Dynamic analyses of system responses during sudden change in input power for single-phase natural circulation loops are only a very recent concept. The present study theoretically investigates the transient behaviour of rectangular loops, subjected to direct heat addition and convective cooling. Non-dimensional forms of governing equations have been used alongside suitable closure relations. Three fundamental signals, having step, ramp and exponential profiles, and a newly-defined modified exponential signal have been applied on input power during power increase and decrease. Step and exponential changes have been found to lead the system towards instability during power up-surge, due to the impulsive nature of transition. The system also takes long time to attain steady-state for power decrease to a stable state following a step change. Step signal can be broken into equal increments for better performance. Increasing the number of increments and time-lag between successive steps, higher power operation can be sustained for a longer duration. Modified exponential signal has been found to be superior compared to others, both during power increase and decrease. It allows a moderately increasing gradient during power transients and hence the rate of oscillation growth is much smaller. (C) 2013 Elsevier Ltd. All rights reserved.