Drought? Saving Water with Residential Hot Water Distribution Systems

There are people out there who capture the cooled-off hot water in buckets and then use it for toilet flushing, plant watering and the like.  However, human nature being what it is, most people don't bother.  Let's face it, humans like convenience.  Rather than worrying about running the water down the drain, humans are more likely to be wishing they could get the darned hot water faster! 

Good news! There are products that can solve this very problem!  They can even save you energy depending on your hot water usage patterns.  Let's see what they are and how they work.

Residential hot water circulating (re-circ) systems

Residential hot water recirc or circulating systems slowly pump water through the hot water piping and into a dedicated return pipe back to the water heater.  This keeps the hot water pipes full of hot water and provides nearly instant hot water at the fixtures. 

The down side is that you have two new sources of energy consumption. The heat lost from the piping, and the energy to run the pump.  The pumping energy is fairly small, but it's there.  The energy lost from the piping can be considerable even with insulated pipes. 

The pump can be placed on a timer so it only runs during times that the hot water is likely to be used, but that creates a problem when you need hot water at an irregular time, and still uses a lot of energy.

Examining the energy losses

A report done for the California Energy Commission (Hot Water Distribution System Research – Phase I Final Report, California Energy Commission, May 2005.  Available here: http://www.allianceforwaterefficiency.org/default.aspx),  found that when hot water is circulated through a 1/2" diameter bare copper pipe with a flow rate of 1 gallon per minute and an inlet water temperature to the piping of 135 F, it resulted in a heat loss of about 2,000 Btu/hr.  

So the loss from a long piping run, say about 75 feet from the heater as the crow flies resulting in a round-trip piping run of around 200 feet, would amount to about 35,000,000 Btu/year. If the water heater is a natural gas water heater with an efficiency  of say 60%, then the amount of wasted energy will be over 58,000,000 Btu/year!

At $1.25 a therm, that 58,000,000Btu would cost about $725 per year.

Insulated pipes would cut that in half.

Pumping energy

Although the pumps often run 24 hours a day, they are low power pumps and only consume 50 watts or less. At 50 watts running 24 hours a day the pump would use about 400kWh/year, or at 13 cents per kWh, about $56 a year.  Obviously putting the pump on  a timer would reduce the energy consumption considerably.

Cold water return type circulating systems

Cold water return circulating systems use the cold water line as the return line. Since they use the cold water line as the return, they can't be filled with hot water. So what they do is circulate warm water.  "The pump and a valve is located at the furthest sink from the water heater or the end of a main trunk line. The inlet to the pump is connected to the hot water line, and the outlet to the cold water line. When the temperature drops to 85 degrees the pump comes on, and when the water temperature at the pump reaches 95 degrees it turns off. 

Grundfos, Laing, Taco Inc, RedyTemp and other companies are marketing these types of systems.  In Grundfos comfort systems the pump is located at the water heater and only a temperature activated valve is mounted at the fixture.

Your hot water isn't “hot” water, since the upper set point is 95 degrees, lower than body temperature.  The water in the cold water lines is cool, not cold.  The hot water will arrive more quickly than because less heat is absorbed from the advancing hot water than if the pipes were cold, but the hot water still has the same piping to travel through before you get “hot” water.

Reduced energy wastage

Because the water in the pipes is considerably higher than ambient temperatures, about 85 - 95 degrees.  With bare ¾” diameter copper pipe the heat loss for 100 feet of piping would be conservatively about 16 BTU/hr per foot, or about 14,000,000 BTU/ year, less the traditional type systems, but still an energy waster.  This type of system does not produce enough flow to operate a tankless water heater.

Examples of these type of systems are the Grundfos comfort system, the Laing Autocirc1, and the RedyTemp TL-4000-N and TL-5000-N.  The Grundfos comfort system has the pump at the water heater and a temperature controlled valve at the fixture. They all do the same thing; fill your piping system with tepid water.

Hot water demand systems

ACT Metlund Inc, Taco Inc, Chilipepper Sales, and other companies manufacture hot water distribution systems that eliminate the water wastage and the lost energy.  These systems are known as "Demand hot water systems".  Like the tepid water systems previously discussed, they connect between the hot and cold water lines at the fixture furthest from the water heater, or at the end of a main trunk line, and the pump turns on when you push a button “demanding” hot water.  The system turns off when the hot water arrives at the pump.

The pump only fills the hot water pipe with hot water when you need to use hot water.  It’s the same thing that happens when you want hot water if you don’t have a pumping system.  Therefore you don’t waste any energy except for the electricity to run the pump. But since these pumps typically run less than a minute and only when you want hot water, they typically consume only about $1-$2 per year in electricity.

Not all hot water demand systems are equal. Some will work with tankless water heaters, and others such as the Metlund S-02 don't produce enough flow to turn on the tankless heaters. Tankless water heaters usually require a flow of at least ½ gallon per minute.

Circulating system water savings

For those of you in drought stricken areas, these systems will save you significant amounts of water. The more often you use hot water the more you will save. Grundfos states on their website that  hot water demand systems can save up to 16,000 gallons of water per year for a four-member family.  According to a Dept. of Energy study, the average family of 4 wastes up to 14,000 gallons of water each year waiting for hot water.

Metlund has three models, the S-50T, the S-70T, and the S-0T.  Chilipepper Sales has one, the CP6000. RedyTemp has 4 models, the TL-4000-D, TL-4000-N, TL-5000-D, and the TL-5000-N. The “D” models however need dedicated return lines. The Chilipepper CP6000 appears to be the best bargain, with a 5 year warranty, the highest power motor, the lowest cost at less than $200, and the only one that has a 30 day money back guarantee.  The CP6000 will work with any tankless water heater and can be used with a dedicated return line if desired.

Fight the drought, install a hot water demand system