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The design day temperature he used, was a modestly cold number but nowhere near the record. I recalculated it based on the record outdoor low temperature but used a linear relationship to the delta T.

A bit surprising perhaps, but you should not use the absolute coldest temperature. You use the design temp.

If you are sizing conventional fired systems, using the design temperature is what to do. This usually will cover 98% of the heating season. However, if using heat a heat pump, its much more complicated. The problem is that as you drop below design temperature, the heating capacity of the heat pump is steadily dropping I.E. ....Here in Chicago, the design temperature is about 0F, However, we have hit -25F as records. A fired system that is designed right on will be able to keep a building warm at 70F when it is 0F outside with let's say 100,000 btu/hr, so for the most part it will be able to maintain the building at 45F if it is -20F outdoors ( more or less, depending on ground contact tempering for example), also needing 100,000 btu/hr. However, with a heat pump, when you're below design temperature (0F) the heat pump capacity is now lower, so you can no longer producing 100,000 btu/hr so you can no longer maintain a 70F differential between the outdoors and indoors. The interior temperature will now drop lower than the 70F differential. This issue seems to currently be ignored in design calculations. As an example, we have a client that had a Heat Pump system installed to heat a large space. They could heat to 70F when it was 0F outdoor. However, when we had some extreme extreme outdoor temperatures ( -10 to -12F) a year or two ago, and they could not get the building above 40F. I have not been able to find what the COP is for a heat pump at 0F versus -25F, but I bet it is quite significant in a percentage or reduced heating capacity.

I just can't think of any reasons why young people would avoid the construction/mechanicals trades...must be something wrong with them

Social stigma, risk of injury/cancers, lack of trade schools/summer jobs/learning from parents about trades that would interest them.

Middle and High Schools don't focus on any kind of exploration of trade work, they push everyone to aim toward going to college.

Vocational education used to be a larger part of public education and it fell by the wayside thanks to Department of Education policy sine its inception and general push by Congress during the Cold War to push everyone towards college thinking that it would increase the number of scientists & engineers without at thought that doing so could compromise the industrial/skilled trade base.

Employers also used to be able to test applicants for technical skills and do more in-company training for various skills similar to the military. Then there were multiple court rulings challenging these tests and in-company credentials, which lead to the gate keeping and credentialism being outsourced to universities or third party trade/occupational associations with a vested financial interest in shrinking the labor pool.