How I repaired my COSY central heating thermostat

The Cosy central heating thermostat is a ‘smart’ device that controls your central heating via an app on your phone. We had our Cosy installed some time ago and found that it really did reduce our heating costs. We were really impressed by the thermostat unit that displays the current temperature to one tenth of a degree and the phone app that lets you set up the schedule for switching the heating on and off.

Cosy central heating control thermostat gets hot and shows a high temperature

The Cosy thermostat unit has become unreliable, our heating is not coming on and off like it used to and the Cosy thermostat is showing a temperature that is much more than the room temperature. When you pick the Cosy thermostat up it feels hot on the base and the temperature is showing over 24°C, if that was the temperature we would be stripping off and passing out because of the heat. Normally the thermostat would be showing a temperature in the region of 20ºC.

To summarise the symptoms . . .

1. The central heating was erratic, and the house felt cold.
2. The Cosy thermostat unit felt hot and showed a temperature above the temperature of the room
3. The Cosy thermostat went blank about 2 minutes after being removed from its base unit.

The reason for all these problems with the Cosy control is that the rechargeable battery inside the thermostat unit has failed.

Here’s an explanation of what is happening . . .

The house felt cold

Because the control is not working properly the heating is not staying on as long it should to raise the house temperature up to the set temperature.

The Cosy thermostat gets hot

The rechargeable battery has a charger unit built into the thermostat unit. The charger keeps the battery charged. However, when the battery failed, all the electricity that should have been stored in the battery was converted into heat; that heats up the Cosy thermostat so it feels hot to the touch. It also means that the heating is switched off because the system thinks that the house is up to the required temperature. As the thermostat gets hot, of course it shows a higher temperature.

Cosy thermostat goes blank shortly after removal from charge base.

Because the rechargeable battery has failed, when the thermostat is removed from the base unit there is very little or no power in the battery, so the thermostat can’t show the temperature until it is replaced on the charger base.

Next page – part 2 – how to replace the rechargeable battery in the Cosy thermostat

No temperature shown on Cosy thermostat after removing it from base unit

Here’s my problem, the Cosy control thermostat is not showing any temperature. This could be the same problem, the battery inside the Cosy unit has failed, or it could be a different problem with the unit.