Is there a manometer available to use with Arduino?
Bruno, there are a number of sensors available, try an Ebay search, to do what you want.  However, you need to determine what pressure range you are looking at.  As you mention manometer I assume you are working in the mBar range but to me that would be at odds with the pressure typically seen in whole building systems.  An Ebay search on 'Pressure transducers' will yield a good range of devices, likely to be primarily industrial types and some using wheatstone bridge type outputs.  you may need an op-amp circuit to buffer / amplify the output signal but virtually all sensors can be interfaced to an Arduino with the correct conditioning cicuitry.  As you are aware to measure pressure all sensors will need to be invasive into the pipework.