Heat recovery ventilators (HRVs), 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 (ERVs) are closely related, however ERVs also transfer the humidity level of the exhaust air to the intake air. (see countercurrent exchange)
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) (see image)
- rotary heat exchanger (requires motor to turn wheel)
- heat pipes / thin multiple heat wires
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.
The EAHX is particularly problematic 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 must be a smooth bore internally to minimise pressure loss and rentention of water.
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.
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.
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