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Hot Water

Hot Water heating consumes approximately 15% of a homeowners utility bill. So, it is important to address this in your quest for energy efficiency.

Before one looks at the hot water heater itself, it is best to address consumption and distribution.  Once you have a handle on that, you can address how to best heat the water.  By looking downstream of the hot water heater, there are several things you can do do minimize the use of hot water, and thus affect the sizing of the unit.  That will save you money initially and throught the life of the home.

Firstly, choose water saving and energy efficient dishwashers and washing machines.  Front Loading Washers are extremely water efficient.  Of course, educating homeowners on when to use hot water for clothes washing is important as well.  Next, is the use of water saving fixtures.  This includes all faucets and shower heads.  They typically limit water flow to 1.2gpm or less.

Then comes the design of the hot water distribution system.  Much hot water is lost, just waiting for hot water to arrive when you turn on the faucet or shower. It tests your patience as well as using the hot water you paid to heat, just to fill the pipes with hot water. Then, when you turn off the water, all the hot water in the pipes between you and the hot water heater now cools down, thus, wasting the energy you used to heat it.  Plumbers tend to oversize the piping used to feed water to faucets and they often use a branch circuit with several faucets being connected to each circuit.  To keep water flowing at full pressure when all faucets on the circuit are on (almost never happens) the size the piping in the branch circuit can be fairly large.  If you calculate the amount of water contained in the pipes, you will see it is substantial.  For example, if the pipes to a faucet contain 1 gal of water and your faucet is rated to flow 1gpm, then it will take you a full minute to get the cool water in the line flushed out and replaced with hot water.  Then you use 10 seconds worth of hot water (1/6 of a gal) to wash you hands.  When you're finished the gallon of hot water in the pipe just cools off again. Hmm... That's about 1.2 gallons just to wash your hands and is a waste of both water and hot water and money.

We changed up the design to use a distribution manifold located near the hot water heater and then make an individual direct run from the manifold to each fixture.  Because the water flow of most water using appliances is very low, we've downsized most of the piping size to 3/8" PEX tubing.  Some plumbers will say that's too small, but if you do the math and look at pressure drop and water velocity in the tubing, you will find all the numbers work out just fine.  Now, with the direct runs and smaller tubing, there is much less water in the lines and one has to wait much less time before water is flowing at the faucet.That saves water and hot water!  

Now that you've addressed the fixtures and appliances, it's time to see how best to heat the water.  But before we get into details, let's look at efficiencies of various methods:

Type of Water Heater Efficiency
Gas- Convential Tank .67
Gas - Instant .7
Electric - Conventional Tank .9
Electric - Instant .9
Electric Heat Pump 2-2.5
Solar 100+


My recommendations are:

  • Don't use either gas or electric tankless units. Their energy savings is typically overstated, they are too expensive to install and maintain, and provide inconsistant hot water if demand gets too high.  See Consumer Reports evaluation of Tankless Hot water heaters:

  • Consider solar hot water if you can locate a system appropriate for your area and at the right price.  We've use salvaged solar panels from military bases and work with a distributor for the remaining components.  We've done the installations using our volunteers.  However, solar HW systems can be a bit pricey and with the new Heat-Pump water heater and the reduced costs of Solar PV systems, you may find it difficult to justify Solar HW systems.

  • Look very closely at Heat-Pump waters heaters.  They are getting more popular and are more that twice as efficient as conventional electric hot water heaters. There is at least one Manufacturer that makes one that has the ability to duct the supply and return air to the heat pump.  In the summer, the cool air given off by the heat pump could be circulated into the house to lower the overall cooling load.  In the winter it could then be switched to discharge the cool air to the outside.   See  They are considerably less expensive than solar HW systems and are making the decision to use solar HW systems a bit more difficult to justify.

    A great combination is to use a heat-Pump water heater to lower power consumption of water heating by about 2/3, then use Solar PV panels. The money you save with a smaller Solar PV setup will more than pay for the extra cash for the water heater.  You will need to look at the installation requirements of the water heater as some require the heater to be located in a room of about 100 sq ft.

  • use Conventional Tank -Electric HW Heater and add an extra blanket of insulation around it to reduce standby losses to a minimum.  Use of electricity means that at least some portion of the electricity can be provided by renewable energy located on the grid. Fuel for gas HW heaters of course is not renewable.  

    If your house has solar panels, then all of your energy consumption is provided by the sun.  With use of solar PV, the cost of the hot water heater and the solar panel portion to cover hot water heating is most likely cheaper than a solar hot water system as well.