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Sunday, October 2, 2011

Banjo cooling house with attic!

Important concepts:
 - Heat rises.
 - Air flow requires a high-pressure point and a low pressure point.
 - A closed room doesn't have air flow - it's like blowing air into a paper-bag; once the bag is full of air, no more air can be blown in.  A hole in the other end will allow air flow.

We live in the south, so our home picks up a lot of heat during a hot day, so even after the sun goes down, we need to cool the house.  It would be nice to open the windows for flow-through ventilation, but that frequently isn't enough cooling to remove the heat, and our whole house cooling fan is so loud we don't like to run it.

A lot of evenings, we would like to switch from our air-conditioned cooling to outside air, but we lack enough air-flow to perform this activity.  Opening windows on both floors allows some flow-through ventilation, and some draft from the first floor windows up to exhaust through the 2nd floor windows, but without any wind or breeze, it's not enough.

Recently, it dawned on me to open up our attic's access door on the 2nd floor.  By opening this door, it allows hot air to flow out of the attic.  However, if the rest of the house's windows are closed, then the only air flow into the attic (if air is flowing out, then there has to be air flowing in, otherwise you would create a vacuum) is from the attic's little vents around the bottom underside of the roof - the outside air vents (there's a word for these, but right now, I can't find it).

So, if you open the attic's access door, and open the downstairs windows, then hot air will flow out of the attic, and air will be pulled in from the windows to flow into the attic.  So you will have increase airflow coming into the house, cooling the rooms where the windows are located, and also removing the heat from the attic, which can get really hot in the summer.

So it's a win-win:
- increased cool air flowing into the rooms from the windows
- very hot attic gets cooled, reducing the thermal heat pushed into the ceilings of the upstairs rooms from the attic/ceiling.

- Your house will not be any cooler than the outside temperature.  There can't be any lowering of temperature, just increase airflow of outside air.
- The attic, being the highest point in the house, and also the hottest point in the house, creates a chimney effect and actively pulls air into the out and exhausts it through the air vents in the top of the attic.
- You can 'tune' the amount of air coming into any room.  At night, in order to increase the airflow into and through our bedroom, we close off all other windows in the house, leaving the windows in our bedroom open.  This means all of the air that is exiting through the attic is pulled in through our bedroom.
- for air to flow into our bedroom and out through our attic, it requires our attic access door (outside our bedroom, in the hallway of our 2nd floor) to be open, so air can flow up into the attic from the 2nd floor.
- for air to flow out of our bedroom, it has to be able to get to the attic.  That means our bedroom door has to be open so the air can come in through our bedroom windows, flow out through our bedroom door, flow up to the attic access door, and up and through the attic to exit on our roof vents.

Using this, except for really hot nights, we've been able to significantly reduce the use of our airconditioning on summer nights.  In addition, it has allowed us to open up the house on days that aren't too hot and use the attic to increase the airflow into the house via the windows.

Hope this helps!

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