> What is convection and how it effects your heating costs

Trombe Wall, what is it?

A Trombe wall simply put is a very thick, south-facing wall (or North facing wall if in the Southern Hemisphere), which is painted black and made of a material that absorbs a lot of heat (say concrete, stone or brick). A pane of glass or plastic glazing, installed a few inches in front of the wall, helps hold in the heat. The Trombe wall heats up slowly during the day. Then as the wall cools gradually during the night, it gives off its heat inside the building.

Trombe walls like this work via the basic greenhouse principle that heat from the Sun in the form of higher-energy ultraviolet radiation passes through glass largely unimpeded to heat up the material behind it. 

Trombe walls are a great 'passive' (as in no mechanical parts) way of providing heat to a room or space. plus they are completely silent in operation. They are often easily built from readily available materials, very reliable, and of course cheap to operate. Trombe walls can successfully contribute to reducing heating and cooling costs all year round if set up correctly. As a result they are one of the core techniques used in Passive Solar Architecture.

Trombe Walls have an extra benefit, in that the heat is radiated in the infra red, which is more penetrating and pleasant than the traditional convective forced air heating systems.

Trombe Wall, who invented them?

Trombe walls are named after the French inventor Felix Trombe, due to his work on them in the late 1950s.

Trombe Walls, how do they work?

With reference to the diagram on the right. The simplest form of Trombe wall consists of a glass pane held against a wall with an air space behind it. Connecting this air space with the inner room are two vents, one at the top and one at the bottom of the air space. During the day the Sun heats first the air in this space, then the solid wall behind. Once the air is heated it rises and enters into the room, giving it additional heat. Also the rising air pulls in cooler air from the room below to then be heated. The real trick with this though, is that for sometime after the Sun goes down the now hot wall will still keep heating air and exchanging that heat into the room.

Though once the wall is cold you need to stop the cold of the outside interacting with the inside of the property, so a one way flap is used on the bottom vent to stop the cold coming back into the room and creating a cooling cycle with the room.

The great thing with a Trombe Wall, is if you have an existing solid wall in the right place, then it can be converted to a Trombe Wall by the addition of vents and external glazing - so this is great energy saving improvement to an existing building. Although remember the wall has to be solid, this won't work with double brick walls, they will require extensive modification.

Note: There is of course an assumption here that the wall is facing directly to the Sun and has good Solar access (i.e. no trees, building or structures that obstruct access to the Sun as it tracks across the sky).

So how do you stop the Trombe wall heating the room in Summer?

This is where the roof overhang comes in - if it is deep enough the higher Summer sun will be able to heat the glass. Although by doing this you a missing a cooling trick...  Basically if an opened vent to the outside is put at the top of the outer side of the air space and you only open that and close the top wall vent in Summer, you create a Solar Chimney that sucks air out of your house to the outside, giving you ventilation for free!

Trombe wall design, variations

There are several ways you can change or improve on the simple Trombe Wall design to suit your circumstances:

• Use double glazing - basically this makes the solar collection of the wall a lot more efficient as more heat is trapped in the inner air space.
• Use low-e glass - this is special glass which has a lower rate of heat transmission, similar in effect to double glazing. Although do check that you still get a usable level of solar transmission through it for your purposes; this could be a useful trade off if you find yourself loosing heat at night.
• Forced fan - basically using a fan to drive air through the air space, this improves the rate of air exchange. Often controlled by a timer and a thermostat.
• Dark colour - if you paint the inner wall black or a dark colour its heat gain potential will improve drastically.
• Movable blinds - these are placed over the glass and used to limit solar gain and can help with reducing heat loss at night.
• Tubes or water tanks as part of a combined solar hot water system.
• Fish tanks used as a form of additional thermal mass.
• Insulation - if the wall areas not behind the glass are insulated on the outside it avoids heat loss at night, improving the longevity of heat gain after dark.
• Sunspace (conservatory) - in effect the glass area in front of the Trombe Wall is sloped to create a space with two surfaces, the Trombe Wall itself and the floor area in the Sunspace. They can heat spaces both through radiation and convection.
• Half-height Trombe Wall - there is no requirement that a Trombe Wall must be full height; rather you can have a Trombe Wall be half height to allow light and direct solar gain into the adjacent living space. A half-height Trombe wall is a simple way to increase solar storage capacity in a passive solar home, whilst still allowing views of the winter Sun. Such half-height walls are usually constructed 4 to 6 inches (100-150 mm) from the inner window surface, such a gap allows blinds or curtains to be used to reduce heat loss on Winter nights and heat gain during the Summer.
• Underfloor ventilation - the air flows through channels in a floor, which warms the floor and then the building. This also makes good use of the thermal mass of the floor to even out temperature changes. See Barra System .

Trombe Wall design challenges

Trombe wall construction guidelines to remember

• The space between the thermal mass wall and the glass should be a minimum of 4 inches;
• Vents used in a thermal mass wall must be closed at night;
• Trombe wall thickness should be about 10-14 inches for brick, 12-18 for concrete, 8-12” for adobe or other earth material and at least 6 inches for water. For a 16 inch thick brick wall it will take about 8 to 10 hours for the heat to reach the interior wall surface.

Trombe Walls alternative uses

Trombe walls can also be used to create ventilation in sub floor spaces. If you have the height in the sub floor space set it up so that the top vent goes into the sub floor space and that the bottom is open to the outside (instead of the inside). In effect you have made a solar chimney feeding into the sub floor space - this will raise the average temperature in the sub floor area that should lower the relative humidity. Also you can use a basic Trombe wall as a solar chimney to "suck" air out of a space, basically bottom goes into the sub floor space and the top vent is outside. These are not true Trombe walls, but essentially use the same principals to do the work for you.