Modern homes have greatly reduced levels of ventilation which reduces heat loss and improves energy efficiency. A well designed mechanical ventilation system can provide the right amount of clean, fresh air that is vital for the health and wellbeing of a household.
Having adequate ventilation in your home is important for good health. It removes moisture and stale air along with odours and pollutants and replaces them with fresh air to breathe.
Usually, ventilation is achieved by simply opening windows and doors (known as ‘natural’ or ‘passive’ ventilation) and by using extractor fans in steamy rooms such as kitchens and bathrooms. Although this works well, it can account for around one third of space heating energy demand in an average home as a lot of warm air flows outside.
Draughts create a similar problem. While they serve to ventilate a home, they are a major source of heat loss and can make rooms feel uncomfortably chilly.

Modern energy efficient homes are more airtight (less draughty) than older buildings because they have to follow specifications for air-tightness in the Building Regulations. Because of this modern homes are sometimes fitted with Mechanical Ventilation with Heat Recovery (MVHR) systems also known as ‘whole house ventilation’ systems. 
The outlets for MVHR are small and unobtrusive, and look like ordinary extractor fans

Whole-house MVHR systems extract warm, damp air from the home and draw in fresh air from the outside. The warm, extracted air is passed through a heat exchanger to recover the heat before being expelled outside. The cool, fresh outside air is also passed through the heat exchanger, without coming into direct contact with the pollutant air where it is pre-warmed before being pumped in to the property.

These systems tend to comprise of concealed ducting in ceiling voids leading to the heat exchanger unit in the loft or another void in the home. Some units have an automatic or manual boost setting which can be used when generating excessive moisture whilst cooking or using the bathroom. Air filtration is commonly built into MVHR systems to prevent pollen and other particles from entering the home which then provides a more comfortable environment for allergy sufferers. Systems typically run continually at 80% efficiency and are inaudible during normal use.

MVHR systems do require servicing as all equipment such as filters and fans must be kept clean to ensure effective operation. MVHR systems can cost in the region of €1,800 to €3,600 but an accurate quote can only be provided based on your individual property.
Because the fresh air is pre-warmed, heat loss from ventilation is largely avoided. This means the householder can spend less on heating their property, reducing costs by as much as €240, therefore saving money and ensuring a healthy, well ventilated home.

The benefits of heat recovery systems that recycle the energy in stale air and transfer it to the new, fresh air coming in are numerous:

• Constant fresh air.
• Cleaner air will be better for you.
• Reduces humidity and condensation that can cause health problems and affect walls and structures over time.

• The improved air quality means that your house isn’t filled with irritants like pollen and pollution as the ventilation is filtered throughout the building.
• lower carbon emissions
• Reduced energy bills (less work on heating system)

Video: https://www.youtube.com/watch?v=VN3jMtzRbpo