Chelsea Green Publishing

Chelsea Green Blog

Celebrate the Winter Solstice by Using Windows to Heat Your Home

By James Kachadorian, author of The Passive Solar House, Revised and Expanded Edition: The Complete Guide to Heating and Cooling Your Home

Most people think that solar heating involves some sort of complicated roof top add on feature to a home. In a passive solar home, windows are the primary means of collecting free solar energy. South faced windows are extremely efficient solar collectors. Clear dual glazed (double pane) windows allow up to 91% of the incident sunlight to pass through them. Once the sunlight is in the home and strikes an object, the sunlight turns to heat. The heat is then trapped in the house. This is referred to as the “greenhouse” effect. The greenhouse effect, as it relates to our planet, is written about extensively as a negative effect but the greenhouse effect is quite beneficial to a passive solar home. A passive solar home works without any mechanical assist; that is, a passive solar home collects solar heat and stores the heat for use when the sun goes down. The low angle of the winter sun “turns” vertical south faced windows “on” as solar collectors and the high angle of the sun in summer “turns off” south faced vertical glass in summer. Today is the winter solstice which means that the sun is at its lowest angle in the sky. At north latitude 40 degrees, a south faced window collects more than twice the amount of solar heat in December than it does in June. As can be seen in the solar home pictured, the low winter sun will penetrate the home 22’ at solar noon on December 21 and conversely will only penetrate inches in the same home at solar noon, June 21.

All the home owner has to is face the home true south and properly size the amount of south faced glass. Too much south faced glass will actually overheat a solar home in winter. Too little glass will make the home dark and cave like. The idea is to have just the right amount of glass for the size and location of the home.

We can expect a passive solar home to be about 50% efficient in northern New England and the 60 – 70% efficient in Virginia. This means that up to half the heat needs to come from another source. Recently wood pellet stoves have become a popular way to provide the balance of the heat needed in winter. Wood pellet stoves are an attractive option for several reasons:

  •   Wood pellets are made from wood products that normally go to waste.
  •   Wood pellets burn very clean and are a renewable source of energy.

The stove pictured has a rear hopper into which the pellets are loaded. The stove has two small electric blowers – One to circulate the heat and one to feed the pellets into the burn chamber.

Another nice advantage of the wood pellet stove is that it runs automatically. Slight disadvantages are that you can hear the small electric motors running and the stove does need electricity to run. A stove such as the one pictured will use about 100 watts an hour to run amounting to pennies a day in electrical cost. For those of you that are using my book, The Passive Solar House, to calculate the amount of wood pellets needed per season, select the wood back-up heat option on the included CD. Determine the number of cords of wood needed and multiply it by 2/3 to obtain the number of tons of wood pellets needed per season.


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