Heat Recovery Ventilators

Posted October 3, 2008

In modern airtight homes, stagnant air and high humidity can become problems particularly in the winter when the windows and doors are closed. In older houses, the natural air leaks would cause the air in the house to be exchanged with air from outside, however, newer homes don’t have that natural air exchange.  In order to bring in fresh air, a device called a heat recovery ventilator (HRV) is used.  The HRV will bring in fresh air from outside and heat it up using the warm stale air from inside.

The HRV will have two fans.  One will draw in fresh air from outside and the other will push out stale inside air.  The fans push the air through a chamber made of series of air passages in which the direction of airflow alternates.  These air passages are separated by a conductive membrane, aluminum in the better units, that allow the heat from the warm air to transfer the colder air. In a house with forced air heating, the warm air is drawn from the return air duct and fresh air supplied a few feet downstream.  A better system draws the stale air from the kitchen and bathrooms, which are the main source of pollutants and humidity, using a separate duct system.  In a house with hot water heat, a full ducting system is used with the warm air  drawn from the bathrooms and kitchen and the fresh air is returned in the bedrooms and other living areas.  In the winter, since the fresh air is dryer, it will drop the humidity in the house down so there is little or no condensation on the windows

When looking at an HRV there are two main factors to consider in terms of efficiency.  The first is the sensible efficiency, which is the amount of heat moved from the warm air to the cold air and can vary from less than 40% to more than 80%.  A higher number is better.  The other factor is the amount of electricity the unit uses.  The units can vary from under 40 watts to more than 200 watts for similar sized units.  This is mostly due to the type of motors used in the units.  You want the unit that uses the lowest watts.  A listing of most HRVs is available from www.hvi.org

The HRVs are sized according to the size of the house, with a larger house needing a unit that can move more air (higher Cubic Feet per Minute orCFM ).  This can be determined by taking the square footage of the house and multiplying it by the ceiling height, giving the volume of the house.  This is then multiplied by the desired air changes per hour (usually 0.3 -0.5) and then dividing by 60 to get CFM.

The first winter we were living in our house, we had not yet installed the HRV and had condensation problems so bad that we had to have towels at the bottom of all the windows.  Since we have installed the HRV, we have had next to no problems with condensation, except for the coldest days, and even then it is minor.  I selected the Lifebreath HRV as I found they had the lowest power usage and one of the best sensible efficiencies.

Posted under Devices

Passive Solar Shading Options

Posted October 1, 2008

In a passive solar design, you need to have shading on the south facing windows during the summer and have direct sun on the windows during the winter.  There are a few different ways to accomplish this.

One way is to plant deciduous trees and shrubs on the south, east and west side of the building.  During the summer the trees will shade the building and block the heat from hitting the house.  It also can have a cooling affect around the building due to the transpiration from the plants.  In the fall and winter, the trees will loose their leaves and will let the sun shine in, heating the house.  The advantage to using trees is that they closely track the temperature changes during the seasons, with the leaves budding in the spring once it has warmed up and falling off in the fall once the temperature has dropped.  The disadvantage to using trees and shrubs is that the woody mass of the tree will always shade the building, reducing the amount of sun in the winter.  The trees also take some time to grow to the point where they will be an effective shade.  They can also be a problem if solar panels are installed on the roof and are shaded by the trees.  This last problem can possibly be avoided by having the solar panels ground mounted beyond the shade of the trees.

A second way to shade the house is to use overhangs. You can calculate the depth of the overhang by finding the angle of the sun at the summer solstice – June 21 (90° – latitude + 23.5° = ss) and winter solstice – December 21 (90° – latitude -23.5° = ws).  Then take the distance from the bottom of the window to the bottom of the overhang (wh) and the distance from the bottom of the overhang to the top of the window (oh).  Then use the formula wh/tan(ss) to get the best overhang for the summer solstice and the formula oh/tan(ws) to get the optimum overhang for the winter solstice.  You want the overhang to be less than the winter solstice calculation and more than the summer solstice calculation.  For an example, I will use a house at 44°N latitude. The bottom of the window is 78″ from the bottom of the overhang and the top of the window is 18″ below the bottom of the overhang.

The angle at the summer solstice = 90° – 44° + 23.5° = 69.5°
The angle at the winter solstice = 90° – 44° – 23.5° = 22.5°

Summer overhang = 78″/tan(69.5°) = 78″/2.67 = 29.2″
Winter overhang = 18″/tan(22.5°) = 18″/.414 = 43.5″

So the overhang should be between 29.5″ and 43.5″.  Since you want shading for some time on either side of the summer solstice add about 6″ to the overhang.  In this case I would use a 36″ overhang, which would give complete about 6 weeks on either side of the summer solstice.  The disadvantage of a set overhang is that the temperatures are not the same the months before and after the summer solstice, which means there is more shading than you want before the solstice and not enough after the solstice.  The advantage is that you have the shading immediately after the house is built, and the shading is predictable.

Another option would be to build a trellis overhang, which would be a hybrid of the two above systems.  The trellis which would be built the same depth as an overhang would give some shading by itself, but if it is covered with a deciduous vine, such as grape, the leaves would give additional shading during the summer and in the fall the leaves would drop and give you more light before the winter solstice through the holes in the trellis.  You would have the added bonus of grapes to harvest.

Posted under Design

This post was written by Ward Edwards

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