[whateveracct]

I think the comment was saying below 30F and below 10F. Much warmer than you're saying.

Also..

> It is warmer than -16C/3F at Chicago (O'Hare) for 99% of the time (i.e., except for 4 days a year), and warmer than -18.7C/-2F for 99.6% of the time (2 days).

If my heat doesn't work for those days, I'm kind of boned. Four days per year without a working heat pump? That's a mess.

[ssl-3]

At face value, then in the worst case that's just 4 days per year of using resistive heat to keep a home warm.

Which is, of course, very expensive to use -- but it's only expensive for those 4 days. Resistive heat can be avoided for the other 361.2425 days in a year.

In the US (as of August of 2025), the average price of residential electricity per delivered kWh is $0.1762 [1].

If using resistive heat averages 4kW during each of those 4 days (it's probably either more than that, or less than that, but ballparks are ballparks), then that's about $16.92 for each of those days. Or: $67.66, per year.

Not so bad, right? Or at least, not "boned."

[1]: https://www.eia.gov/electricity/monthly/epm_table_grapher.ph...

[throw0101a]

> 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):

* 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...

[ssl-3]

I understood all of that when I wrote my comment.

The choir's appreciation towards this unnecessary lesson is not very good.