They won't work very well when they don't have good southern exposure, they won't circulate a lot of heat if you have rooms and doors.

Meanwhile, the direct operating costs for natural gas heat aren't all that high.

Inserting something like this into the duct work of a normal heating or cooling system is fairly simple. The real issue is they increase construction costs and most buildings are built by people that don't use them. Further, they don't look like something out of a Norman Rockwell painting.

> most buildings are built by people that don't use them

Having rented for the past 6 years in 4 different houses, I've become convinced that this is one of the most significant factors for why so many house aren't enjoyable to live in.

Even if the overall design and layout of the house is good (I'm talking about the Christopher Alexander sense of good design), there are always small and not-so-small features that would never have been built that way had the person designing or building the house been intending to occupy it. Or had they moved in, said problems would be remedied in short order.

A problem of insertion into ductwork of a passive-design system component, is that when the heating system runs at night, it will cause air to pass through the panels, and thus cool the building at night in winter.

Since the solar panel-system is passive in design, insertion into ductwork would be a new design issue.

"good southern exposure".

In which case you can fit normal windows and achieve the same or better.

No, windows fail, as can be seen by reviewing the linked pages.

Windows are not the same:

- Windows do not passively aid circulation of warmed air, which the panels are particularly designed to do.

- Windows also do not passively reduce or eliminate overnight-losses of energy (heat) on a cold night (consider negative-centigrade temperature or less), which the linked-to design does do, by passively preventing circulation of air that would cool the building overnight

The design linked-to is specifically designed to passively deal with the prevention of the transfer of heat outside on a cold night, among other things, including limited cost, simple construction, and rapid payback over the life of the building in a cold winter environment. Windows fail as a passive solar source.

Diurnal intervention required for windows.

Because this is better than building code, and builders really don't have a huge incentive to go beyond code.