Page 1
L641A - Cylinder Thermostat
Installation Instructions
The L641 is a water temperature thermostat designed for surface mounting on domestic hot
water cylinders. It is supplied complete with mounting strap for rapid and easy installation,
so draining of the system or tapping the cylinder is not necessary. The desired temperature
control point is set by the clearly scaled temperature selector on the front cover.
6050
40
1
Mounting Position
The cylinder thermostat
should be mounted
of the way from the bottom
of the cylinder.
70
80°
min
L641A1039
C
to
Height
to
40
L641A1039
min
Features
6050
70
80°
C
• SnapactionSPDTMicro
Switch™.
• Wiringterminalsareeasily
accessible.
• Tamperprooftemperature
setting.
• Optionalmanualsetting
knob supplied
2
Pre-insulated cylinder
Specication
Switch Action: SPDT, Type 1 CL.
Switch Rating: 230V~ 4(2)A 50...60Hz
Temperature Setting Range: 40...80°C
Differential: Approximately 10°C non-adjustable
Ambient Temperature Range: 0...55°C
Cylinder Surface Temperature Range: 0...95°C
42007492-001 R6
preparation
Remove a section of
insulation as specified.
Clean cylinder surface
to ensure good surface
contact.
45mm
80mm
3
Cover Removal
Caution: Isolate
power supply and
make safe before
wiring the unit to
prevent electric
shock and equipment
damage. Installation
should be carried
out by a competent
person.
Cover installation is the reverse of above
5
Wiring Diagram
Use connections C and 1 when
system operation is required on
temperature fall
* Terminal 2 used on
Y-Plan systems
230V~
50..60Hz
3 AMPS
Timer/Programmer
L641
C
1 2
*
Heating Load
4
Wiring to terminals
All wiring must be in accordance with I.E.E. regulations.
Use heat resisting flexible cable rated at 85°C.
A switch, having contact separation of at least 3mm in all
poles, must be incorporated in the fixed wiring as a means of
disconnecting the supply.
The thermostat is a class II (double insulated device) requiring no
earth connection.
The heating system must be appropriately fused with a fuse rating
of no higher than 3 amps.
For use in normal domestic environments.
EMC compliance considerations
Keep AC mains supply/load cables separate from signal wiring.
Refer to Code of Practice EN61000-5-1 and -2 for guidance.
max
8mm
Page 2
6
Mounting
Pre-formed insulation
Hot Water
Cylinder
Fixing hooks
Important Note:
Ensure good clean
surface for thermal
contact.
Locate mounting strap in cover slot
7
Fixing Strap and Hooks
Cut strap to size (if required) to ensure
thermostat is held firmly against cylinder.
Screw hooks into strap and link together
to secure around cylinder.
8
Temperature Setting (Tamperproof option)
Normal set point 60°
9
Optional Manual Setting Knob (Non tamperproof option)
Important Note:
Ensure slot in knob is aligned with arrow then push home firmly
What is a cylinder thermostat? …an explanation for householders
A cylinder thermostat switches on and off the heat supply from the boiler to the hot-water cylinder. It works by
sensing the temperature of the water inside the cylinder, switching on the water heating when the temperature
falls below the thermostat setting, and switching it off once this set temperature has been reached.
Turning a cylinder thermostat to a higher setting will not make the water heat up any faster. How quickly the
water heats up depends on the design of the heating system, for example, the size of the boiler and the
heat exchanger inside the cylinder.
The water heating will not work if a time switch or programmer has switched it off. And the cylinder thermostat will not always switch the boiler
off, because the boiler sometimes needs to heat the radiators.
Cylinder thermostats are usually fitted between one quarter and one third of the way up the cylinder. The cylinder thermostat will have a
temperature scale marked on it, and it should be set at between 60°C and 65°C, then left to do its job. This temperature is high enough to kill
off harmful bacteria in the water, but raising the temperature of the stored hot water any higher will result in wasted energy and increase the
risk of scalding.
If you have a boiler control thermostat, it should always be set to a higher temperature than that of the cylinder thermostat. In most boilers, a
single boiler thermostat controls the temperature of the water sent to both the cylinder and the radiators, although in some cases there are
two separate boiler thermostats.
IMPORTANT: No user serviceable parts.
This product and its associated documentation and packaging are protected by various intellectual property rights belonging to Honeywell Inc and its subsidiaries and
existing under the laws of the UK and other countries. These intellectual and property rights may include patent applications, registered designs, unregistered designs,
registered trade marks, unregistered trade marks and copyrights. Honeywell reserves the right to modify this document, product and functionality without notice. This
document replaces any previously issued instructions and is only applicable to the product(s) described. This product has been designed for applications as described
within this document. For use outside of the scope as described herein, refer to Honeywell for guidance. Honeywell cannot be held responsible for misapplication or the
product(s) described within this document.
EMEA,Z.A.LaPièce16,1180Rolle,Switzerland,byitsAuthorisedRepresentativeHoneywellInc.
ManufacturedforandonbehalfoftheEnvironmentalandCombustionControlsDivisionofHoneywellTechnologiesSàrl,ACS-ECC
Honeywell Control Systems Ltd.
Arlington Business Park,
Bracknell
Berkshire, RG12 1EB
Technical Help Desk: 08457 678999
www.honeywelluk.com
42007492-001 R6
© 2010 Honeywell International Inc.
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