Determining the appropriate size of a furnace for a 1500 sq ft house requires considering several factors beyond just square footage. The insulation of the house, climate, number of windows, and ceiling height all play a role in heat loss and gain. However, as a general guideline, a rough estimate is to allocate around 25-30 BTUs (British Thermal Units) per square foot for heating. For a 1500 sq ft house, this would result in a furnace size of approximately 37,500-45,000 BTUs. Keep in mind that this is a rough estimate and other factors may influence the final size requirement. To get a more accurate sizing, it is recommended to consult with a professional HVAC technician or contractor who can perform a heat load calculation. They will assess various factors specific to your home and provide a more precise recommendation to ensure proper heating capacity and energy efficiency. Choosing the correct furnace size is crucial for optimal comfort and efficiency, so consulting with a professional is highly recommended.