©
EUROHEAT
DISTRIBUTORS (H.B.S) LTD. March 2006
Technical Guide IN1087 Edition C
14
If the appliance manufacture has provided the specific pressure readings required it is important when
taking measurements that the pressure of the combustion chamber itself is measured. Any baffle or
other intentional flue way restriction will result in a higher reading within the flue than the combustion
chamber. whilst this difference will be dependent on the relative sizes of the restrictions to the size
of the air inlet, and may be very small, it is important to eliminate all potential measurement errors.
If a suitable test point is not provided, make one, or use a little ingenuity to access the combustion
chamber directly; it will be worth the effort.
The relative remittances to air
flow of all restrictions A, B will give
differing pressures between the
flue and combustion chamber.
A
B
C
D
By keeping a record of the
combustion chamber pressures at
D, faults not apparent by testing the
flue C will be easily detected.
If you are using the “draught” gauge mentioned earlier, by the time you have connected the connecting
tube to the gauge’s outlet point you will have noticed that the side showing pressure rather than air
speed has two scales. By leaving the air inlet to the gauge open, the air flow is unrestricted and the
lower scale on the left hand side is used, but by covering the inlet with a finger the air has to pass
through a much smaller orifice at the side of the inlet, and the higher scale on the right hand side
should be read.
If the inlet is left
open, the left hand
scale is read
If the inlet is closed
and the inlet is through
the restricted orifice
the right hand scale is
used
.02
.01
.03
.04
.05
.06
.07
.08
.09
IN
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.05
.1
.2
.3
.4
.5
.6
.7
.8
.9
1.0
IN
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Before lighting the stove an indication of the flues natural state can be obtained. Ensuring all the
stove’s openings are closed and any draught stabilizer is in a closed position, connect the gauge to
the test point and with the gauge being held in an absolute vertical position note the position of the
ball and its movement. Any pressure reading at this time will be caused by wind and the temperature
difference between the house fabric causing the flue to be at a higher temperature than that outside
the house.
If no pressure is detected it is useful to put the hose onto the air inlet which will give a reading of any
positive pressure. Never do this with when the stove is dirty or the dirt will contaminate the ball and
tube making all readings unreliable until the tube has been cleaned and the ball replaced. If a positive
pressure is indicated, no attempt should be made to light the stove until this has been reversed. If
the outside air is warmer than the room in which the stove is installed, the flue can be warmed with
a hair dryer or fan heater directed into the stove for a few minutes, taking care to remove the gauge
before doing so. If the pressure cannot be rectified or is caused by wind, indicated by the gauge ball
showing changing positive pressure, the flue terminal and all house ventilation should be looked at
to find the cause. Do not attempt to light the stove until a negative pressure is measurable.
