Heat Recovery Ventilators

Commercial building codes normally require some amount of outside air be introduced into a building.  To accomplish this, outside air is normally introduced into the return of a building's HVAC system.  This air is then mixed with the return air of the system and heated or cooled as required and introduced into the conditioned space.  To prevent the building from becoming over pressurized from the addition of the outdoor air, some of the return air from the HVAC system is exhausted to the outdoors.

This design adds an additional cost to the installation and operation of an HVAC system.  Since the outside air introduced into the system needs to be heated or cooled, a larger HVAC system is required adding to the original installation cost and operating cost.  Also, energy is wasted as the return air which has already been conditioned is simply exhausted outdoors.

There is a method to recoup some of the additional cost required to introduce outdoor air into a building.  A heat recovery ventilator (HRV) can be installed.  A basic HRV is a heat exchanger which uses the exhausted return air to pre-condition the outdoor air being introduced into the HVAC system.  The outdoor air first passed through the HRV before entering the HVAC system, at the same time the exhausted return air is passed through the HRV.  In the heating mode, some of the heat energy from the return air is transferred to the outdoor air, increasing its temperature and reducing the load on the HVAC system.  In the cooling mode, some of the heat energy from the outdoor air is transferred to the return air, decreasing its temperature and reducing the load on the HVAC system.  The return air and the outdoor air do not come in direct contact with each other, so only the heat energy is transferred.  Any pollutants contained in the return air do not transfer to the outdoor air being introduced into the system and are exhausted outdoors.  Below is a simplified diagram of the process.