# Efficiency rating for water heaters: What does it really mean?

I’m working on buying a water heater (natural gas, 50 gallon tank) to replace our current one of the same type which has a slow leak. One of the considerations in deciding which model to purchase is the efficiency rating, which is a number less than 1. The higher the rating is, the more efficient. I’m finding a considerable difference (~15 to 20%) in price between an “energy efficient” (e.g. Energy Star) water heater and less efficient models. However, their efficiency ratings are almost always 0.62 and 0.58, respectively, which doesn’t seem like that big a difference. But how do these efficiency numbers translate into annual monetary savings per year? (or energy used per year, and I can translate that into money) One salesman told me \$5 to \$7 saved per year. Is this about right?

I’m not familiar with the ratings in the format that you mention, for gas appliances I’m used to the efficiency stated as the output as a percentage of the input. For instance, if you have a standard 80% efficient heater, you get 800 BTUs of heat out of every 1000 BTUs that are burned by the unit (the remaining 200 BTUs are carried away by the flue gas). As you get to greater and greater efficiencies, the temperature of the flue gas drops to the point where it can now longer be exhausted by a standard gravity vent and required a fan to be removed, and then condensate needs to be dealt with at some point.

That’s getting besides the point though, but an 80% efficient unit will provide 800 BTUs of heat for every 1000 BTUs it burns, while a 96% efficient unit will provide 960 BTUs of heat for every 1000 BTUs it burns.

Now take the efficiency of your old unit and look at a years worth of old bills to see how many BTUs you burned (1 CF of natural gas has about 1000 BTUs if you need a conversion factor) and find out how many BTUs your old heater actually provided to you. Then find out how much less gas the new unit will burn to provide the same amount of heat, figure out how much that gas is worth, and there is the actual savings you should expect.

Also, if you’re in an area with a lot of sun, you might want to consider solar water heating.

Ech… Personally, I’m a big fan of the on-demand tankless water heaters. Whether in electric or gas, they are much more efficient then keeping 50 gallons of water hot all the time. They are also much much smaller, and produce a never ending supply of hot water if you need.

Depending on the layout of the house, usually two or three smaller units in appropriate places can supply hot water wherever it is needed. Say, a 3 gallon per minute unit for a remote bathroom, and then a 5 to 7 GPM unit to service the kitchen, laundry, and another bath.

Plus, with a centrally located tank you waste all the water that it takes to flush the pipe every time you turn the water on after the water in the pipe has cooled.

Is there any sort of tax credit for getting a water heater with a ‘better’ efficiency rating?

We bought some doors recently and the least expensive, the most attractive, and almost certainly the most energy efficient did not have any rating hence it did qualify for a tax credit.