If the HVAC systems are old for more than 10 years, then the cost of repairing would be very large, it would be costly to repair the old system. This works on a simple algorithm; if the price of installing a new set of the HVAC system is near to the repairing and fixing of an old set, then it is best to replace the system, but if the repairing is way cheaper because of the latest models, then it is best to fix the old rather than spending bucks on installing a new system.