A modest 3 bed and 2 bath home will be built in a cold climate with a winter design temp of -20 F.
As in many houses today, the common spaces are one large flowing series of volumes, and the largest of them is the great room with a big tall vaulted ceiling, averaging 17 feet in height. Flanking that space is a tall wall of windows with a frenchdoor exit, and this bunch of glass all faces south.
The place will be occupied typically in the July through October timeframe each year, with occasional week-long visits in the snowy winter, for ski vacations.
The intention is to build and insulate it well, and focus on best practices for sealing and ventilation, and heat it with a small mod-con boiler, LP-fired, supplying 160 F. water (max) to panel radiators throughout.
The system will be in the crawlspace foundation under the main floor, and that will be either built with ICF (R-25) blocks, or conventional poured 'crete, and insulated and sealed very well. No ventilation. We estimate that the presence of the boiler and system in that space will condition it so that there is essentially no heat loss into it from the living spaces above.
I used Slant-Fin's Hydronic Explorer 2 package to calculate the room-by-room heat loss, and arrived at a total a little under 40,000 Btu/h at the design temp, with over half that amount happening in the large common greatroom.
The smallest mod-con I can find, using web searches, is the Knight Lochinvar wall-hung at 50k input, with a 5:1 turndown.
What might you recommend?