1. A Heat Flow Model (HFM) node used in a Heating, Ventilation and Air Conditioning (HVAC) fault detection graph comprising: a FwdIn edge configured to receive parameter ranges from a downstream direction; a FwdOut edge configured to output parameter ranges in the downstream direction; a RevIn edge configured to receive parameter ranges from an upstream direction; a RevOut edge configured to output parameter ranges in the upstream direction; and node specific configuration data that defines the functionality of the node.