The HRV System

Components of an HRV System

An HRV system generally consists of the following equipment:

• insulated ducts for incoming (fresh) and outgoing (stale) air, along with exterior hoods;
  
• ductwork to distribute fresh air throughout the home and to return stale air to the HRV;
  
• fans to circulate air throughout the home and to exhaust stale air to the outdoors;
  
• a heat-exchange core, where heat is transferred from one air stream to the other;
  
• filters to keep dirt out of the heat-exchange core;
  
• a defrost mechanism (some units use a preheater) to prevent freezing and blocking of the heat-exchange core when the temperature of the incoming air is cold (not shown);
  
• a drain to remove any condensation from inside the HRV (may not be required with all models); and
  
• operating controls to regulate the HRV according to ventilation needs.

Figure 1
Components of an HRV

 
(Note:All the parts shown here may not be found on all HRVs.)

During the heating season, an HRV recovers heat from the outgoing, stale household air and uses it to preheat incoming, fresh outdoor air. The HRV then distributes the incoming air throughout the house.

In the example shown in Figure 1, the flow of air in and out of the house takes place simultaneously (note: the path of the airflow may vary from one type of HRV to another). The two air streams are always kept separate within the HRV. Fresh outdoor air is filtered before it enters the HRV core, where a circulation fan distributes the air throughout the home via ductwork. A separate ductwork system draws the stale indoor air back to the HRV, where it is filtered and pushed by a fan through the heat-exchange core. Here, the stale air releases heat that is transferred to the fresh air being drawn into the house.

During the air-conditioning season, the HRV reverses this heat-exchange process, removing some of the heat from the incoming air and transferring it to the outgoing air (see the box on Energy Recovery Ventilators).

Air Distribution

There are two standard ways to distribute fresh air throughout the home – through ductwork installed specifically for the HRV (direct ductwork) or through the ductwork of a forced-air furnace system.

A direct-ducted system is commonly found in homes that do not have forced-air heating, such as those with electric baseboard, hot water or radiant heating. In this case, the fresh air is distributed through ducts to the bedrooms, living room, dining room, basement and other rooms, where it disperses throughout the house (see Figure 2). Exhaust air ducts take the stale air from rooms that have high moisture and pollutant sources back to the HRV and from there to the outdoors.

Figure 2
An HRV with direct ductwork

An HRV can also be installed to work in conjunction with a forced-air furnace system, as illustrated in Figure 3. In this case, the HRV's fresh-air duct is connected to the furnace's main return air duct. The fresh air enters the furnace and is distributed throughout the house using the regular system of ductwork. In such a configuration, the furnace blower should run continuously at low speed when the HRV is operating to ensure a regular flow of fresh air throughout the house. The furnace blower can also be wired to operate at the normal higher speed for heating and cooling. However, this higher speed can be noisy and may make the rooms feel cool if used to distribute ventilation air continuously. Separate, additional ductwork may be needed to transfer stale air from the damp rooms to the HRV (as shown in Figure 3).

Figure 3
An HRV installed in conjunction with a forced-air furnace system

 
*Furnace return air may come from more than one location.

Checking Airflow Room by Room

As a general rule, a direct-ducted system should provide at least as much airflow per room as shown in Table 2. An HRV system connected to a forced-air furnace distributes the fresh ventilation air to each warm air supply duct proportionally. Keep in mind that if you adjust the dampers in a forced-air heating system to increase or decrease ventilation airflow, you will also affect the amount of heat (cool air during summer) that reaches the room.

The Canada Mortgage and Housing Corporation has developed a simple technique to determine if each room connected via ductwork to an HRV system is being provided with the correct amount of fresh air. The technique requires a common household plastic trash bag (66 cm x 91 cm [26 in.x 36 in.]), a wire coat hanger, tape and a watch. Twist the coat hanger into a rectangular shape and tape the open end of the trash bag around the wire. Gently deflate the bag, place it over the air register and time how long it takes for the bag to fill with air.

Table 3 provides the approximate relationship between inflation times and airflow rates. Although this technique is not precise, it will help you estimate the rate of airflow to each room and allow you to make appropriate adjustments.

Table 3
Time to inflate a plastic trash bag
(66 cm x 91 cm [26 in. x 36 in.]

The Need for a Balanced System

HRVs are designed to operate in a balanced state – the same amount of air should be drawn into the home as is being exhausted. An unbalanced system results in poor airflow and poor heat recovery and can lead to other problems, including an undesirable, continuous air-pressure difference between the inside and outside of your home.

System imbalance is usually caused by differences in the amount of ductwork used in the fresh air and exhaust air streams; however, imbalanced airflows may also be caused by a clogged filter, a blocked intake or exhaust hood, or a malfunctioning damper or fan.

Withdrawing more air from the house than comes in creates excessive negative pressure, as illustrated in Figure 4. Excessive negative pressure can cause spillage of carbon monoxide and other combustion by-products from fuel-burning appliances (e.g., a furnace, water heater or fireplace). Instead of leaving the house via the chimney, these combustion by-products can be pulled back into the house where they may be inhaled by occupants. Negative pressure can also increase the rate of entry of undesirable gases and moisture from the soil surrounding the basement (see Table 1, “Common pollutants and their sources”). Either scenario can threaten the health of the occupants.

Pulling more air into the home than is exhausted, or excessive positive pressure, can cause moist air from the home to be pushed into the walls and roof of the house, where it can condense and lead to deterioration of the building materials. This hidden problem often goes undetected until severe damage has been done. As well, moisture escaping through exterior door locks can freeze the lock mechanism, making it difficult to enter or leave the house. An imbalanced airflow can also cause the HRV core to develop frost or freeze, restricting or completely blocking further airflow (see “Considerations for Heating Season Operation”).

FIGURE 4
Effects of air pressure on a house

Although your HRV should have been balanced during installation, it is a good idea to have the system checked by a qualified contractor once a year. If you are purchasing an existing home that is equipped with an HRV, consider having the system serviced, including a balance test.

Do-It-Yourself Balance Check

You can perform your own rough check of your HRV's airflow balance using the plastic trash bag technique. In this case, tape the open end of an extra large trash bag (i.e., the type used for leaf collection) to a coat hanger. Place the deflated bag over the HRV's exhaust hood located on the outside of your house and time how long it takes for the bag to fill with air. Then hold the inflated bag over the HRV's air intake hood and time its deflation. On a well-balanced system, inflation and deflation times should be equal, within a matter of seconds.

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Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency