Every good heating system design starts with a room-by-room heat load calculation based on realistic indoor temps and the 99% outside design temp for your location. If zip 80526 Fort Collins that design temp is +1F in town, but if you live in the hills subtract 3F per 1000' above 5000'. That information will determine how much burner you actually need.

In a passive solar house it's not possible to determine the heat load based on fuel use. The information you would need for the rough cut is the "whole-wall" R-value and area of exterior wall in each room, the "whole-assembly" R-value of the roof/attic assembly (where appropriate) and the ceiling area, and the window U-factors & area. If you set up a spreadsheet with the wall/ceiling/window area & U-factor of the windows, with the wall & attic construction & R-values we can cook up reasonable whole-assembly R values to generate a U-factor for the walls & upper floor ceilngs.

The basic calculation of the heat load is:

U-factor x temperature difference x square feet= BTU per hour.

Add up the numbers for wall/window (and door)/ceiling for each room to come up with a raw heat load for the room. Add all of the room loads on each zone to come up with the zone load, and add up the zones for a whole-house load. Since the shop zone is intermittent use only, keep it separate, or only calculate it with a 45F indoor temp (or whatever temp you keep it at when not in use.)

To fine-tune it you'd have to add a fudge-factor for infiltration/ventilation rates, and subtract out interior heat sources (250BTU/hr per sleeping human, 150BTU/hr per refrigerator, etc.), but in a tight passive solar house using just the raw conducted loss numbers should be close enough.

Then you will need to measure up the linear feet of SlantFin baseboard in each zone. The ratio of heat load per foot of baseboard will determine the water temp required to deliver enough heat to cover the load.

With the heat loads and water temp requirements then (and only then) is it possible to come up with appropriate & reasonable heating equipment suggestions for meeting that load (and how to set it up.)

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