I am planning the second floor of a new build home in climate zone 4 which i am completing in stages due to money constraints. The building envelope is hot roofed with 4" cc spray foam on the roof deck and 5.5" of rock wool beneath that, the walls are 2x4 with cavity insulation and 2" of XPS on the outside of the wall sheathing. The walls are further air sealed with a combination of Knauf eco-seal and cc spray foam and the basement walls have 2" of cc spray foam so the envelope is reasonably tight. Fuel source available is propane which is already piped into the home.
The first floor and basement are heated and cooled with their own dedicated forced air system on 3 zones with a new 3 ton Carrier Infinity Greenspeed Heatpump and electric resistance back up with a Manual J calculated heating load of about 50k BTUs. DHW downstairs is on a dedicated condensing tankless water heater.
My question is, how would you recommend heating the second floor which has a Manual J calculated heating load of about 40k BTUs and DHW requirement for 2 bathrooms? Assume Minisplits and plumbing based hydronic heating are not options for design reasons and i want to minimize the electric load because i backup the house to a whole house 20kw generator which i do not want to overload. Would you recommend a setup similar to the Rinnai hydronic furnace and tankless heater featured in Building Science RR-1206? (link: http://www.buildingscience.com/documents/reports/rr-1206-measure-guideli...)? Or perhaps a Goodman 45k BTU furnace and a/c unit like in your prior blog entry? or a different approach?