281 Issue 1 - Service Manual - BX3
19
5.3 Two Flashes - Pressure Switch Inactive
Reason: Pressure switch inactive
A pressure switch within the appliance provides proof of combustion airflow. During the “ON”
phase of the heating cycle the pressure switch has opened, indicating a loss in vacuum
pressure. The heating cycle will be forced to its “OFF” state and a restart initiated. If this
occurs continuously the unit will lockout and manual reset will be required
.
During a heating
cycle, the unit responds to the state of this switch in the following ways.
Pressure switch fails to open.
Once the unit is calling for heating the control board will first ensure the combustion air
pressure switch is open (no air flowing) before energising the combustion fan. However If the
pressure switch is closed then the unit shall wait for the switch to open for as long as the call
for heat is present. If the pressure switch remains closed longer than 30 seconds the
corresponding two flash warning flash code is displayed. If the pressure switch remains closed
longer than 4 minutes, the unit shall lockout and the four flash fault code is flashed on the
status LED. Once lockout occurs, the combustion fan turns off.
Pressure switch fails to close.
If the combustion fan pressure switch does not close shortly after the combustion fan has
been energised the unit shall keep the combustion fan energised for 4 minutes, until either the
pressure switch closes or the call for heat is removed. If the pressure switch remains open
longer than 30 seconds, the 2 flash fault code is displayed. If the pressure switch remains
open longer than 4 minutes, the unit shall lockout and the four flash fault code is flashed on
the status LED, once lockout occurs the combustion fan turns off.
Service Check List
Vacuum Pressure Checking Procedure
Measure vacuum pressure at the pressure switch during pre-purge (or force an overheat
condition, by breaking the overheat switch circuit, to operate the combustion fan). Perform these
pressure checks without the pressure switch connected to the pressure hose.
If vacuum pressure is above specification
If the flue has no blockage and you find the vacuum pressure is above the pressure switch’s
specified value and pressure switch error still occurs, then the pressure switch may be faulty and
should be replaced.
If vacuum pressure is below specification
1. Vacuum pressure hoses are not fitted correctly – check hose connections
2. Vacuum pressure hoses have perished and cracked causing vacuum pressure loss –
check for deterioration.
3. Water trapped in the vacuum pressure hose – check for water traps or presence of
moisture in hoses. Make sure there is no water in the hose when servicing the system.
4. Excessive heat exchanger leakage resulting in vacuum pressure loss – check heat
exchanger for leakage.
5. Combustion fan may be faulty – check according to procedure in section (Combustion
Fan)
6. Flue piping is too restrictive – check against allowable flue pipe configuration
specification including the flue cowl.
7. Flue pipe has collected excessive condensate – check for potential collection points
(there must be a continual fall back to the heater – 20mm rise per Metre).
8. Check pressure switch pressures on 2 flashes pressure switch lockout.
