Heat recovery ventilation

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Heat recovery ventilation (also known as a heat exchanger, air exchanger or air-to-air exchanger) is a ventilation system that employs a counter-flow heat exchanger between the inbound and outbound air flow. HRV provide fresh air and improved climate control, while also saving energy by reducing the heating (or cooling) requirements.

Heat recovery ventilators (HRV's), as the name implies, recover the heat energy in the exhaust air, and transfer it to fresh air as it enters the building. Energy recovery ventilators (ERV's) are closely related, however ERV's also transfer the humidity level of the exhaust air to the intake air. (see countercurrent exchange)

As building efficiency is improved with insulation and weatherstripping, buildings are intentionally made more air-tight, and consequentially less well ventilated. Since all buildings require a source of fresh air, the need for HRV's has become obvious. While opening a window does provide ventilation, the building's heat and humidity will then be lost in the winter and gained in the summer, both of which are undesirable for the indoor climate and for energy efficiency, since the building's HVAC systems must compensate. HRV technology offers an optimal solution: fresh air, better climate control and energy efficiency.

HRV's and ERV's can be stand-alone devices that operate independently, or they can be built-in, or added to existing HVAC systems. For a small building in which nearly every room has an exterior wall, then the HRV/ERV device can be small and provide ventilation for a single room. A larger building would require either many small units, or a large central unit. The only requirements for the building are an air supply, either directly from an exterior wall or ducted to one, and an energy supply for air circulation, such as wind energy or electricity for a fan. When used with 'central' HVAC systems, then the system would be of the 'forced-air' type.

[edit] Earth-to-air heat exchanger

Heat recovery ventilation, often including an earth-to-air heat exchanger, is essential to achieve the German passivhaus standard

The air coming into the heat exchanger should be at least be above 0 degrees Celsius (32 Fahrenheit). Otherwise, the condensed water from the outgoing air would freeze and block the outgoing air. Therefore, it is necessary to warm the incoming air to at least 0 degrees C. This can be done by an earth warming pipe, usually about 10 m long and 20 cm in diameter. It is buried about 1.5 m below ground level. In Germany and Austria this is a common configuration for earth to air heat exchangers.

[edit] Summer bypass flap

During the winter, the earth at the depth of the earth to air heat exchanger is ordinarily much warmer than the air temperature. The air becomes warmed by the earth before reaching the air heat exchanger.

In the summer, the opposite is true. The air becomes cooled in the earth to air exchanger. But after passing through the earth air exchanger, the air is then warmed by the heat recovery ventilator using the warmth of the outgoing air. In this case, the heat exchange to warm up the cold air from the earth is undesirable.

The problem is solved with the summer bypass flap. The flap blocks the path of the incoming ground-cooled air to the heat exchanger and opens a direct path to the interior of the building.