In a well insulated/sealed home actual heat demand is not easy to pre-calculate because of how much lifestyle secondary heating can affect things. The home we currently live in I rebuilt with half the super insulation planned for the new one. It is a modest sized two story dollhouse with finished attic(very hard to insulate well and is cooler,winter/warmer,summer than first floor. But even this house has to have the window opened on zero degree days when cooking all day and adding 10-12 more people for a family meal( and the heat never comes on). But with now just two of us, gone most of the time the furnace runs more. In a super insulated space even the change from to incandescent lighting to LED may be significant. Go induction cooking, microwaving over oven and many of the other conservations that lower "waste" heat and most of the heating demand rules are in question.
As I really want hydro radiant floor heat and UL listed geo HP's are not yet readily available scaled to the low heat demand living space I see a HP DHWH as a workable alternative if I can supply the warm air it needs with out paying to heat it during heating season. I also don't want to go to the extreme of just wood boiler heating as many in my area are doing because it being "free" is relative in time or money. Being rural elec a small wood(free) or propane(getting very expensive) back up heater seems wise.
Pushing the envelope of different often means guessing your best(from the most info you can gather) and than adjusting/tweeking from there for DIYer's. We seldom have the budget for full scale engineering studies first. From this comes most real innovation(not planned profits).
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