Controlling central heating with outside temperature?

I have Tado thermostats. Can they be set to percent open instead of temperature?

I’d like to clarify your request. Does your heat exchanger thermostat control the temperature of water being distributed to the radiators? Are you attempting to implement something like an outdoor reset curve where the water temperature goes up as the outdoor temperature goes down?

Yes exactly. I wanted to control my central heating temperature of the radiator water by the outside temperature. I solved it by putting an external temp sensor on the radiator heating pipe from the central heating, an external temp sensor outside and a Tado thermostate on the central heating control. I then build a small smart app to monitor outside temperature and adjust the radiator heating water with the tado thermostate. I did not find a way to control the tado stepwise - it seems to only allow binary heating or not heating. I have tried to build a small hack that I hope allows gradual heating - but it is not documented. Instead of setting the tado to heat I measure its temperature sensor and then set the target one degree above if I need to heat or one degree below if I need to cool. The app runs every five minutes.

It works, but the radiator temperature forms a sinus curve +/- 3 degress around the target temperature and many thermostat adjustments drain the bateries. I would like it to be more stable and need less adjustment, but have not found a way yet.

If you have a smarter idea of how to get an external sensor to work with tado, I would love to hear it.