Hi John, the AHS is a gravity feed version of the AA boiler, the boiler vessels are otherwise virtually identical.
do you have a desire to have a very large coal supply hopper so you only have to 'look at' the supply hopper once a week or month? keep if full with a tractor loader? Or do you not mind the idea of hand filling a hopper every few days ? if you want the big hopper, then you want an AA or an EFM, you can use the AHS, but an add on auger must be purchased.
You could design your out building with a tractor-fill able hopper on one end, and the coal boiler at the other end, with a pedstrian door access, and an ash dump nearby.