The time frame between emptying ashes is in direct proportion to the depth under the door - the less the distance, the greater the frequency of ash emptying. If the emptying of ashes is a chore to be avoided then purchase a forced air draft furnace. The depth of the firebox below the door surface is very important. This space is needed to allow the ashes to build up. Yes, it is harder to empty ashes from a recessed floor, but if the furnace does not have this dropped area then the space for wood will quickly run out.