Connecting the EMOS thermostat to the boiler: terminals and OpenTherm

The EMOS thermostat connects to the boiler in one of two ways. Which one it will be depends on the boiler, not the thermostat.

Non-polar contact

The most common case. The boiler has a pair of terminals on the terminal block labeled as room thermostat, often TA, RT or numbered 3 and 4. These are shorted by a small wire from the factory. The bridge is removed and two wires from the thermostat, or from its receiver, are connected to the terminals. The thermostat only opens and closes the contact, no voltage is applied there.

OpenTherm

Boilers with this bus have terminals labeled OT. The wiring is also two-wire and the polarity does not matter, but data communication is transmitted. The thermostat tells the boiler what water temperature to maintain, so the burner does not run at full and does not switch on and off, but modulates smoothly. The difference in consumption is most noticeable in spring and autumn.

What to be careful of

• Before working, turn off the boiler and the circuit breaker, the terminal block is often under voltage.
• Do not run the thermostat wire in the same conduit as the 230 V power line, interference may occur.
• With the wireless version, place the receiver near the boiler, not in a metal distribution box, metal attenuates the signal.
• In the reference room, leave the radiator without a thermostat head or fully open the head.

If you are unsure about the wiring, leave it to the boiler service technician. Incorrect connection to terminals with voltage will damage the thermostat and such damage is not covered by warranty.