The code for a gas appliance is that it must be elevated 18″ off the ground.
Why? If a car is parked in the garage and it leaks gas, or if you accidentally tip over gas or flammable liquid, fumes can quickly reach the pilot light and cause a fire.
We built a proper foundation and installed a 92% Ruud™ furnace. We also replaced all the ducting in the attic. The hot water heater is elevated properly to code now. The whole system is now air tight and efficient.