The safe operation of any gas appliance
requires an adequate air supply to the
appliance the safe operation of an open
flue gas heater can be compromised by a
negative pressure environment.
This video
explains what negative pressure is and
its effect on gas appliances
AS/NZ S 5.601.1 203 Clause 6.4.1 Adequacy of air
supply states that gas appliances shall
be installed in locations with adequate
ventilation for complete combustion of
gas proper operation of the flue and to
maintain the temperature of the
immediate surroundings at safe limits
under normal operating conditions
This
diagram shows the correct operation of
open flue heaters the arrows show how
air is drawn into the appliance from the
room for combustion the products of
combustion exit via the flue and into
the atmosphere if there is insufficient
ventilation in the room or building the
installation is non-compliant and
additional ventilation is required.
ASNZ S5.6.0.1 part 1 2013 Clause 6.3.1
adverse effect of air movement
systems states gas appliances shall not
be installed with the operation of any
ventilation system air distribution
system fan or air blower could under any
circumstances a deprive the gas
appliance of the air required for
combustion and draught diverted dilution
or be otherwise adversely affect the
operation of the gas appliance.
A negative pressure environment can be
created within the room or building when
exhaust fans are turned on if there is
inadequate permanent ventilation in an
attempt to equalize the pressure in the
room air is drawn into the building from
in the room to equalize the air pressure.
Room air is drawn into the building from
external openings in an open flue heater.
The flue can operate as an opening to
relieve the negative pressure. To prevent
this from happening, install ventilation
in the room to equalize the air pressure
this will enable the appliance to
operate safely.
To achieve the greatest negative
pressure effect close external doors and
windows in the building then open or
close internal doors to create the
maximum potential negative pressure
switch on all extraction fans.
Now carry out a smoke test.
A smoke test must be carried out when
your gas appliance is off and the flue
is cold.
Using a smoke match check that smoke is
not drawn away from the appliance draft
diverter relief openings or from the
appliance combustion air intake.
If smoke is drawn away from the appliance, slowly
open a window in the room where the
appliance is installed to equalize the
air pressure. the open window will act as
a guide to how much additional fixed
ventilation is required.
when the smoke is drawn towards the downdraft diverter
opening the heater in general or
straight up towards the ceiling then the
gap in the window opening is providing
sufficient ventilation to overcome the
effects of the negative pressure caused
by the extraction fans.
The size of the window opening is an indication of the
fixed ventilation required to be
installed. please be mindful that on
windy days you may see intermittent
disturbances of smokes at the downdraft
diverter from wind gusts at the flue.
This is not a sign of negative pressure.
Negative pressure will draw a constant
stream of smoke away from the heater.
It's time to conduct a CO spillage test
once you have established your
installation has sufficient air supply
for combustion and that extraction fans
are not having an adverse effect on the
operation of the appliance.
Reviewed 12 September 2023