A quasi-steady-state model has been developed to evaluate 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 simulation method was developed to simulate a building heating loop connected to the solar loop via a thermal storage tank or counterflow heat exchanger. The model was validated with published data and with a parallel TRNSYS simulation. The results of the investigation show that optimizing flow rate has significant potential to improve thermal efficiency. This benefit was found to be dependant on ambient and process loop conditions, and was limited to the counterflow heat exchanger case rather than that with a thermal storage tank.