Do I really need a solar collector to heat my home, or can I get just as much heat by adding a south-facing window?
Windows and solar hot-air collectors are about equal in collection capability. I think the choice boils down to which works best for your situation. To me, these are the pros and cons.
• Collect heat and provide daylight.
• Nice bright interior.
• With good thermal mass (material that absorbs and maintains heat) in the house, they can carry some heat into the evening.
• High heat loss at night and on cloudy days (can be somewhat overcome with thermal shades).
• Can be undesirable in some situations because of glare or loss of privacy.
• Can lead to overheating unless you have overhangs for summer and a house with adequate thermal mass to absorb the heat.
• To be effective, your south wall has to have a good view of the south sky.
• You need a floor plan in which it makes sense to have windows in the south wall.
• It’s harder to distribute the heat into the parts of the house where you want it.
That’s a fairly large list of cons, but solving these problems is what passive solar house design is all about. If you do it right, the windows both collect heat and make the house a bright and pleasant place to be. In a retrofit situation, you just have to be lucky that you can place the windows where you need them for passive solar.
Solar Hot-Air Collector Pros:
• More flexible in location — south roof, south wall, even detached from house — and easier to find a location with good sun.
• Can move the heat from the collector to where you want it more easily than with windows.
• It is possible to store heat for later use (at the cost of more complexity).
• No night heat loss problem.
Solar Hot-Air Collector Cons:
• You lose the daylight and the views.
• Often have controls and fans that must be maintained.
• Typically they don’t look as good as windows.
It’s not really an either/or situation — you can (and should) use both on the same house. You can even mix them on the same south wall. I did not include cost as a pro or con for either option, since it can vary so much depending on how you go about it.