What does BTUH rating mean on water heaters?

[DesignGuy]

I'm getting ready to replace my Rheem 75 gallon gas water heater with a Kenmore Power Miser of the same size.

However I noticed that the Rheem has a rating of 75500 input BTUH, and the Kenmore has rating of 55000. I'm not clear if a loer or higher number is better, as some explanations I've read seem to indicate that the Rheem would use more BTU's to heat the water than the Kenmore (thereby making the Kenmore more efficient).

Is my reasoning correct or is the reverse actually true?

Thanks in advance.

[dpb]

...

No and no...that rating is related to the "first hour rating" and correlates to how fast it will heat cold water initially. The higher the input, the faster the water will get hot (and reheat when recharged).

The overall energy efficiency is the energy factor (EF) and comprises recovery efficiency and standby and cycling losses.

See (watch wrap)

for an overview. There's a rating on each unit that compares operaing costs that correlates (roughly) to efficiency but that's not the only criterion in making a selection.

for

[Wayne Whitney]

The reverse is probably true. The BTUH rating doesn't tell you anthing about efficiency, it just tells you the rate at which the unit can draw and burn gas. If the units are of comparable construction, they likely have comparable efficiencies. In this case, the higher BTUH unit will simply reheat its hot water tank faster after hot water is drawn out of it.

For efficiency, look at the water heater's "ER" rating. This will be a number between 0 and 1 that basically tells you how much of the theoretical energy available in the gas the water heater burns actually makes it into heating the water. It reflects inefficiences in the burner itself as well as standby losses from the storage tank.

Cheers, Wayne

[Keith Williams]

BTUH == BTU per Hour. A BTU (British Thermal Unit) is the heat needed to raise a pound (pint) of water from 39.1F to 40.1F (love them English units! ;). A BTUH is then the total number of pints*degrees it'll raise the temperature of the water per hour.

The Kenmore unit will take about 40% longer to raise the water to temperature than the Rheem. During that time they'll take exactly the same energy (ignoring insulation losses). If they had the same insulation (unknown) the cost to run would be very nearly the same but the Rheem would have a 40% faster recovery.

[DesignGuy]

Thanks guys, that makes it clear.