Heat Recovery Ventilators

The development of new residential building designs, such as R2000 homes, has led to the introduction of heat recovery ventilators or HRVS.  These devices are used year round to exhaust stale air from the home, while bringing fresh air in from the outside.  In the winter, HRVs also transfer heat from the exhaust air stream to the fresh air stream.  While in the summer, the exhaust air cools the incoming warm fresh air.  By using these balanced-type ventilation systems, homes maintain comfortable indoor relative humidity, alleviate pressure differentials, and conserve energy.

Components Of A Heat Recovery Ventilator

A/ Housing. The housing of the HRV is suitable for being installed on the wall, floor, or suspended from the ceiling.  There are removable panels on the housing to allow the technician to change the two washable air filters and service the unit.  A defrost mechanism within the housing prevents freeze-ups during the winter.  To ensure quiet operation, the unit contains thermal acoustic insulation, vibration isolation collars and hanging straps.  Each HRV has two ducts on either side of the housing (six inches in diameter for most residential applications).  One set is used for fresh air supply and the other set is used for the exhaust of stale air.

B/ Fan Blowers.. Two high pressure, centrifugal blowers are used to draw in fresh air and exhaust stale air for the HRV.  These fan blowers are balanced to extend the life of the motor and for silent operation.  Each blower has built-in dampers for balancing air streams and regulating airflow.  In addition, the fan blowers can be set up to run on continuous low speed and increase to high speed when there is a call for heating or cooling.  HRVs usually have 120 VAC, 60 Hz, Permanent Split Capacitor (PSC) motors that are permanently lubricated and designed for various duct configurations.

C/ Controls. An HRV operates on a 24 volt control system that has a integral off/low/high switch.  When the unit is energized, the thermostat sends an electrical signal to the fan blowers to run on either continuous low speed or on high speed (heating or cooling).  During the heating or cooling cycle, slight positive pressure is generated and some outdoor must be kept out.  As a result, a dehumidistat activates the fan blower's high speed and correctly regulates the operation of the HRV.

D/ Heat Exchanger Core.. Each HRV contains a non-enthalpic air-to-air core that transfers energy.  The exhausted stale air and fresh ventilation air pass through the. cross flow heat exchanger core where they do not directly mix.  Instead, sensible and latent heat is transferred indirectly from one air stream to another.  The fresh incoming air is automatically preheated or precooled depending upon the time of year.

Operation Of A Heat Recovery Ventilator

Heat Recovery Ventilator uses two fan blowers to exhaust stale air and supply fresh air via the heat exchanger core.  The fresh air is drawn in at approximately the same air flow rate as the stale air is exhausted.  In the core, the fresh air stream is automatically preheated or precooled (depending on the season) by the exhausted air and distributed to the living space of the home.  Once the fresh air has circulated around the home for a period of time it becomes stale.  This air is then removed from the building by the HRV and the process starts over again.

Performance

Heat Recovery Ventilators have significantly improved the energy efficiency of residential homes.  HRVs are quiet ventilation systems that can replace utility and bath room exhaust fans in a home.  These devices recover 60 to 75 percent of the heat in the stale air and provide a

minimum airflow of 0.35 air changer per hour.  In addition, some HRVs maintain indoor relative humidity by recovering moisture from exhaust air in the winter or dehumidifying fresh air in the summer.  While these ventilation units are not utilized in older residences, they have become vital to new airtight homes.