Ahve they actually had it diagnosed or are they just figuring because it's age they will replace it?
Nat Gas, propane or electric?
Agree and True........but...........Typical garage furnace at 20 years old...my recent DIY experience the board burnt. $100 for the part. Poke around just about every part was rusty. Exhaust pipe transition joint was paper thin. Exhaust power fan was a a mass of rust.
At less than $400 for a new unit it made no sense to rebuild the old unit.
Each case will be different.......but........with HVAC contractor the labor alone on repair will QUICKLY tip the scale Towards replacement.
I would DIY the troubleshooting and hope a Simple safety switch, pressure switch, presure switch hose cracked or igniter failed before a call to the pro. Read the board codes and look around at the unit for its overall condition.