Posted by: SherlockHomesSY | 30 Nov 2010

Common Complaints: Wasted Hot Water


Energy in hot water systems is lost by . . .
– Waiting for hot water
– Heat going down the drain
– Water cooling in water pipes
– By heating it
– Overheating
– Recirculation

After heating and cooling, water heating is the second largest source of energy use in our houses.

There are two sources of waste of hot water: behavior and hot water system design.  One we each control.  The other we may be able better control without replumbing the entire house.

Waiting for Hot Water. We’re willing to wait 3-5 seconds for hot water, and occasionally 10 seconds, but not several minutes.  However, we live with what we’ve got.  My “favorite” reaction is turning on the hot water and going away to do something else while waiting.  Even if the fixture doesn’t overflow, the water’s been hot a long time before you came back.  This is behavioral waste.

Restricted Flow. Some well meaning bureaucrats have made the problem worse, not better, by restricting water use at our fixtures.  We left the water pipes the same size while lower the water flow through the faucet and showerhead.  Now, before we can use the hot water, we have to waste 2-4X the amount of water to get hot water where we want it in the first place and have to wait up to 18 X as long to get hot water.

In my frustration, I adjusted by running water in the through the high flow-rate tub faucet before turning on the sink.  Little did I know that not only did I save considerable time, I saved a lot of water too!

The bureaucrats wanted us to restrict our use of hot water once we got it.  What they did was make us waste more energy, water, and time.  Don’t throw away your new fixtures though.  I’ll let you know how to fix it!

The other reason we have to wait is because some of our pipes are too long.  Since 1970, the average length of the longest run of pipe grew from 30 feet to 80 feet.

Heat Going Down the Drain. Unless you have a multistory house and want do a small amount of re-plumbing, there isn’t much you can do about it besides lowering the water temperature and reducing the amount of water used: both excellent things to do anyway!  There are Drain Waste Heat Recovery devices, but they require an 8 foot drop below the shower or tub.

Water Cooling in Water Distribution Pipes. The wait time for hot water results from having to clear the once heated water, but now cool water, from the water distribution pipes.  Without re-plumbing, the best option is to insulate the entire hot water line with at least R-4 insulation.

Insulation doesn’t help much when water hasn’t been used for over an hour, but it can make a huge difference during your busy periods, such as getting ready for work and cooking dinner.  Insulated ¾ inch copper pipe will keep water in the pipe hot 3X longer (60 minutes) than it would if it were uninsulated.  It doubles the cool down time for ½ pipe.

Water Heating. You’ll probably stick with the water heater you have until it has to be replaced.  Water heating technology is improves significantly between average water heater changes. Someday, you’ll need to replace your water heater.  When you do, get the best efficiency you can afford.  Do your research, first!

Sometimes, it makes sense to have more than one water heater, especially to reduce the water distribution pipe length to the water fixtures.  If you have a big house, you might be better off!

An idea that intrigues me that I really hadn’t thought of before is passively preheating the cold water before allowing it into the water heater.  Why should I have to pay for the water heater to do what I can let either the sun or indoor air temperature do for much less cost?  The water coming out of the ground is colder than the temperature inside my house.  With a solar pre-heater, I can probably get the temperature even higher before injecting it into the water heater.

Overheating. Some people set their water temperature way too hot with the misguided idea that they well get hot water faster.  Physics says it won’t happen.  The same amount of cooled water has to be pushed out of the way.

Overheating water does allow people to have hot water longer because less of it is used to mix with the cold water for a shower.  However, this extra-hot water is wasted most of the time—waiting for the next use!

Hot water needs to be between 110 and 120°F at the water heater.  This is set by the Building Code.  It helps prevent scalding, while lowering hot water energy loss.

Recirculation. On-demand hot water recirculation is the best way to shorten wait times.  In addition, it’s the most energy efficient.  Pumps that run continuously wastes 90% of its cost in water heat loss and 10% from pump operation.

The best option is to place at least one on-demand water recirculation pump at the fixture farthest away from the water heater.  You may need more, depending on how the plumbing is laid out in your house.

The on-demand option allows you to tell the pump when to recirculate water.  The cool water from the hot water distribution line will be rerouted back to the water heater.  This is the safe way to turn the water on and go do something else while waiting.  It also saves water and energy.

Demand controlled water circulation systems are available from these manufacturers:

ACT Inc. Metlund Systems


Diagnosing and correcting water heating issues can be found on my website.

Click the picture at the top of this page for the class notes I used to create some of this article.  My instructor, Gary Klien, is the authority on hot water systems worldwide.  My notes are from a class taught for Pacific Gas & Electric (PG&E)

In Conclusion

Next time, Mold, Mildew, and Musty Odors: Part 1 of 2, Outdoor Sources


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