This report details a study undertaken to evaluate the carbon emission impact of air-source heat pump-based HVAC systems in comparison to traditional natural gas furnace HVAC systems in single-family homes in Ontario. Twelve hourly energy models were created to evaluate three different home types, each with four different HVAC systems: conventional HVAC system (gas furnace, gas DHW, and DX air conditioning), air-source heat pump with electric back-up & electric DHW, air-source heat pump with gas back-up & gas DHW, and variable refrigerant flow air source heat pump with gas back-up and gas DHW. The annual hourly energy use developed by each model was then used to calculate operating costs and carbon emissions, based on both specific hourly generation source energy data (provided from the IESO), and assuming a marginal energy generation source of natural gas only. Key findings include: all the models using ASHP systems (2, 3, and 4) consumed less energy than the model using the conventional natural gas furnace HVAC system (1); from an annual operating cost perspective, the conventional HVAC system (1) was the lowest in all cases due to the significantly higher cost for electrical energy as compared to natural gas energy, with system 2 having the highest operating cost; & using the specific hourly generation mix provided by the IESO resulted in lower CO2 emissions for all the ASHP based HVAC systems. Emission calculations were also conducted using a marginal natural gas generation; the study’s results highlight the significant impact that generation mix assumptions have on the forecasted CO2 emissions, with additional study recommended to better understand the impact and optimum ASHP sizing in relation to the heating load from both a cost/benefit and C02 emission perspective.