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Wayne Whitney

On 2008-03-17, wrote:

standard tanks actually have very low standby losses, just got touch
your tank hot hot is it?

That's not really true, a standard 80% combustion efficient tank
heater has an energy factor of around 0.60, so of the theoretical
heating value of the fuel burned, 20% goes up the flue, and the other
20% is roughly standby losses from the tank. Even an electric tank
water heater, which has a 100% "combustion" efficiency has an energy
factor of 0.91-0.93, so 7%-9% of the energy is lost as standby.

Also, a conventional tank water heater has most of its standby losses
up the flue, which travels through the middle of the tank. This is
why the standby losses are much higher than an electric tank. You
wouldn't notice this by touching the outside of the tank.

current high efficency condensing tanks are over 90% efficent. that
should be enough for anyone

Combustion efficiency is not the same as energy factor. AO Smith
doesn't have an energy factor rating for the Vertex, they say that
anything about 65,000 BTUs/hr input doesn't need to get rated. One
can guess that the standby losses are less than a conventional water
heater (due to the helical flue in the Vertex), but still more than an
electric. So the energy factor is maybe 0.75-0.80.

For a tankess gas water heater, the standby losses are zero, so the
energy factor is equal to the combustion efficiency. So an 80%
combustion efficient tankless has an energy factor of 0.80.

Yours, Wayne