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. 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 it cools gradually during the night, it gives off its heat inside the building.
Trombe walls are a great 'passive' (as in no mechanical parts) way of providing heat to a room or space. 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.
An extra benefit of Trombe Walls is that the heat is radiated in the infra red, which is more penetrating and pleasant than the traditional convective forced air heating systems.
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, 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.
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
Variations on the Trombe wall design
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.
- 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.
- 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.
Important 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;
- Thermal 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.
Alternative Uses for Trombe Walls
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 use essentially the same principals to do work for you.
BTW if you are reading this page and know of any equations or formula that can be used to work out how to dimension/design a Trombe Wall - please let us know via the contact page - thanks!