Weatherspark has 6 years of past data for Angel Fire. No others nearby, but that 6 years averages -25.5.

Sounds like -25F would be a good design temp then, eh?

If the heat loss calculation methods overestimate by even 10% (most do), and you sized the boiler EXACTLY for the calculated -25F load you'd sail through a sustained -35F without a problem.

If you're 15% oversized for the -25F load it wouldn't give up any ground even at a sustained -40F.

That sounds like a good temp to me as well. Thanks for the help! I do appreciate it!