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11.1 Fault finding, negative-head pumps (STN, SSN and STL 2.0 CN)
Before starting work on the pump, make sure that the electricity supply has been switched off and that it cannot be
accidentally switched on.
Fault
Possible cause
Remedy
1. The pump fails to start. a) The electricity supply is switched off.
Switch on the electricity supply.
b) The fuses are blown.
Replace the blown fuses. If the new ones blow too, the
electric installation should be checked.
c) The built-in thermal protection has switched
off the motor.
The thermal protection resets automatically within
1 to 2 hours.
d) Isolating valves are closed.
Open the isolating valves.
e) Strainers are blocked.
Close the service isolating gate valves, clean the
strainers and re-open the service isolating gate valves.
f)
Pump installed with non-vertical discharge
ports (flow switches).
The pump must be installed with vertical discharge ports
(flow switches).
g) Dry-running protection operated.
Switch off the electricity supply for 10 seconds to reset.
2. The flow from the
shower drops.
a) Blocked strainers.
Clean the pump strainers.
3. The pump runs
continuously.
a) A flow demand still exists.
Check the installation for leaks.
b) Flow switch or reed switch faulty or
incorrectly set.
Adjust or replace as appropriate.
c) Faulty pressure switch.
Check the pressure switch.
d) The float in the flow switch is stuck in the
up (on) position.
Make sure that the flow switch can operate correctly and
there is no debris in the flow switch.
4. The pump pulses.
a) The use of other taps in the house may
cause the pump to start momentarily.
Contact your installer.
b) Leaks in the system.
Check the system for leaks.
c) Faulty non-return valve.
Check the non-return valves and replace, if necessary.
5. Pump stops when there
is a demand.
a) Flow demand below 0.4 l/min.
Increase the flow demand.
b) Flow switch or reed switch faulty.
Replace the flow switch or reed switch.
6. Pump hunts when
shutting down.
a) Incorrect tank air-pressure setting.
Adjust the tank air-pressure setting.
b) Insufficient run-on time.
Replace the printed-circuit board (PCB).
7. Pump hunts when
starting up.
a) Flow rate below 0.4 l/min.
Increase the flow rate.
b) Flow switch or reed switch faulty or
incorrectly set.
Adjust or replace as appropriate.
Grundfos.bk Page 8 Wednesday, September 3, 2008 1:36 PM
