The present invention relates to a method for controlling a pump for feeding fluid (F) into a heating system (1000). The heating system has a hot fluid tank (HFT) receiving fluid from an associated fluid reservoir line (5) with an incoming fluid mass flow rate (dmcw/dt). A pump (P) receives fluid from the line, and pumps the received fluid with a mass flow rate (dmc/dt). A heat exchanging unit (HX) transfers heat (Q) to the fluid (F) from a medium (R). The transferred heat (Q) is maximized by controlling the pump (P1) in response to this information indicative of the transferred heat (Q), the fluid mass flow rate delivered by the pump thereby having a minimum as a function of the incoming fluid mass flow rate (dmcw/dt) when maximizing the transferred heat. The invention provides significantly improved heat transfer to the fluid and power savings for the pump. The invention also relates to a heating system, e.g. a heat pump system.