Your 19 year old carrier 3 ton heat pump with anywhere from 7-10 seer needs replacing can you use a 13 air handler?

No, you cannot use a 13 SEER air handler with a 19 year old carrier 3 ton heat pump with anywhere from 7-10 SEER. The SEER rating of the air handler must match the SEER rating of the heat pump in order for the system to operate efficiently.