This is winter-design temps & wind chills in the upper Midwest with which to check the heating Btuh performance of your Oil or Gas furnace.

With a notebook; list the temp & wind velocity; list Burner-Run-Time; then list the time of next Burner Start; divide the Total-Cycle-Time (TCT) into the burner run-time, X's the listed burner OUTPUT listed on furnace tag.

Then do a heat-loss heat-gain calc http://www.loadcalc.net/ & see how the results of each compare.

Burner runs 15-mins it's 60-mins to next start-time; 15-BRT / 60-TCT is .25% * OUTPUT of furnace is 57,000-Btuh = 14,250-Btu/hr. Burner Runs 30-mins TCT to next start 60-mins; .50% * 57000 is 28,500-Btuh.
Do the same during hottest summer weather with around 15-mph winds... This will help to correctly size your next A/C or heating system replacement; also illustrate where you can reduce energy usage..