Trained Eye

 
 
 

HRV Operation

Question:

I bought an older house, blew cellulose into the attic and installed a high efficiency gas furnace. The result is high level of humidity. Please explain in basic terms how an HRV works? I have had advice on how to build my own version of it. I could buy a pipe fan, place it on the basement floor and blow the cool basement air outside, and also install a fresh air intake. The pipe fan would be regulated by a humidistat. Am I on the right path?

Answer:

This is an excellent question you have put forth, as Heat Recovery Ventilators (HRV) or Energy Recovery Ventilators (ERV) have been widely used for many years across the country, but have not been very popular in our area in the past. This is rapidly changing, as homes become tighter and indoor air quality concerns are more in the forefront. Still, most people, like yourself, do not know the exact function or use of these systems. I will try to give you a brief, simplified description of the way an HRV works.

What an HRV does is provide a regular exchange of stale house air with fresh outside air, which accomplishes two main objectives. Firstly, the stale house air may contain contaminants and pollutants from cooking, cleaning, bathrooms and other normal household activities. This exchange removes these unwanted products from the home and replaces the stale exhaust air with fresh air from outside. Secondly, the warm heated air in the home often contains a high amount of moisture, relative to outside, again due to cooking, cleaning and bathing. The fresh air brought in through the ventilation unit will be relatively dry, in comparison, and will help to reduce the Relative Humidity (RH) within the home.

The homemade method you suggest will probably help to reduce the RH in your home, somewhat, but is flawed in a few different ways. The first problem is that you are only going to be drawing air from the basement, which is not the source of most of the moisture in the home. A properly installed HRV will be connected to the main ducting of the furnace as well as exhaust vents in the bathrooms and often the kitchen. This type of arrangement will draw air from the rooms with the highest source of added moisture as well as the entire heated area of the home. In this way it will be effective in complete removal of excess moisture and contaminants from the whole house, not just the basement. Often the HRV controls are connected to the furnace, so that the small fan within the unit operates in conjunction with the furnace blower fan. This allows enough pressure to move the air through the entire house, a task not possible with your small fan or the one installed in the HRV, alone.

In most newly built homes they have a system that is halfway between your suggestion and a true HRV. They have a central exhaust fan connected to the bathroom exhaust vents that blow the stale air outside. This is often attached to a humidistat and another control that allows regulation of the volume of air moving through the system. There will also be an insulated open duct, near the furnace, that brings fresh air into the home. The volume of fresh air will automatically increase when the exhaust fan is in operation. While this system is superior to older homes with isolated exhaust fans and no controls for balancing incoming and outgoing air, it has the same fundamental flaw as your proposal. They are both very inefficient in relation to energy conservation and heat loss.

A Heat Recovery Ventilator is so named because it is designed to recapture a substantial portion of the heat from the warm air that is exhausted from the home, before it is blown outside. This is accomplished by passing both the warm air leaving the home and the cold air coming in through a heat exchanger. This heat exchanger draws some of the heat from the exhaust air and transfers it to the cold incoming air, reducing heat loss considerably. This heat exchange also prevent condensation in the furnace ducts due to the introduction of freezing cold air, had this air not been preheated.

To work effectively a properly installed HRV must be properly balanced. To achieve this balance, the amount of fresh air entering the home should be roughly equal to the amount exhausted. This will prevent a low or high pressure situation in the home, relative to the exterior, which can have major effect on the building if this pressure differential becomes too large. If the pressure is too low in the home, more cold air may be drawn in through cracks, including doors and windows, cooling the home and reducing energy efficiency. If the pressure is too high, warm air may be forced into the walls and attic of the home causing moisture problems and damage to building components.

Installing homemade contraptions, like the one you suggest, often do more damage to the home than any problems they may solve. These problems may be undetectable for many years, when the damage is discovered. Properly installed and balanced HRV units should be installed by qualified HVAC contactors specifically trained in design and installation of these specialized systems. More information can be found in an excellent booklet produced by the Natural Resources Canada Office of Energy Efficiency (OEE) entitled Operating and Maintaining Your Heat Recovery Ventilator. This booklet can be viewed and ordered free from their website at www.oee.nrcan.gc.ca.