The potential for energy saving in existing solid wall dwellings through mechanical ventilation and heat recovery

Mechanical ventilation with heat recovery (MVHR) is increasingly
being promoted in the UK as a means of reducing the CO2
emissions from dwellings and installers report growing activity
in the retrofit market. The behaviour of a whole-house MVHR
system installed in an experimental house, purpose built to
typical 1930s standards, has been simulated, at a series of air
permeability values corresponding to those achieved in the retrofit
upgrading process. There is a critical value below which
the air permeability, as measured in a 50 Pa pressurisation test,
must fall before MVHR makes an overall energy saving. In the
house considered this is 3–5 m3/m2.h (4–6.5 ach-1), although
more significant savings of 9-12 % are achieved when the
air tightness is improved to Passivhaus standards of 0.63 m3/
m2.h (0.6 ach-1). When the CO2 emissions are considered, the
air permeability needs to approach Passivhaus standards to
ensure an overall reduction, because the electricity needed to
run the MVHR system has a CO2 intensity almost three times
that of the gas used to heat the house. Airtightness is a critical
factor in achieving energy and CO2 reductions and it is easy for
designers to over-estimate the potential savings.