As we finally move forward with detailed design, the house will be broken into four large zones (further subdivided of course). This gross division will be: garage storage-probably rarely heated; a mid level section of bedrooms and media room, used only when a large volume of company is visiting, ( both these first two levels are built into hillside completely with south facing wall); third level is main living space and will be occupied at all times; fourth level two bedrooms used only when small number of guest are present. Each of the lower levels are 2800 feet, (including the garage ;-) ), 4th story is 800 ft. Since we've designed to be able to shut down the area we will not constantly occupy, would it still make sense to use a single boiler? This is 9000 degree day climate. Having two levels into hill eases the loads for those levels, but still large variability based on zones in use.

To properly size a residentiall space heating boiler one starts with a total (block) load and then looks for the high load zone (greatest load per square foot of floor space). From this information the a condensing boiler with the appropriate minimum fire and output can be chosen. In most cases this is one boiler for space and water heating. All zones will present a load since they must all be kept from freezing and this the rare case were a larger boiler may make sense since you will have to pickup the cold zones when needed. We use the IBC boilers here in Minneapolis for applications where large eccentric loads and micro-zones with low output floors are present.

The IBC condensing boiler features a 5 to 1 turn-down meaning a lower low-fire and a higher output high-fire. But only the heat load can tell for sure.

I would put some insulation between the areas.

I will of course get the loads calculated. I had another post regarding outside design temp so I can begin to work those out. I didn't have a clue what kind of boilers were out there and what range could be had. I appreciate the information especially that boilers can be had with a large variation in output.

Jonr, Since it is radiant in the floor, I certainly plan of insulating underneath. Probably 2" foam board between levels 2/3 and 3/4. Between 1/2, I'd insulate heavier as level 1 will normally be unheated.