Heat recovery ventilation

Benefits

As building efficiency is improved with insulation and weather stripping, buildings are intentionally made more airtight, 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 heating, ventilating/ventilation, and air conditioning (HVAC) systems must compensate. HRV introduces fresh air to a building and improves climate control, whilst promoting efficient energy use.

UK building regulations require one air change every two hours (0.5 ACH). With traditional extract-only ventilation that means a house boiler would need to warm up a house-full of cold air 12 times a day.

Technology

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 fans and electronic control system. 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 types of air-to-air heat exchanger that can be used in HRV devices:

Incoming air

The air coming into the heat exchanger should be above 0 °C. Otherwise humidity in the outgoing air may condense, freeze and block the heat exchanger.

A high enough incoming air temperature can also be achieved by

Earth-to-air heat exchanger

This can be done by an earth warming pipe ("ground-coupled heat exchanger"), usually about 30 m to 40 m long and 20 cm in diameter, typically buried about 1.5 m below ground level. In Germany and Austria this is a common configuration for earth-to-air heat exchangers.

In high humidity areas where internal condensation could lead to fungal/mould growth in the tube leading to contamination of the air, several measures exist to prevent this.

The pipes may be either corrugated/slotted to enhance heat transfer and provide condensate drainage or smooth/solid to prevent gas/liquid transfer.

Air quality

This is highly site dependent.

Radon

One critical problem of using earth-to-air heat exchanger is being located in soils with underlying rock strata which emit radon. In these situations the tube needs to be airtight from the surrounding soils, or an air to water heat exchanger needs to be used.

Bacteria and fungi

Formal research indicates that Earth-to-Air Heat Exchangers (EAHX) reduce building ventilation air pollution. Rabindra (2004) states, “The Earth-Air Tunnel 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 earth-to-air heat exchangers 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-to-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”.

Earth-to-water heat exchanger

An alternative to the earth-to-air heat exchanger is the earth-to-water heat exchanger. This is typically similar to a geothermal heat pump tubing embedded horizontally in the soil (or could be a vertical pipe/sonde) to a similar depth of the EAHX. It uses approximately double the length of pipe Ø 35 mm i.e. 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 exchange fluid which is slightly more efficient and environmentally friendly than polypropylene heat transfer liquids.

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 micro-heat pump in reverse (an air conditioner) with the evaporator (giving heat) on the air inlet after the HRV heat exchanger and the condenser (taking heat) from the air outlet after the heat exchanger will suffice.

Seasonal bypassing

At certain times of the year it is more thermally efficient to bypass the Heat recovery ventilation-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 external air directly. In this case it is helpful to have a bypass such that the EAHX is disconnected and air taken directly from outside. A differential temperature sensor with a motorized valve can control the bypass function.