164 homebuilding.co.uk
Whichever
system
you opt for
I would
urge you to
design it
early onAirtightness is simply another term
for draughtproofing and it must be
emphasised that it does not happen by
accident. Air moves through the fabric
of the building at all penetrations, such
as windows, doors and services pipes, as
well as all junctions, such as plate levels
where the floor meets the wall and where
the roof meets the wall, and through the
fabric of the building itself. We would
not want to draw humid air from the
bathroom to then simply allow cold air to
ingress through the ceiling area. All these
areas must be designed to be airtight from
the outset because if these details are not
on the drawings they’re probably not
going to get built.
There is a lot of ductwork to
accommodate and in order to make it
discreet you really want to hide it away in
open web floor/ceiling joists, studwork or
loft areas. Too little ductwork, too small
ductwork or too many twists and bends will
increase the resistance and result in higher
fan power and therefore higher running
costs and increased potential for noise.What are the alternatives?
If airtightness cannot be guaranteed or
the duct runs cannot be designed and
installed with low resistance (which can
be a challenge in existing homes) then
perhaps MVHR is not for you... but there
are other options that may be available.
In 1972 Nuaire developed the concept
of Positive Input Ventilation (PIV) to
specifically tackle the ventilation issues in
existing properties. The system works by
gently supplying fresh, filtered air into the
property from a unit installed in the loft
area, and a distribution diffuser mounted
in the ceiling. The continual supply and
slight positive pressure result in the air in
the property being continually diluted,
displaced and replaced to create a
healthier indoor air quality.
The airtightness of the property does
play a part as air will travel the path
of least resistance and you don’t wantit escaping out the nearest window and
having no effect on rooms further away
from the diffuser. And it does also rely
on the availability of some loft space to
put the unit in, so is not ideally suited to
apartments or homes with vaulted ceilings
or flat roofs. You will also need to make
sure that Building Regulations Part F is
complied with, as the PIV unit does not act
as an extract ventilator.
MVHR units are designed to run
continuously all year round and another
alternative option is known as ‘on
demand’ ventilation. This is a process
whereby mechanical ventilation (without
heat recovery) is installed but only used
when required, or to draw more air from
areas of higher occupancy. So it is only
the stale air that is exhausted and not a
continuous flow. Simple versions of this
are decentralised extract fans in bathrooms
and kitchens. If we combine these with a
passage of airflow from another area of the
property we can introduce mechanically
managed cross ventilation.
Further improvements would be to
add a small number of extract ducts that
converge to a single fan unit. This is known
as a mechanical extract ventilation
(MEV) unit, which requires the fresh
air to enter the home through carefully
positioned airbricks and trickle vents. So,
an MEV system is almost the opposite of
a PIV system. Ventilation manufacturer
and system designer Aereco has also
introduced humidity-sensitive trickle
vents and airbricks. This feature allows the
apertures to open when there is humidity
present (occupied rooms) and close when
the humidity levels drop, thus creating
different paths of airflow according to
changing occupancy.
What’s more, by using an MEV system
that has a refrigerant heat exchanger we
are then able to extract the heat from the
stale exhaust air and enhance it and use it
as useful heat for hot water preparation.
The Earth Save Products Ecocent hot
water cylinder (overleaf) does exactly thisthe Build