So, this passive solar home had a lot of features that were good in theory but need some changes in practice. The rock pile is the main one.
Theory: The rock pile is supposed to gather solar heat during the day and vent it into the living room (via floor vents) at night. There are also huge cold air returns to keep the air circulating. There are vents or louvers from the collector to the rock pile, and louvers on the hot ducts (but not the cold air ducts), again, to direct the flow of air.
Reality: There are several issues.
- All ducts blow air all the time. This is especially bad on windy days. Even the cold air “returns” have air blowing out of them into the living room. This is not so bad in the summer – sort of a natural fan, if a bit humid – but in the winter, it’s awful. Video of vent blowing air
- The rock pile doesn’t ever get warmer than about 65-70 degrees in the cold weather. It was about 80-85 in the summer – but you don’t want it hot then, of course. This may be because the louvers from the collector to the rocks are stuck in the “closed” position – we can’t tell until we open up the collector, and that’s not happening this year. So, we’re stuck with a 65 degree breeze, which doesn’t feel warm at all – it just feels drafty.
- The louvers on the hot air ducts don’t fill the entire duct space. So, if the duct coming off the rock pile is 48″ wide, the louver mechanism might only be 40″ wide, leaving a huge gap with no way to control the flow of air.
- There’s a large (1″) gap between the concrete slab and the wood floors. Meaning the cold air can circulate under the floor at will.
- Snakes really like the rock pile. And also our living room.
The workaround: For the moment, we are wedging rigid foam insulation into all the ducts to seal off the rock pile from the rest of the house, and we’ll probably spray some foam in the gaps between the ducts and the floor. Next summer, we will probably take the solar collectors apart (they need to be re-glazed desperately) and we’ll see what we can see while we’re down there. We might be able to get it functioning better, but I suspect this is one of those “better in theory than reality” items.
Fortunately, the solarium (“sunroom,” “greenhouse”) works incredibly well, building up heat during the day and releasing it to the house. For first-floor heating, we just open the French doors wide. The vents to the upstairs need some work (we need to be able to seal them tightly and open them easily), but if it’s sunny and at least 40, we don’t need any additional heat in the house during the day. If it hits 50 and is sunny during the day, we can actually go several days and nights without supplemental heat, even if it’s in the mid-20s overnight.