Honeywell Cylinder Stat L641A1039

Wired 230V Analogue White

The Honeywell L641A1039 is a strap-on temperature controller designed for domestic hot water cylinders. It is a surface-mounted "aquastat" that senses the water temperature through the cylinder wall to control the flow of heated water from the boiler via a motorised zone valve.

Key Technical Specifications

Model Number: L641A1039

Temperature Range: 40°C to 80°C (Adjustable via the external dial).

Switching: SPDT (Single Pole Double Throw), allowing for both "heat demand" and "satisfied" signals.

Switch Rating: 10(3)A at 230V AC.

Differential: Fixed at approximately 8°C.

Mounting: Supplied with a spring-loaded tensioning strap (suitable for cylinders from 225mm to 450mm diameter).

Ingress Protection: IP40.

Why the L641A1039 is the Industry Standard

Reliability: It uses a high-quality liquid-filled sensing element for accurate temperature detection.

Versatility: The SPDT switching means it is compatible with all standard "S-Plan" and "Y-Plan" system architectures.

Easy Installation: No need to drain the system; it simply straps onto the cylinder after removing a small square of insulation to ensure metal-to-metal contact.

Installation & Wiring Tips

Placement: Mount the thermostat approximately one-third of the way up from the bottom of the cylinder. Placing it too high will result in the boiler shutting off before the full tank is heated.

Contact: Ensure the sensing base is firmly touching the copper or stainless steel of the cylinder. Use a small amount of heat-conductive paste if the surface is uneven.

Wiring Terminals:

1 (Common): Live feed from the wiring center/programmer.

2 (Call for Heat): Connects to the "brown" wire of the motorised zone valve.

3 (Satisfied): Used in Y-plan systems to signal the valve to close/divert.