Velocity Shadow Series, Installation And Operating Instructions Manual

Page: 77 / 104

76
B. Troubleshooting when the Display is Blank
Use the flow chart in Figure 14.5 to locate the problem when the display is blank or is not readable.
C. Operation and Troubleshooting of Boiler Limits, Switches and Sensors
For the location of the controls described below, see Figure 14.6.
Thermal Fuse – The thermal fuse is designed to prevent boiler operation in the event that the heat exchanger is damaged by
excessive flue gas temperatures. It is set to open at 358F and is a one shot non-replaceable device. If this fuse opens, the heat
exchanger must be replaced. The thermal fuse is essentially a back-up to the flue gas sensor, which is designed to prevent boiler
operation if the flue gas temperature exceeds 210F. Note, however, that it is possible for the thermal fuse to open even though the
flue temperature limit is working properly. If the thermal fuse is suspected of having opened, unplug the wire heading to it and
check continuity directly across the thermal fuse. If no continuity is present, the heat exchanger needs to be replaced. If continuity
is present, the fault lies elsewhere.
Sump Pressure Switch (SPS) – This pressure switch is normally closed and monitors the difference in pressures between the
boiler cabinet and the sump (the sump pressure is very close to the pressure at the boiler’s vent connection). This switch is piped
as shown in Figure 14.7. The SPS is calibrated to open at 3.15 +/- 0.10 inches water column. A blockage in vent system that
causes the vent pressure to exceed the SPS set point will cause the burner to shut down and recycle. Likewise, a blockage in the
air intake system that causes the vacuum in the cabinet to fall below -3.15 inches will also cause the boiler to shut down and
recycle. If this switch opens, look for a blockage in the vent or air intake system.
Air Proving Switch (APS) – This pressure switch is normally open and is connected to the outlet tapping on the gas valve as
shown in Figure 14.7. During pre-purge, when the gas valve is closed, the APS is monitoring the pressure drop across gas-air
mixer (venturi for the 80-120, swirl plate on the 150 and 180). Greater air flow through the mixer causes this pressure reading
to increase. The APS is calibrated to close when this pressure rises to 1.05 +/- 0.05” wc, which is a pressure at pre-purge
corresponding to adequate air flow. After the gas valve opens, this pressure will drop very close to zero, regardless of the actual
air flow rate through the boiler. For this reason, the APS is ignored after pre-purge. In the event that the burner fires continuously
for two hours, the boiler control will recycle the burner to verify that the APS still closes and that adequate airflow is still present.
If the APS opens, look for the following:
• Blockage in the vent or air intake system.
• Disconnected, loose or blocked APS tube.
Flow Switch – The paddle type flow switch is calibrated to close at a water flow rate of approximately 3.2GPM. This switch is
intended to do two things:
• Prove that there is water in the boiler (no water means no flow at this switch)
• Prove that there is enough flow for the supply and return temperature sensors to accurately measure the water
temperatures entering and exiting the heat exchanger.
The minimum flow rate required for all models is significantly above 3.2 GPM (Table 9.1). The boiler control verifies that there
is adequate flow through the boiler by monitoring the temperature rise through the boiler; the flow switch merely assures that the
measured rise is accurate. The boiler control starts to limit the firing rate when the temperature rise exceeds 53F and the boiler
goes into a Hold if the rise reaches 63F. If the flow switch does not close, look for the following:
• No water in the boiler
• Trapped air in the boiler or system - make sure that a steady stream of water exits the manual air vent shown in Figure
11.1 when it is opened.
• Valve closed in boiler loop piping
• Fouled Y strainer in boiler loop piping
WARNING
Asphyxiation Hazard. Fire Hazard. Do not attempt to jump or replace the thermal fuse. Doing so may
cause a breach of the heat exchanger casing resulting in property damage, personal injury or loss of
life.