Underfloor and radiators are not an ideal mix. The underfloor acts as a large heat sink and absorbs all of the heat generated by the heat pump and holds the radiator inlet water temperature low. This means that the radiators will not give out any heat until the underfloor is up to temperature. If an underfloor zone opens requiring heat this will also drop the temperature to the radiators meaning they will stop emitting heat. Ideally it would be better to use underfloor throughout or if this is not possible, radiators throughout.
