A method of controlling an orifice of a valve in an HVAC system to regulate the flow (Φ) of a fluid through a thermal energy exchanger of the HVAC system and adjust the energy transfer rate ( Q̇) of the thermal energy exchanger in response to a demand value (d), the method comprising: controlling the orifice of the valve in a first mode of operation where the flow (Φ) of the fluid through the thermal energy exchanger is regulated within efficiency constraints on the energy transfer rate ( Q̇) with respect to a first efficiency threshold value ( FT 1), the efficiency constraints being determined by comparing a temperature difference Δ T = T in - T out, between a supply temperature T in of the fluid entering the thermal energy exchanger and a return temperature T out of the fluid exiting the thermal energy exchanger, to the first efficiency threshold value ( FT 1), adjusting the orifice of the valve in order to attain the demand value (d), if the temperature difference Δ T = T in - T out is at or above the first efficiency threshold value ( FT 1), and adjusting the orifice of the valve in order to maintain a current flow (Φ) of the fluid through the thermal energy exchanger, if the temperature difference Δ T = T in - T out is below the first efficiency threshold value; and upon receiving an override signal, controlling the orifice of the valve in a second mode of operation where the flow (Φ) of the fluid through the thermal energy exchanger is regulated within the efficiency constraints on the energy transfer rate ( Q̇) with respect to a second efficiency threshold value ( FT 2), the second efficiency threshold value ( FT 2) being lower than the first efficiency threshold value ( FT 1), adjusting the orifice of the valve in order to attain the demand value (d), if the temperature difference Δ T = T in - T out is at or above the second efficiency threshold value ( FT 2), and adjusting the orifice of the valve in order to maintain a current flow (Φ) of the fluid through the thermal energy exchanger, if the temperature difference Δ T = T in - T out is below the second efficiency threshold value ( FT 2). controlling the orifice of the valve in a first mode of operation where the flow (Φ) of the fluid through the thermal energy exchanger is regulated within efficiency constraints on the energy transfer rate ( Q̇) with respect to a first efficiency threshold value ( FT 1), the efficiency constraints being determined by comparing a temperature difference Δ T = T in - T out, between a supply temperature T in of the fluid entering the thermal energy exchanger and a return temperature T out of the fluid exiting the thermal energy exchanger, to the first efficiency threshold value ( FT 1), adjusting the orifice of the valve in order to attain the demand value (d), if the temperature difference Δ T = T in - T out is at or above the first efficiency threshold value ( FT 1), and adjusting the orifice of the valve in order to maintain a current flow (Φ) of the fluid through the thermal energy exchanger, if the temperature difference Δ T = T in - T out is below the first efficiency threshold value; and upon receiving an override signal, controlling the orifice of the valve in a second mode of operation where the flow (Φ) of the fluid through the thermal energy exchanger is regulated within the efficiency constraints on the energy transfer rate ( Q̇) with respect to a second efficiency threshold value ( FT 2), the second efficiency threshold value ( FT 2) being lower than the first efficiency threshold value ( FT 1), adjusting the orifice of the valve in order to attain the demand value (d), if the temperature difference Δ T = T in - T out is at or above the second efficiency threshold value ( FT 2), and adjusting the orifice of the valve in order to maintain a current flow (Φ) of the fluid through the thermal energy exchanger, if the temperature difference Δ T = T in - T out is below the second efficiency threshold value ( FT 2).