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Heat free with passive solar Trombe wall

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A Trombe (rhymes with prom) wall creates a convective loop using a glass covered wall facing the winter sun and a vented thermal mass wall. Air heated by the sun rises through the vents in the wall causing cooler air to pull into the space through vents in the bottom of the wall.

The wall works best when made with fairly high thermal mass materials like concrete, brick or stone usually painted black on the exterior to absorb solar heat. The glass is set in notched wood frames about 2 to 5 cm from the mass wall. Heat is absorbed and lost at the rate of one inch every two hours as a rule. So a four inch thick mass wall will absorb and lose its heat in about 8 hours, or 8 inches in 16 hours.

Trombe walls, also known as solar walls, have vents at the top of the wall to let heat flow into the living space. At the bottom vents the living space's cooler air flows into the Trombe wall air compartment. Vents can have flaps to keep convection from going in the wrong direction as when the wall space cools during nighttime in colder months. In hot summer climates, vents can be external to dispose of hot air at night and internal vents are left closed. When properly designed with proper roof overhangs to shade the air space from summer sun, it will not get heated in the hot summer months. Trombe walls can be left unvented and depend on conduction through the wall to heat interior living space.

A Trombe wall works best in areas that receive days of direct sunlight with high diurnal (from day to night) temperature swings and few clouds in winter. One installed at Zion National Park’s Visitor Center in Utah (see pictures on the website) provides most of the heating for the building.

The advantages of a Trombe wall are:

  • it provides environmentally-friendly free heat
  • it does not use fossil fuels
  • it is quiet and reduces drafts
  • it creates evenly distributed, natural radiant heat
  • it does not blast dust and allergans through ductwork so provides better indoor air quality
  • it uses existing wall space
  • it is simple to build yourself
  • it is quite effective
  • to be even more environmentally-friendly, it can be made of rammed earth, recycled materials like recycled glass, crushed recycled concrete or urbanite, and painted with zero VOC paint. Concrete is not a green product.

The disadvantages are:

  • it can be expensive to install, especially if the house needs modification for the added weight
  • it must be carefully designed, preferably by an expert
  • it uses up wall space where windows could be and blocks any views.

Menelaos Xenakis from EcoArchitects estimated the cost of 12-inch thick concrete Trombe house wall at $734, including support systems, vents, and concrete. The glazing window would add another $912 for a total of $1648. Building codes would need to be checked because some wood framing could not support the added weight of a masonry wall. Calculate the new weight load to see if the house needs to be modified to meet code which would add more cost.

Edward S. Morse first patented the idea in 1881. The Trombe name comes from French engineer Félix Trombe who with architect Jacques Michel developed the concept in 1964. Interest in Trombe walls increased in the United States in the 1970s in large part to research from New Mexico's Los Alamos National Laboratory.

There is a good pdf of the July 2004 pamphlet Trombe Walls in Low-Energy Buildings: Practical Experiences.

Clark Snell mentions Trombe walls in his book Building Green with Tim Callahan and would be the expert in western North Carolina in the Asheville area. Contact Clarke on Facebook.

Watch the attached video where Sister Nancy Hoffman explains how the Trombe wall at Prairie Woods works. There is also a YouTube video of how Keven Bell retrofitted his house with a rough Trombe wall using 100 percent recycled materials.

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