Setting the heat for the second floor and cooling for the first floor when it's 79°F (26°C) outside in May may not be the most efficient use of your HVAC system. Typically, during warm weather, it's more energy-efficient and comfortable to set the air conditioning on both floors rather than using the heat on one floor and cooling on the other.
If you were to set the heat for the second floor and cool the first floor in such conditions, the HVAC units would have to work against each other, resulting in inefficiency and potentially causing strain on the system. The cooling unit on the first floor would be working to lower the temperature, while the heating unit on the second floor would be trying to raise it. This conflicting operation could lead to increased energy consumption, reduced system performance, and potentially even malfunctions or breakdowns over time.
Ideally, it's best to set the HVAC system to cool both floors when it's warm outside. If you're experiencing temperature imbalances between floors, it might be worth examining the zoning settings, insulation, and ventilation of your home to address the issue more effectively. Consulting with an HVAC professional can help you optimize the system and make it work efficiently throughout the year.