"Percival P. Cassidy" wrote:
We have a RuudGlas PaceMaker gas water heater (installed by a previous
owner) that never feels warm on the outside, yet the EnergyGuide label
on it shows it as using about 35% more energy than the most efficient
one of similar type.
Obviously we're not going to replace it, but I'm curious about what
could have been done to make it more efficient.

Does the flue feel hot when the heater is on (hint: be careful testing this)?

Anything to take the heat that goes up the flue and instead applies it to the
tank water increases efficiency. Nothing practical a homeowner can do as a
retrofit, 'tho.