Tankless Water heater with solar thermal backup

Tankless water heater installed

Tankless water heaters are getting very popular in the United States.  I first wrote about them in May of 2009 with our wonderfully titled post, "Tankless Water Heaters: Not Sure if I'd Demand Them"  In that post I was hesitant to recommend tankless water heaters because while they are efficient, they are not always economical.

For example, here in Atlanta, GA, we have access to natural gas to heat our water.  Right now natural gas is VERY cheap!  Below is a graph of my natural gas consumption in 2009 and the beginning of 2010 taken from what we're working on at

Along with heating our water, we also heat our home and cook our food with natural gas.  So to get an estimate of how much natural gas we use just to heat our water, I have to make some simple assumptions.  You can see how much our consumption spikes in the winter (and this last winter was a cold one).  In the summer we are using zero natural gas for heating our home, and all of it is used to heat our water and cook with.  I will assume that 2/3 goes to heating our water and 1/3 goes to cooking our food.  For the summer months we average about 9 therms of consumption.  Right now I'm paying about .85 cents/therm.  That means that if my assumptions are correct, I'm paying about:

$$$ to heat water each month= 2/3 * 9 therms * $.85/therm = $5.10

So I'm only spending $5 bucks a month to heat our water!   Even if a tankless cut our natural gas consumption needed to heat our water in half, I'd only be saving $2.50 a month.  In the winter, this cost to heat the water will go up a little because the incoming water temperature will be lower.  In my situation, I would rather use any money that would be spent on a tankless water heater on something that could save more energy and water and provide a greater return on investment.

But not everyone heats with natural gas and a tankless water heater does save energy and money!  Some use electricity or propane.  Also, some people have more than 2 people living in their home, so their going to be using more natural gas.  For instance, Guy Marsden was heating his water with propane in a standard hot water tank and a solar thermal backup.  Then he installed a tankless water heater made by Rinnai.  His post discusses his installation, includes pictures, and graphs.  But he really nails his closing argument.  In discussing how living with a tankless water heater differs from a regular water heater he says:

So clearly tankless water heaters require a change in usage strategy in order to prevent unexpected temperature dips!  It's not a design failure it's a failure of the humans who refuse to adapt.

Learning to live with a tankless heater is like learning to drive a hybrid vehicle.  If you don't change your habits you will suffer - either with cold water - or with poor gas mileage!  I have seen people who have bought a Toyota Prius and have gotten very poor MPG simply because they refused to learn how it works and how to change their driving style to optimize it's performance.  I call this the "Lazy American Syndrome".  We have become so accustomed to the unlimited cheap resources that we have, that we forget how to think frugally.

If you are thinking of installing a tankless heater or you already have one installed and want to share your thoughts, head over to the forum on the topic tankless water heater and let us know your thoughts!

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With the energy credits the government offered I installed several tankless systems. So far the clients seem very happy. One wanted one because of the need for extra capacity for a new soaker tub and a lack of space for a higher capacity storage unit. Good article hope to do more tankless work in the future. Happy plumbing! Willis
Hi, I don't think your calculations are correct. Are you saying you only use $7.50 worth of gas each month, with 2/3 ($5) going for water heat?
ckmapawatt's picture
Tommy, What about my calculations do you think are incorrect? I'm looking at my June 2010 bill. I used 8.128 therms, and I pay $0.859/therm, so I used $6.98 worth of gas, which is actually less than I said. Now, I have other miscellaneous charges like the Base Charge, Customer Service Charge, and Sales Tax, but those charges will be there no matter what.

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