Measure the heat going out the stack (stack temperature).
Measure the waste heat. Eff == Eout/Ein and Eout = Ein-Ewaste
No, the waste heat can be measured, too.
If your service tech is any good he'll measure the efficiency. The point is
that the energy input is well determined by the amount of fuel used. This is
a pretty basic design point, so is specified. Everything else is measured
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