2. small boilers that will mount on a wall above a storage tank: Monitor, Weil McLain, Burnham
3. small boilers that are mounted on top of hot water tanks. Teledyne-Laars
4. small boilers that will fit into the closet space on a shelf and send hot water as a zone to heat a water tank mounted below or elsewhere.
Many of these do not require a chimney because they will vent through the wall using stainless steel vents directly to the outdoors. Local codes may permit some of these boilers to be mounted in the attic.
Warm air units can do none of the above. You can then concentrate on making the cooling unit do its job from the ceiling level with shorter ducts. As a backup, a hot water coil can be installed with the air conditioning blower to give supplementary heat from the air conditioning ducts.
To review some of these concepts with pictures, go to the free air conditioning and multizone lessons at http://www.hydronic.net site.