A method, system (100), and a computer program product comprising a non-transient computer-readable medium (190) having stored thereon computer program code configured to control one or more processors (180) of a computer (170) for operating an HVAC installation (200, 300), wherein a set of enthalpies (H1,i, H2,i, H1,o, H2,o) and flow rates (ϕ1, ϕ2) as variables of the HVAC installation (200, 300) is monitored and used for controlling the operation of said HVAC installation (200, 300), comprising the steps of: (a) dividing said set of enthalpies (H1,i, H2,i, H1,o, H2,o) and flow rates (ϕ1, ϕ2) into a first and second subset; (b) measuring each variable of said first subset with a related sensor (110, 120) arranged in said HVAC installation (200, 300); and (c) determining the variables of said second subset from the measured variables of said first subset by using a mathematical and/or empirical relationship between the variables of said first and second subset.