Post date: May 9, 2015 2:25:56 AM
The heating method is borrowed from Vermont. I read a book on Passive Solar by James Kachadorian, who had a business for 20 years building ‘kit houses’ using his ‘solar slab’ in a factory heated by the same method. Basically the sun shines in windows and heats objects and the air in the house. This heat is stored in the floor slab.
True passive solar heat requires the sun to shine on the concrete floor. With the ‘solar slab’ a furnace fan pulls that warm air under the concrete floor slab through concrete blocks laid on their side - so the holes line up to form air ducts.
At night the moving air warms the house. This method was developed 40 years ago, and Kachadorian’s book has a lot of what is now passé information. My house will be better insulated than his were and the furnaces are more advanced now - with continuously variable fans and gas burners and very sophisticated controls.
No, the method is not totally passive. It does require an electric fan running continuously. In the garage there will be a fan but no furnace, and the garage airflow is completely separate from the house (by fire code, too).
My heating engineer was skeptical at first, saying (correctly) that it is easier to pump water than to pump air. But a solar water system requires about $10,000 more investment — with ugly panels on the roof and a means of keeping the water in the panels from freezing. In addition, water tubes in the concrete floor (hydronic heating) is much slower to respond to demand for more heat. (It took the house I’m staying in two full days to rise from 50 to 70 degrees.)
However, I figure in a few years batteries will be cheap enough so that I can install photovoltaic (PV) panels on the roof to supply my base electric needs: furnace fan, Energy Recovery Ventilator (ERV), and refrigerator. And I will have a record of my electricity use so that I can size the system based on my actual use. With batteries I won’t depend on a subsidy that is paid by the poor (who cannot afford PV panels).
We have improved on Kachadorian somewhat. He used sheet metal ducts under the floor to distribute the air from the floor registers to all the concrete blocks. We put a baffle in the row of ducts with precisely sized holes to ensure even airflow - which will be less expensive and quieter.
I will be running the airflow downward through the rooms — the opposite of most hot air systems but the way it is done in hospitals to settle the dust rather than float it. A drawback to this is that either the ducts fill with dust or we install filters in every floor register (a pain when it comes time to change all of them) rather than a single filter at the furnace intake. But water-heated floor slabs do not filter the air at all.