Heat recovery ventilation

Ventilation is necessary in buildings to remove ‘stale’ internal air and replace it with ‘fresh’ outside air. This helps to:

• Moderate internal temperatures.
• Reduce the accumulation of moisture, odours and other gases that can build up during occupied periods.
• Create air movement which improves the comfort of occupants.

Very broadly, ventilation can be ‘natural’ or ‘mechanical’.

• Mechanical (or ‘forced’) ventilation tends to be driven by fans.
• Natural ventilation is driven by ‘natural’ pressure differences from one part of the building to another.

Ventilation systems may also include heating, cooling, filtration and humidity control.

Ventilation has become increasingly important because of the tendency to ‘seal’ modern buildings. However the process of extracting internal air, which may have been conditioned, and replacing it with air that has not is inherently wasteful.

This can be mitigated by heat recovery, the process of collecting and re-using heat that would otherwise be lost, which can help to reduce energy consumption, reducing running costs and carbon emissions.

Heat recovery ventilation (HRV or ventilation heat recovery (VHR) or mechanical ventilation heat recovery (MVHR)) uses a heat exchanger to recover heat from extract air, that would otherwise be rejected to the outside, and uses this heat to pre-heat the ‘fresh’ supply air. Very efficient heat exchangers can recover as much as 98% of the ‘waste’ heat.

Typically heat recovery ventilation works by transferring heat between the incoming and outgoing air streams by blowing them in opposite directions between adjacent flat plates in air-to-air heat exchangers. Heat is absorbed by the plates from the 'hot' air on one side and released to the 'cold' air on the other. Cellular heat exchangers, which can achieve greater efficiency, drive the incoming and outgoing heat through adjacent square tubes, increasing the heat transfer surface area.

Air to ground and air to water heat exchangers use the earth or water as a heat exchange body by blowing air through tubes surrounded by earth or water.

Heat can also be recovered from other processes and components, such as refrigeration units and chiller units, boilers, power generation plant, plant cooling systems, hot liquid effluents and high-temperature exhaust gasses. Recovered heat can also be used for drying processes, power generation, pre-heating combustion air for furnaces or boilers and so on.

Heat recovery is also possible with heat pumps, which essentially reverse the refrigeration cycle of chiller units to provide heating, rather than cooling.

It is also possible, although relatively complicated to include heat recovery in natural ventilation systems.

[edit] Related articles on Designing Buildings Wiki.

• Air conditioning.
• Air handling unit.
• Chiller unit.
• District energy.
• Geothermal pile foundations.
• Heat pump.
• Heat recovery.
• HVAC.
• Mechanical ventilation.
• Thermal labyrinths.
• Variable refrigerant flow.

[edit] External references

• Heat recovery: A guide to key systems and applications.
• Carbon Trust, How to implement heat recovery in heating, air conditioning and ventilation systems.
• The Future of Heating: Meeting the Challenge.
• The potential for recovering and using surplus heat from industry.
• Harvesting energy: body heat to warm buildings