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.
Energy recovery ventilators (ERVs) are closely related, however ERVs 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 consequently less well ventilated. Since all buildings require a source of fresh air, the need for HRVs 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.
HRVs and ERVs 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.
Air to air heat exchanger
There are a number of heat exchangers mostly used in HRV devices :
* cross flow heat exchanger up to 60% efficient (passive)
* counter current heat exchanger up to 99% efficient (passive)
* rotary heat exchanger (requires motor to turn wheel)
* heat pipes / thin multiple heat wires
Earth-to-air heat exchanger
The air coming into the heat exchanger should be at least 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 °C. This can be done by an earth warming pipe, usually about 30 m to 40 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. It can also be achieved by recirculating the air (loss of air quality) when required or using a very small heat pump 1 kW on the air outlet (stays above 5 degrees Celsius) after the HRV heat exchanger dumping heat to the air inlet after the heat exchanger. This may, 1.5 kW is sufficient, be also used to supplement the solar hot water system. Photovoltaic panels can run the heat pump during daylight.
Additional measures may need to be employed in high humidity areas where internal condensation can lead to fungal/mould growth in the tube and contamination of the air. Precautions can be taken such as running the pipes to a low (drainage) point, cleaning the tubes regularly, and using pipes with an imbedded bactericide coating such as silver ions (non-toxic for humans), using air filters F7 / EU7 (>0,4 micrometres) which traps mould (size between 2 & 20 micrometres), and/or a UV lamp system. The pipes may be either corrugated/slotted to enhance heat transfer and provide condensate drainage or smooth/solid to prevent gas/liquid transfer. This is highly site dependent.
That being stated, formal research indicates that Earth-Air Heat Exchangers reduce building ventilation air pollution. Rabindra (2004) states, “The tunnel [Earth-Air Heat Exchanger] is found not to support the growth of bacteria and fungi; rather it is found to reduce the quantity of bacteria and fungi thus making the air safer for humans to inhale. It is therefore clear that the use of EAT [Earth-Air Tunnel] not only helps save the energy but also helps reduce the air pollution by reducing bacteria and fungi.” Likewise, Flueckiger (1999) in a study of twelve Earth-Air Heat Exchangers varying in design, pipe material, size and age, stated, “This study was performed because of concerns of potential microbial growth in the buried pipes of ground-coupled air systems. The results however demonstrate, that no harmful growth occurs and that the airborne concentrations of viable spores and bacteria, with few exceptions, even decreases after passage through the pipe-system”, and further stated, “Based on these investigations the operation of ground-coupled earth-to-air heat exchangers is acceptable as long as regular controls are undertaken and if appropriate cleaning facilities are available”.
An alternative to the earth to air heat exchanger is the "water" to earth heat exchanger. This is typically similar to a geothermal heat pump tubing embedded horizontally in the soil (or could be a vertical sonde) to a similar depth of the EAHX. It uses approximately double the length of pipe Ø 35 mm ie around 80 metres compared to an EAHX. A heat exchanger coil is placed before the air inlet of the HRV. Typically a brine liquid (heavily salted water) is used as the heat exchanger fluid.
In temperate climates in an energy efficient building, such as a passivhaus, this is more than sufficient for comfort cooling during summer without resorting to an airconditioning system. In more extreme hot climates a very small air to air heat pump in reverse (an air conditioner) on the air inlet after the HRV heat exchanger dumping heat to the air outlet after the heat exchanger will suffice.
At certain times of the year it is more thermally efficient to bypass the HRV heat exchanger or the earth to air heat exchanger (EAHX).
For example, 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 EAHX, the air is warmed by the heat recovery ventilator using the warmth of the outgoing air. In this case, the HRV can have an internal bypass such that the inflowing air bypasses the heat exchanger maximising the cooling potential of the earth.
In autumn and spring there may be no thermal benefit from the EAHX, it may heat/cool the air too much and it will be better to use the external air directly. In this case there is a bypass such that the EAHX is disconnected and air taken directly from outside. A differential temperature sensor with a motorised valve can control its functioning.