Dear Solar Guy,

I am an architecture student and have an assignment to design, fabricate
and install a solar chimney for attic heat extraction. Would appreciate
any information you can provide on the design of these.

Thank you,

Phillip Neal

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Hi Phillip,

We all know that "hot air rises", because it is less dense than
surrounding air. When the effect is used to ventilate buildings,
it is known as "stack effect".

Ventilation driven by stack effect can be very effective. I refer
you to the ASHRAE Handbook of Fundamentals for a discussion
and equations of the expected flow rates.

A JavaScript calculator that does the calculation can be found
at http://chuck-wright.com/calculators/stack_effect.html

The recommended method of attic ventilation is use of a continuous
ridge vent and a continuous soffit vent, combined with a radiant
barrier under the decking. Given the recommended free ventilation
areas and the ASHRAE equations, this approach gives only
marginal ventilation (but better than nothing).

Although I have no plans for solar chimneys, here are some things
to think about. Flow rate is enhanced by increasing temperature
difference and by increasing height. You will most likely place
an extended vent at the ridge of your roof, and arrange it so that
it will absorb maximum sunlight and deliver this to the air. A
problem you may encounter is facilitating this heat transfer without
impeding air flow. A poorly designed solar chimney could hurt more
than it helps. Of course, making it look good might also be a
challenge!