Siemens room thermostats and sensors including QFA3171 humidity sensor, RDS110 digital thermostat, rotary dial thermostat and CO2 sensorSiemens room thermostats and sensors including QFA3171 humidity sensor, RDS110 digital thermostat, rotary dial thermostat and CO2 sensor


Thermostat wiring usually belongs to one of the following types: two-wire for basic on-off switching; three-wire that includes the common or neutral wire; and four or five-wire for multi-function thermostats used together with a programmer. Correct connection according to the wiring diagram will save you from two of the most frequent errors, which are heating system that cannot call for heat and heating system that cannot shut down.

Types of Room Thermostat Connections

A two wire thermostat is the simplest arrangement, switching a single circuit on and off directly and requiring no separate neutral connection. In the case of three-wire thermostats, there is an added common or neutral wire that is essential when there is a requirement of power to be supplied permanently to the thermostat, which is the case of displaying units, electronic devices, and so on. Four or five wire thermostats are installed when there are more than one functions for the thermostat to control.

Wiring Diagrams

A generic two wire diagram runs a switched live from the thermostat directly to the heating system's call for heat terminal, with no neutral required at the thermostat itself. A generic three wire diagram adds a neutral connection back to the thermostat, alongside the switched live, to power the thermostat electronics independently of the heating circuit. A four wire diagram with a programmer typically separates the time control signal from the temperature control signal, allowing the programmer to set when heating is available and the thermostat to set the temperature within those hours.

Be sure to check out the wiring diagram of your thermostat model first before connecting as each manufacturer varies on their wiring terminals and colours despite using the same wiring setup.

Understanding the Common Thermostat Terminal Labels

L is the live connection, carrying the incoming supply. N is neutral, completing the circuit for the thermostat electronics. E is earth, required on any thermostat with an electrical connection for safety. Com is the common terminal, the shared reference point for the switching contacts. NO stands for normally open, a contact that closes to call for heat. NC stands for normally closed, a contact that opens to call for heat, used less often but present on some heating and cooling switchover applications.

Wiring Room Thermostat to Zone Valve

A thermostat switching output is directly connected to a zone valve motor, making the valve open only when there is a call for heat from the thermostat. The zone valve, once fully open, typically feeds back an auxiliary switch signal to the boiler and pump, ensuring the heat source only fires once the valve has actually opened. This is one of the most searched wiring combinations because it sits at the exact point where thermostat faults and valve faults, covered fully in our zone valve troubleshooting guide, are easy to confuse.

Solving the Issues With Thermostat Wires

If there is no call for heat at all, check the Com terminal connection first, since a loose or incorrect connection here is the most frequent cause of a thermostat that appears completely dead. If the heater won’t shut off, make sure that the wiring on the NO and NC terminals is done correctly, because the wrong wiring may keep the circuit closed.

Questions & Answers

Which wires go where on a room thermostat?

It depends on whether your thermostat is a two-wire, three-wire, or four-wire thermostat because each one will have its own diagram that needs to be referred to for correct terminal labelling.

Do I need a neutral wire for a room thermostat?

A simple two wire on and off thermostat does not need a neutral, but three wire and four wire thermostats with electronic displays or additional functions usually do.

Can I replace my thermostat myself?

If you are competent working with low voltage heating controls and the wiring is straightforward, many replacements are straightforward, though anything involving mains wiring should be carried out or checked by a qualified electrician.

Recommended Replacement Thermostats

For fan coil and universal applications needing a 3 wire or programmable option, the Siemens RDG160T gives a 7 day time switch with DC 0..10V or on/off output, suited to heat pump and universal heating setups. Where a digital display and Green Leaf energy saving functions are wanted, the Siemens RDG200T range extends this to fan coil, universal and VAV applications with built in air quality monitoring. For a straightforward two wire on/off replacement, the Siemens S55770 electromechanical thermostat covers 8°C to 30°C with volt free contacts, suitable for heating only or cooling only systems without a neutral connection. Browse the full range in our Thermostats & Humidistats category.

Shop replacement room thermostats at iACS Store, including Siemens and Danfoss.