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10. Negative-head pumps
Pump types:
STN, SSN and STL 2.0 CN.
10.1 General
Automatic negative-head shower pumps are designed to operate
automatically in installations where the pumped system outlets
are above the stored water level.
Shower pumps, types STN and SSN, feature independent
pressure and flow controls, with built-in non-return valves and
stainless-steel expansion tanks in each pump end. This system
ensures complete hot- and cold-water system isolation with
stable control over a very wide flow range.
The pump is fitted with dry-running protection. Should the water
supply fail, the pump will stop to prevent damage to the shaft
seals.
10.2 Dry-running protection
STN, SSN and STL 2.0 CN negative-head pumps feature dry-
running protection to avoid damage to seals caused by water
starvation. This condition is possible if there is insufficient stored
water or the pump strainers are blocked.
If the pump runs for 60 seconds with an insufficient water supply,
the pump will stop and "lock out".
Resetting the "lock out"
To reset the pump, switch the electricity supply off for
10 seconds.
When the electricity supply is switched on again, the pump will
run for a few seconds to recharge the tanks. If it continues to run
and there is no demand, this would indicate that one of the
following is still present:
• No water in the system.
• Blocked strainer.
• Air which has not yet cleared.
10.3 Connecting the pump
For the pump to work correctly, it must have:
• Air-free supply of hot water – the use of Surrey flanges is
recommended.
• 22 mm (minimum) feed pipes, gate valves and flexible hoses.
• Minimum head of water to supply the pump 2 metres (6'6").
However, the STN and SSN pumps will function with a supply
head as low as 600 mm (2 ft), providing the water make-up
system has unrestrictive pipework, and provision is made to
avoid air locks.
• Maximum hot-water temperature of 60 °C conforming to
BS 6700: 2006.
• Maximum static head of 8 m, i.e. the highest point of the
system must be less than 8 m above the pump. See fig. 4,
page 14.
10.4 Installation instructions
• The hot-water cylinder must have an expansion pipe or be
fitted with a vent pipe connected to the pump hot-water supply,
to allow air from the hot-water cylinder to escape.
Excessive air entering the pump will cause faulty operation
and damage to the pump seals which will cause serious water
leaks.
• Feed pipes, hot and cold, must be minimum 22 mm with
22 mm service isolating gate valves. The flexible hoses
supplied
must
be used. The hoses must be connected without
kinks which will restrict water flow. The maximum bend angle
is 45 °.
• Ensure an adequate head of water and non-restrictive make-
up pipework.
Inadequate feed head will cause pump cavitation, unstable
water temperature, noise and ultimately pump and seal
damage.
• All air must be thoroughly purged from the system. If air is
present in the pump, it will be unable to generate sufficient
pressure to operate the pressure switch. This will cause the
pump to run or pulse continuously.
•
Lavatory cisterns
If the pump is to supply a lavatory cistern, certain precautions
should be taken to avoid frequent starting and stopping of the
pump due to low flow rates.
If possible, best practice is to supply the cistern from the
mains water supply. Failing this, a "Torbeck" or equilibrium
valve is recommended. If either of these are not possible, the
following instructions should be followed:
–
A:
The pump is supplied with two expansion or pressure
tanks. They have capacities of 500 ml and 160 ml.
The larger tank must be used on the cold side of the
pump
.
–
B:
The cistern filling pipework must be as free-flowing as
possible with all flow restrictors removed.
It is possible to change the tanks from one side of the pump
to the other. When doing so, take care to ensure that none
of the fibre washers are lost.
•
Washer dryers
Only brass pumps must be used for these applications.
10.5 Pressure and flow control
Shower pumps for negative-head applications are controlled by
both water pressure and water flow.
Twin-impeller pumps incorporate two separate pressure and flow
control systems, one on each pump end, ensuring complete
isolation of hot- and cold-water systems and consistent operation
over a wide range of unbalanced flow rates.
The control system manifold includes an integral non-return valve
to maintain system pressure when water is not flowing.
Operation
The pressure switches start the pump on falling pressure.
The switch point is factory-set.
Once the pump has been started by the pressure switches, the
flow switches maintain the pump running, providing the flow rate
is above 0.4 l/min. When the flow rate drops below 0.4 l/min. or
the demand is turned off, the pump will stop after first running on
for 3 seconds to recharge the pressure tanks.
10.6 Operating instructions
Taps and valves used on the "pressure system" must be fully
turned off when not in use.
10.7 Maintenance
Washers and seals must be replaced when necessary to prevent
leaks or drips.
Any leak or drip will cause the pump to operate periodically, the
frequency dependent on the rate of the leak.
Tank air-pressure setting
The pressures are factory-set. In the event of a leak, they should
be set as follows:
The smaller tank (160 ml) should be set at a pressure between
0.9 and 1.0 bar. The larger tank (500 ml) should be set at a
pressure between 0.5 and 0.6 bar.
For further information regarding general maintenance and
service, see section
.
Caution
Continued dry running will lead to overheating of
the shaft seals and eventually to a water leak.
Caution
Do not connect the pump directly to the water
mains supply.
Note
It is important that the total head on the pump
is less than 8 metres. Otherwise the pressure
switch can never operate and hence the pump
will fail to start.
Note
In certain applications, e.g. steam cabinets, it
may be necessary to increase the pressure.
Consult the appliance manufacturer.
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