If you know the outside and inside temperature, you can calculate the thermal gradient, i.e. how much temperature your room/ house is losing. then you know how much heat you need to input in order to maintain (or raise) the temperature.

I am halfway there with this project in my house with internal sensors. Next step is to get the external sensor, and some relays on the heating controller, then just write the if then else code. In your case the code would be more complicated, but the concept is the same.

Our house has the problem of single glazing, but replacing all the windows with UPVC would be expensive and ugly. Replacing them with double-glazed sash windows would be probably twice as expensive. So we won’t be able to reduce our heating bills for quite a while yet.

The idea of using computers for heating seems somewhat inefficient to me but I would agree with the concept of trying to harness the excess heat they produce then being used normally.