> At face value, then in the worst case that's just 4 days per year of using resistive heat to keep a home warm.
The design philosophy for using 1% is that you may end up having to run your heating (or cooling) 24/7 to keep up with temperature delta between outside and desired inside, but it will keep up with the demand.
The rest of the time (99%) the mechanicals only run intermittently. Also note that the 1% would not necessarily occur every year: it is just the historical average. The charts also have the 0.4% extremes if you want to be extra conservative, but most building codes specify 1% because that is what experience has shown is a good trade-off.
Part of the process (in the US) is to use what is called the Manual J to determine/estimate/calculate how much energy is needed to maintain a particular temperature (typically ≥70F/21C in winter, ≤75F/24C in summer):
> The Cooling Design Day is effectively the "worst case" day for your air conditioning loads. The "worst case" hour of this day determines equipment capacity, fan sizes, and subsequently duct sizes. This largely impacts first cost. The Design Hour also impacts peak KW demand which often has a huge impact on the utility bill.
The design philosophy for using 1% is that you may end up having to run your heating (or cooling) 24/7 to keep up with temperature delta between outside and desired inside, but it will keep up with the demand.
The rest of the time (99%) the mechanicals only run intermittently. Also note that the 1% would not necessarily occur every year: it is just the historical average. The charts also have the 0.4% extremes if you want to be extra conservative, but most building codes specify 1% because that is what experience has shown is a good trade-off.
Part of the process (in the US) is to use what is called the Manual J to determine/estimate/calculate how much energy is needed to maintain a particular temperature (typically ≥70F/21C in winter, ≤75F/24C in summer):
* https://www.acca.org/standards/technical-manuals/manual-j
* https://www.youtube.com/@HomePerformance/search?query=Manual...
> The Cooling Design Day is effectively the "worst case" day for your air conditioning loads. The "worst case" hour of this day determines equipment capacity, fan sizes, and subsequently duct sizes. This largely impacts first cost. The Design Hour also impacts peak KW demand which often has a huge impact on the utility bill.
* https://energy-models.com/blog/hvac-what-cooling-design-day
* https://hvac-blog.acca.org/sizing-selecting-hvac-equipment-p...
* https://www.airequipmentcompany.com/2021/what-does-design-da...
Here's an overview of the design process for one particular municipal jurisdiction:
* https://www.suffolkva.us/DocumentCenter/View/7362/Understand...