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


Posted September 3, 2008

An important aspect of green building is to provide sufficient insulation in your building.  Insulation is what keeps the heat in during the winter and the heat out during summer.  The most common forms of insulation are fibreglass batt (Pink Insulation), mineral wool batt (Roxul), cellulose, and foam. Two of the main things you want insulation to do is to slow the movement of heat, and to slow the movement of air.  You want to stop air movement, since if the air can move through the insulation, it will take heat with it.

The advantages to fibreglass and mineral wool is the ease of installation.  They are flexible and will allow you to friction fit them into the stud spaces in the wall.  Of the two, mineral wool has a number of advantages.  The first is that it is fireproof, and so will slow the spread of fire in a building.  The second is that it is made of molten basalt rock which is a very common rock that is easily quarried, and slag which is a recycled material.  It also is denser than fibreglass which reduces air infiltration and makes it less prone to slumping.  It is also waterproof, so if it gets wet, it will not lose its insulating properties.  If fibreglass gets wet, it turns into a sodden mass that has little or no insulating properties.  Some people also find Roxul easier to work with.

Cellulose is a recycled material made from ground up newspapers and borax ( a fire retardant and insect repellent).  It is very commonly used as an attic insulation as it is blown in, lowering the labor costs for installation.  It can also be used in walls, but must be either blown in wet, so it sticks together (this requires a special machine that is rarely available to do it yourselfers), or be blown in behind a mesh.  The advantages of cellulose is that it has a lower air infiltration rate than batt insulation, it will get into all the nooks and crannies in a wall, and it is a recycled material.  The major disadvantage is that in walls it is more difficult to install than batt insulation.

The third most common form of insulation is foam insulation.  This is available in a number of forms, including extruded polystyrene (solid boards, usually blue or pink), expanded polystyrene (made of many small beads that are stuck together) and sprayed in place foams (usually polyurethane, but there are others).  The extruded polystyrene (XPS) is one of the best solutions for places where the insulation will be buried, or have weight on top of it, such as under concrete floors in basements.  Expanded polystyrene (EPS)  can also be used for insulating underground walls and is commonly used in the manufacture of Insulated Concrete Forms (ICFs, will be discussed in a later post), however it is not as strong as XPS, but is generally cheaper.  Both XPS and EPS can be used in frame walls, but is generally used on the outside of the wall as an external insulation.  One of the best insulations for frame walls is sprayed in place foam, as it will form an airtight seal and will fill all the nooks and crannies in the wall, however, it is the most expensive and can only be installed by professionals.

In designing a energy efficient house, insulation will play a major role.  For the walls you will want at a very minimum 6 inches of insulation (R20), but more is better.  For ceilings go for as much insulation as you can up to about 18 inches (R60), with a minimum of 12 inches (R40).  There are ways to get more insulation in the walls and attic, which will be discussed in a later post.  Not only is the amount of insulation important, the way it is installed is also important.  Batt type insulation must be installed so that it fits snugly into the wall cavities, but it should not be compressed.  It is the air pockets in the insulation that provide the insulating properties, and if it is compressed, it loses this insulating property.  There must also be no gaps in the insulation, as this provides a direct path for heat to escape.

Air movement, also called air infiltration, must be minimized through the insulation.  Although some heat is lost by conduction through the insulation, most of the heat loss in a building is through air movement through the walls.  With batt insulation the air movement is stopped by the vapour barrier (a polyethylene plastic installed on the inside of the wall).  For the best performance, the vapour barrier must be made as air tight as possible by taping all the seams, using plastic boxes around electrical boxes, and caulking any penetrations through the vapour barrier by things such as wires and plumbing pipes.  The lack of air infiltration is one of the major advantages of foam insulation, as it stops almost all air movement as part of its nature.

Posted under Design