k for a one yr old unit be a good deal? Sorry but don't know detaiils on the used unit but it would be the right size for a 3 level 1400 sq ft condo. Any advice would help me greatly. Thanks!

It sounds like your unit is a combined heat and air rooftop unit. If that is the case the entire unit will need to be replaced unless the defective part can be found. Even if the part can be found it still might be better in the long run to replace the entire unit.

Size and price of unit is a local issue. If you are unhappy with the price get other estimates.