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EON Retrofit Test House: MVHR systems and specifications

Solid-wall, ‘hard to heat, hard to treat’.

Overview

This case study is part of the CALEBRE project, 'Consumer Appealing Low Energy technologies for Building REtrofitting', which focused on solid wall 'hard to heat, hard to treat' properties.

The project considered a range of current, medium and longer-term technologies for domestic energy efficiency refurbishment, with the perspective of the householder at the heart of all thinking and decision making.

Approach

MVHR (mechanical ventilation with heat recovery) systems work most effectively and efficiently when the airtightness of a home is improved.

Two MVHR systems were analysed using dynamic thermal modelling of the Eon Retrofit Test House, a replica 1930's semi-detached property, to determine the annual energy consumption of the house at six levels of airtightness.

The results were also compared to the calculated energy consumption of the house with natural ventilation.

Performance

If a MVHR system is installed in a house with poor air tightness, it can increase the energy consumption as there are then more air changes per hour. This additional air moving through the property has to be heated to maintain the same internal temperature - although this will be partially offset by the heat recovery part of the system. Electricity is also required to power the fans in the ventilation system.

 

For a MVHR system at Minimum Building Standards, it is necessary to reduce the air tightness to 3 m³/h.m² @50Pa to ensure a reduction in energy consumption.

For a MVHR at Best Practice Standards, the system achieves an energy saving when the air permeability is 5 m³/h.m² @50Pa.

Lessons

The airtightness of a property must be improved as much as possible to ensure energy savings are made by the installation of a MVHR system, though draughtproofing can be less onerous (or greater savings can be made) by specifying a MVHR system at Best Practice Standards, above Minimum Building Standards.

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