Honeywell DT90E1012 Digital Room Thermostat

The Honeywell DT90E1012 is a wired digital room thermostat from Honeywell's DT range of domestic heating controls. It measures the air temperature in the room and switches the heating circuit at the set point, holding the space at the chosen temperature rather than letting the boiler run on a fixed schedule. It is a hard-wired control, so it replaces an existing wired thermostat on the same circuit.

• The room reaches the set temperature but the heating does not shut down, or the boiler runs continuously.
• The heating fails to come on at all when the thermostat is calling for heat.
• The displayed temperature drifts noticeably from the actual room temperature, causing short cycling or long cold spells.
• The display is blank, flickers, or the buttons no longer respond to changes.
• The thermostat clicks but the boiler or zone valve does not react.

• Confirm the existing control is a wired thermostat and that the wiring arrangement at the wall plate matches the replacement.
• Check the part number on the old unit against HONEYWELLDT90E1012, as the DT range includes several similar-looking models.
• Isolate the heating system electrically before removing or fitting any thermostat, and confirm the wiring against the manufacturer's instructions.
• Note the existing thermostat's position and cable entry so the replacement can be mounted and connected in the same place.