Balancing an HRV or ERV Air Exchanger — Here's What to Expect

Homes built according to current codes are so airtight that they need an extra HVAC component in addition to their air conditioning and heating systems to blow fresh air into the house and exhaust stale air. This air exchanger may be a heat recovery ventilator (HRV) or an energy recovery ventilator (ERV), both of which have a heat exchanger at the core that transfers heat from the warmer air stream to the cooler one. An ERV differs from an HRV in that it also regulates indoor humidity because of its absorbent core that transfers moisture between the two air streams — while an HRV has a metal core that condenses and drains excess moisture from humid air. You balance HRVs and ERVs using the same procedure, but it's more likely that you'll be balancing an HRV since they are more common.

Because it's an active mechanical ventilation system, an air exchanger has the potential to overpressurize or underpressurize the building, and both conditions have unfavorable consequences such as mold growth or an increased energy bill. To prevent this, the HRV has to be balanced by the installer and periodically rebalanced during its service life. When an HRV is balanced, the incoming airflow — measured in cubic feet per minute (CFM) — is equal to the exhaust airflow within a differential of 10 CFM.

Checking and correcting the balance of an HRV is normally a job for a licensed HVAC pro, who takes accurate measurements of the airflow and makes necessary adjustments to the dampers located on the air intake and exhaust vents or adjusts the blower speed. The equipment used consists of a sensitive probe to detect airflow in the inflow and outflow ducts and a meter to measure it, which are both items a homeowner isn't likely to have. A homeowner can, however, conduct a simple test to diagnose a severely out-of-balance air system, and that will help determine whether or not to call for service.

To get the most benefit from the air conditioning or heating system, it's important for a homeowner to keep all the doors and windows closed, and when you do that in a modern house, there are no gaps through which air can pass to equalize indoor air pressure. All the fresh outside air comes through the air exchanger, which must remove an equal amount of indoor air to maintain the indoor air pressure the same as it is outdoors. If the walls were made of rubber, they would bulge outward when the amount of outdoor air coming in exceeds the amount of stale indoor air being exhausted, and they would collapse inward in the reverse situation.

Walls don't do that, of course, but people in the building can notice the effects of unbalanced air exchange. When there's a net positive pressure, air gets pushed through cracks and gaps through which it wouldn't pass at normal air pressure, losing heat to the outside and driving up the energy bill. Moist air also gets pushed into the walls and insulation, producing condensation and allowing mold to grow.

In a negative pressure situation, air gets pulled into the building from outside, again creating mold and condensation problems and raising energy costs, and it also causes problems with combustion devices, like gas stoves and heaters. Both negative and positive pressure are bad for indoor air quality, increasing the concentration of allergens and other pollutants and reducing the energy efficiency of the HVAC system.

When a new air exchanger is installed, the installer adjusts the balance to make sure the blower is sending the same amount of air into the house as it is exhausting to make the air pressure inside the building roughly the same as it is outside. Various other factors affect indoor air pressure — like the stacking effect, which is when warm air migrates to the top of the building, putting pressure on ceilings and windows on the upper floor. The frequency of use of the exhaust fans in the bathroom and kitchen also has an effect, so balancing the air exchanger doesn't always solve the problem of pressure imbalance.

If you notice your energy bill going up or you feel drafts near closed windows and doors, there's an easy way to test for pressure imbalance. Close all the doors and windows in the house and turn off any exhaust fans and appliances that have an exhaust fan, such as the dryer and range hood. Then, crack a single window to make a 1-inch opening for air to flow. Hold a burning stick of incense next to the window or tape a piece of tissue paper to the top of the window. Let it hang freely and watch what happens.

If the smoke rises straight up or the paper hangs straight down, there's no net movement of air through the window, and the pressure is balanced. If the smoke is drawn outside or the paper sticks to the window, there is positive pressure in the room, and if the smoke is drawn into the room or the paper blows inward, there is negative pressure. In either of the latter two cases, a service call is warranted to diagnose the problem and balance the air exchanger if necessary.

A typical HRV has two inlet ports and two exhaust ports for a total of four ports, although units using concentric ductwork need only two ports. Inside the unit, the air streams cross each other in the core, which keeps them physically separated but allows them to exchange heat. An ERV has an absorbent core that allows it to also exchange moisture, which is a useful feature in moderate climates. Airflow is regulated by a variable-speed blower motor, dampers on the air intake and exhaust ports or both.

To adjust the balance, the technician opens each damper all the way, sets the blower to high speed, cleans the air filters, secures the house by closing doors and windows and turning off all the exhaust fans and tapes any points of leakage in the ductwork. If the air exchanger is connected to the ductwork of the main HVAC system, the technician will turn on the blower in the air handler. To measure airflow inside the ducts, the technician needs to insert a probe, which is done by either drilling a 1-inch hole that later gets taped over or inserting flow collars in the inflow and exhaust ducts. When collars are used, the pipes have to be separated at joints and reassembled after the test is completed.

To conduct the actual adjustment, the technician hooks up a device called a magnehelic gauge. This measures in inches of water, and the technician later converts the results to cubic feet per minute (cfm). With the blower set to its highest speed, the technician first measures airflow in the air duct that feeds indoor air into the air exchanger and then measures flow in the duct feeding the house. Because the supply air from indoors is usually more restricted, the second reading is typically larger, and if the difference is more than 10 cfm, the technician will either adjust a damper or the motor speed to make them equal.