Thank you for your extensive reply.
I had already done the heating calculations for the house based on planned
superinsulation. For a worst-case design temperature of -7C (typical winter
in Vancouver only ever goes down to about -3C, or about 27F for you
Americans), I'm estimating about 25 kBtu/hr, or about 2.1 tons heating. In
principle, I could probably slap several more PV panels on the roof and run
everything off a heat pump (air or GS), but I'd really like to get the
electrical budget down enough that a whole-house battery backup becomes
If I use 6 feet of dirt under one portion of the house which can support
this, I end up with about 7.9 MBtu of heat storage, or about 13 days' worth
of storage. Naturally, I'd get 3 times this if I used water as the storage
medium. I'm still trying to figure out how the Riverdale house manages to
get 2 months' reserve for a similar heat load and a smaller (water) tank,
but I guess they're figuring in additional heat input from the collectors
and historically-based temperature patterns, as opposed to a straight cold
From your explanation, it sounds like the buffer I really need is enough to
get me through sunless stretches based on historical records, and then I
need to ensure there is enough collector area to heat it all back up again
(or at least as much as is practical) in a timely fashion.
I guess there's still the issue of what to do with the excess heat during
the summer if I've increased the collector area to provide heat during the
winter. Running this into a separate heat sink was what led me to consider
the seasonal heat store in the first place. With a drainback system, I
guess I'd just have the collectors empty most of the time.