Heat load derivation

Could one estimate their heat load by clocking the meter and adjusting the gas valve on the furnace?
I turn down the gas valve reducing flame size until the furnace runs continuously on a design temp day, then clock the meter to determine the input btu's. Multiply the input btu by the AFUE factor should provide the output btu.
ie: existing furnace input btu 100k, output 80k
Turn flame down and read meter clocked at 60k (furnace running continuously and maintaining temp)
Effective heat loss would be 60k * .8 = 48k
Would this not be more accurate than a mathematical model assuming the in/out relationship of the existing furnace is still 80%.
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Get a self-regulating digital thermostat and let the technology do this for you. Honeywell makes a good one--their technology is called "adaptive recovery".
You may need to run the ground wire loop to the "G" terminal--better read the manual that comes with the stat to see for sure.
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