Passive Stack Ventilation & Solar Chimneys

rene26

Hi all,

I was wondering if anyone has any experience installing "chimneys" to aid in the ventilation of houses. I often work with an organization in underdeveloped countries (which have year-round hot whether, and where AC is not an option), and we often find that houses retain a ridiculous amount of heat during the evening hours and nighttime, where outside it is nice and comfortably cool.

I was introduced to the concept of passive stack ventilation and solar chimneys as an effective method of ventilating the hot air, and I thought the idea was pretty cool. So we put it to the test. A friend of mine has a one story house, about 40' x 40', concrete construction. He cut two chimneys into his roof, each with a cross-sectional area of about 18" x 18", and each of a total height of 4'8".

We were a little disappointed when, upon trying them out for the first time, we didn't notice a big difference. Sure, some air was being pulled up, but the house remained notably hotter inside than the outside temperature throughout the evening.

So I'm wondering how to help pull in more of the cool evening air. If we raised the chimneys to say, 7' each, would that greatly help? Is there an optimal ratio between cross-sectional area and chimney height to encourage maximum air flow?

Most of the walls have windows which extend from the ceiling height to about halfway down the wall. I realize that it is preferable to have lower inlets, as a greater distance between these and the chimney outlet helps a lot, but we're kind of stuck working with what we've got.

Does anyone have any advice? I'd greatly appreciate and thoughts/comments. I'm all ears.

Cariboocoot

Re: Passive Stack Ventilation & Solar Chimneys

Wow. It's been a long time since I've seen that technology used. It used to be a method for making a "cold room" in houses before refrigeration; wouldn't quite drop the temp to what we consider "safe" levels by today's standards but it would be cooler than the rest of the house. I have heard of some people, usually of the environmentalist type, doing this in the hot climes of the Southwest.

I think you understand the basic principal: using the temperature differentiation to cause a draft that draws cooler air in and exhausts warmer air. But you have to be able to control both inlet and outlet for it to work properly. If the main of the house is open all over the place the air will have all sorts of pathways to follow and up the chimney may not be the one it picks. It's similar to a wood burning stove in operation, but at much lower temperatures.

Maybe there's some plans on Build It Solar http://www.builditsolar.com/ of this sort of thing.

Dill

Re: Passive Stack Ventilation & Solar Chimneys

not to mention that your friends home is made out of concrete, which will retain the day's heat for a long time into the night before it cools off.

bluetick

Re: Passive Stack Ventilation & Solar Chimneys

Reminds me of cellar vents, Having one taller then the other, theory behind it as wind speed is greater and less disturbed by the ground at the higher level allowing it to draw the air out of the building.

Then look to nature for an answer;

http://www.asknature.org/strategy/e27b89ebcdec8c9b5b2cd9ac84b8f8a0

niel

Re: Passive Stack Ventilation & Solar Chimneys

Dill wrote: »

not to mention that your friends home is made out of concrete, which will retain the day's heat for a long time into the night before it cools off.

i agree with dill as you must try to prevent the concrete from absorbing that solar heat during the day in the first place. it might be a matter of vegetation shading the walls or painting the walls white to adding insulation and all 3 of these can be used simultaneously. i have my own place with double walled brick that really gets hot in the sun and everybody knows bricks make for good ovens, but concrete is similar in this respect as a holder of that heat.

westbranch

Re: Passive Stack Ventilation & Solar Chimneys

Another thing that can be done is to allow the top of the 'stack' to be heated, black paint, to increase the flow rate through the entire stack.
We saw a 3 story hospital in Cuba that was a comfortable temperature due to air movement in the stack, which was the entire 100 x100' courtyard of the hospital. The building was built on pillars and 4 feet off the ground. NO fans or other mechanical ventilation, just the sun...

Cariboocoot

Re: Passive Stack Ventilation & Solar Chimneys

Because I can't resist poking around the Build It Solar site: http://www.builditsolar.com/Projects/Cooling/passive_cooling.htm#Towers

GaryGary

Re: Passive Stack Ventilation & Solar Chimneys

Cariboocoot wrote: »

Because I can't resist poking around the Build It Solar site: http://www.builditsolar.com/Projects/Cooling/passive_cooling.htm#Towers

Really glad to hear you can't resist poking around the BIS site

I think that one thing that would help the performance of the solar chimneys would be to glaze the south side (or north in the sourthern hemisphere) and then painting the north inside of the chimney black. This would heat up the air inside the chimney and increase the stack effect.

Another thing you could try is to use a thermosyhon collector with the inlet vent inside and the outlet vent outside. So, this collector is what I use to heat my shop: http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
If you just take the same design, but instead of having the upper vent into the room have it outside --t his will pull air from in the room through the collector to the outside. On this collector, the outlet velocity in good sun is about 140 fpm, and the total exit airflow is (5 sqft)(140 ft/min) = 700 cfm -- that is, a total of 5 sqft of exit vent flowing at 140 fpm gives 700 cfm -- not bad.
Of course, the sun has to be on it to get this.

Agree with others that some form of shading on the outer walls would probably help a lot -- vines, trellis, shade sails, ...

Gary

ggunn

Re: Passive Stack Ventilation & Solar Chimneys

Why not electrically heat the stack to provide more convection?


I'm joking, of course.

inetdog

Re: Passive Stack Ventilation & Solar Chimneys

ggunn wrote: »

I'm joking, of course.

Then you should use the unofficial satire font to make it clear. (My pet crusade.)

ggunn

Re: Passive Stack Ventilation & Solar Chimneys

inetdog wrote: »

Then you should use the unofficial satire font to make it clear. (My pet crusade.)

Sorry, it doesn't mean that to me; it's just another font. How about Zapf Dingbats?

Cariboocoot

Re: Passive Stack Ventilation & Solar Chimneys

You're not allowed to make jokes on here unless you've been issued the Official Forum Jester's Cap and Bells.

inetdog

Re: Passive Stack Ventilation & Solar Chimneys

ggunn wrote: »

How about Zapf Dingbats?

Hard to read! :-)

Comic Sans MS is the funniest font I could find in the list.

ggunn

Re: Passive Stack Ventilation & Solar Chimneys

inetdog wrote: »

Hard to read! :-)

Comic Sans MS is the funniest font I could find in the list.

Well, it does have "Comic" in its name, but other than that, to me it just looks like another font.

ggunn

Re: Passive Stack Ventilation & Solar Chimneys

Oh, right, Bells. At first I thought you said something else...