A quasi-steady state model has been developed to assess of the potential of variable flow strategies to improve the overall thermal efficiency of Photovoltaic-thermal (PVT) collectors. An adaption of the Duffie-Beckman method is used to simulate the PVT, in which the overall loss coefficient and heat removal factor are updated at each timestep in response to changes in flow rate and ambient conditions. A novel calculation engine was also developed to simulate a building heating loop connected to the solar loop via a counterflow heat exchanger that calculates the steady-state conditions for the system at each timestep. The results from PVT simulation are in good agreement with test data obtained from the solar simulator - environmental chamber facility at Concordia University. Further validation for the overall system was carried out via a parallel simulation run in TRNSYS and the model-predicted annual solar heat gains were within 3.6%. The results of the investigation show that a variable flow rate strategy has significant potential to improve thermal efficiency. This benefit was found to be dependent on ambient and process loop conditions, and most effective for systems with greater difference between heating process supply and return temperatures.