The rule is simple: reduce the demand for energy first, and only then replace the source. In practice that means the following sequence:
- Building envelope - insulate the roof or the ceiling below an unheated loft, then the facade, then the windows. Ceiling insulation is usually the cheapest measure per unit of saving.
- Controls and distribution - hydraulic balancing of the heating system, thermostatic valves, lowering the flow temperature.
- Heat source - a heat pump or other efficient appliance, sized for the new, reduced heat loss.
- Electricity generation - a solar array, which makes the most economic sense once a heat pump is already in the house.
If you cannot carry out the whole renovation at once, at least plan it as a whole. Simply leaving prepared penetrations for a future heat pump when the facade is redone, or reinforcing the roof for future panels, can save you thousands later.