A Clever Energy Saving Idea From The Radio

A couple of days ago the NPR call-in program Talk Of The Nation had a segment on innovation in the troubled economy.  I only caught the end of it, but I heard a caller who was starting a business to help people save money by adding a nifty addition to their home water heating setup.  The idea is this:  instead of bringing the cold water line directly into the water heater, install an uninsulated water tank next to the water heater, and then draw from the top of that tank for the heater.  This allows the intake water time to warm to ambient air temperature, thus requiring less energy to further heat it to whatever you have your water heater set to.

I thought this was a very clever idea, and did a quick back-of-the-envelope estimate of what the energy saving should be.  For ease of approximation, I make the following assumptions:

• Assume a standard 40-gallon (151 liter) electric water tank, set to warm to 120°F (48.9°C).

• Assume that I use water in such a way that all the water in the tank has time to completely warm to the air temperature before I draw it into the water heater.  (Otherwise it would be necessary to model usage, flow rate, bring in Newton’s law of cooling, etc.)

• The table at the right gives the median ground water temperature in the United States as about 55°F (12.8°C).  I will use this as temperature of the water going into the heater without the uninsulated tank.

• Finally, assume that the ambient air temperature around the uninsulated tank is a constant 72°F (22.2°C) year round.  This is almost certainly a net underestimate where I live, a net overestimate in some places, and probably pretty close to correct if your water heater is in an air conditioned room (but then the energy use of the whole system is more complicated to calculate).

The specific heat of water is 4186 Joules/kilogram, so without the uninsulated tank the energy to heat one water-heater-full is (4186 J/Kg°C)(151 Kg)(48.9-12.8 °C)= about 22.8 million Joules.  With the uninsulated tank in the system, the equation changes to (4186 J/Kg°C)(151 Kg)(48.9-22.2 °C)= about 16.9 million Joules.  The difference is 5.9 million Joules, or about 1.63 kilowatt hours.  Assume that I use one full tank of water per day.  The current average price of electricity is 11.47¢/kWh, which amounts to a savings of $5.60 per month.

This is a really rough estimate, but it is enough to convince me that this alteration to home water heating is probably in the class of improvements that will pay for themselves in reasonably finite time.  Good on you, clever radio call-in man.


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  1. I think the assumption that the water heater is located inside climate-controlled living space is quite strong.

    In my case (and everyone’s that I know around here), the water heater is located in the basement. Basements generally aren’t heated directly, relying on heat loss through the floor, the heating ducts, etc. for their heat.

    If it’s -10 F outside, the temperature in my basement will be below 40F. That’s not much of an improvement over the groundwater temperature.

    Of course, this fact means I get a much bigger improvement in efficiency by using a water heater blanket and insulating my hot-water pipes.

  2. There is huge variability depending on what part of the country you live in, clearly. My water heater is out in the garage, which isn’t air conditioned. My parents’ water heater is inside, in the corner of the library. So how much benefit you could get from this system will vary a lot. In your case, something with a valve where the uninsulated tank lives outside and you only use it during the summer might be the best bet.

    Regardless, I still think it is a clever idea, and that the guy who wants to start a company to come to your house and figure out these variables for you probably has a viable business model.

  3. Michael Fischer

    March 7, 2009 — 10:13 pm

    Interesting idea. The achievable benefit is likely lower than you estimate because, in most localities, the typical temperature of cold water entering a residence is close to the average annual temperature, which tends to be higher than the groundwater temperature.

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